What is the difference between a chemical reaction and a nuclear reaction? |
Explain the difference between an exothermic reaction and an endothermic reaction. |
Define the three types of chemical reactions; combination, decomposition and replacement. |
What is "standard conditions" and how is it used? |
What keeps two atoms from getting too close to each other? |
What keeps two atoms bonded together from coming apart? |
If the sodium cation (Na+) is attracted to the chlorine anion (Cl-) by electrostatic attraction, what is to prevent their nuclei from coming completely together? |
Explain bond dissociation energy, bond stretching and average bond length. |
Draw and explain a reaction path, activation energy, endergonic and exergonic reactions.
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What are the "rules" for figuring out the (likely) valence number of an atom? |
Explain reduction and oxidation.
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Fire is a chemical reaction. You know that. The word "fire" is used by different people to mean different things. It doesn't refer to a specific chemical reaction but to a group of different kinds of reactions that have the same effect. That is, many things can burn in a fire. The simplest "fire" occurs when methane combines with oxygen to produce carbon dioxide and water.
Write a balanced equation for this reaction. (You should know their molecular formulas by now.)
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Identify the redox "pair" in the equation you just balanced and describe what is being oxidized and what is being reduced. Warning! This is not as easy as it looks. (Notice I have the word "pair" in quotations.) Hint: If you get confused it's always smart to step back and think of the basic ideas. |
Now that you have identified the redox "pair", (in the oxidation of methane) prove that the electrons are balanced by accounting for all of them. That is, prove all the electrons are accounted for by showing which atoms release electrons and which atoms absorb them. Keep track of the numbers. Hint: it helps to set up the reduction and oxidation half-reactions to show all the electrons as they are swapped around. Write a series of redox "pairs" proving that the electrons balance. This isn't easy but you will learn a lot by trying. So take your time.
By the way:
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A very important chemical reaction of the 20th century occurs inside a car's engine. Gasoline (or "petrol" to the British) is a mixture of hydrocarbons (molecules made of hydrogen and carbon). It is "burned" inside the engine to power the car. These type of engines are called "internal combustion engines". (And now you now why!) The huge amount of heat produced in the reaction causes the gases to expand rapidly and push against a movable joint called a piston. A series of these "combustion chambers" are linked together by mechanical parts, turning chemical energy (combustion) into mechanical energy (rotation). Write and explain a balanced equation for the combustion of octane (C8H18). Use your knowledge of "combustion" to figure out the skeleton equation. Then do the balancing, but just balance the atoms. (We'll ignore the electrons. They will balance anyway, if you do the atoms and ions correctly.) Hint: The skeleton equation will look a lot like the combustion of methane. But the balanced equation will look VERY different. If the coefficients you get look big or strange, you are working in the right direction. |
One problem with the internal combustion engine is that it is INTERNAL. Combustion requires oxygen but the car's engine is a combustion chamber. The chamber is a CLOSED container. If it weren't closed, the expanding gas from combustion would not push against the piston and you would not get any mechanical energy. Car engineers design the chamber to open briefly, fill full of air and fuel, and then close before it is "ignited" (from a spark produced by a sparkplug). This all occurs in the blink of an eye. The trick is to get the right mixture of fuel (octane) and air (oxygen) into the chamber before the chamber closes. To do this, engineers designed another mechanical device called a carburetor. A (good) mechanic knows how to adjust the carburetor to give the best combustion. What would happen if there were not enough oxygen? Would there be any combustion? Would the energy output be the same? Would the products be the same? Think about it. |
Carbon monoxide (CO) is made when any hydrocarbon (fuel) is burned without enough oxygen. It is a colorless, odorless, flammable and highly toxic gas. It is produced by all car exhausts. Even the best "tuned" cars make it. Carbon monoxide is a product of "incomplete combustion". Write a skeleton equation, then a balanced equation, for the INcomplete combustion of octane. Hint: The skeleton equation is like the skeleton equation for the complete combustion of octane, but carbon monoxide is made instead of carbon dioxide. Also, once you have the equation balanced, compare it to the balanced equation you figured out earlier for the complete combustion of octane. Explain any differences in the coefficients (number of molecules) between the complete and incomplete combustion of octane. |
What would be the equation that would convert toxic carbon monoxide
into (relatively) harmless CO2?
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The oxidation of carbon monoxide (which you worked on in Q 18)
is a very slow reaction. |
The oxidation of carbon monoxide to carbon dioxide is a slow process. "Catalytic converters" are installed to speed it up. The (hot) "exhaust" gas from the combustion chamber is passed through a pile of pellets coated with either platinum (Pt) or palladium (Pd). Both of these metals can act as a catalyst for the reaction. What is a catalyst? How does it affect the energy path of the reaction? How might you "fit" it into the chemical equation? Use the chemistry of the "catalytic converter" to illustrate your points. |
You may have been surprised that the catalysis of the oxidation of carbon monoxide did NOT produce a lot of extra heat. In fact, the amount of heat produced by the oxidation of carbon monoxide is the same with or without a catalyst. With the catalyst the heat is produced FASTER, but it doesn't make more of it! Explain why the amount of heat is the same whether you use a catalyst or not. Hint: Think about what produces the heat (in any exothermic reaction). |
Nitrogen (N2) is in the combustion chamber because it is sucked
in with the oxygen through the carburetor.
Nitrogen wasn't in the skeleton equation because we weren't interested in it. Nitrogen has nothing to do with the combustion of the fuel. But nitrogen is in the chamber and it can take part in other reactions not (directly) related to the combustion of octane. Side reactions are the name Alchemists give to chemical reactions that are not the ones they want to happen, but which happen nonetheless. Many Alchemists spend their lives trying to stop side reactions. In the very hot chamber of the car engine, nitrogen can combine with oxygen to produce a wide variety of side reaction products that are generally called "smog".
One of the visible components of smog is a yellow to reddish-brown
gas called nitrogen dioxide (NO2).
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The oxidation of nitrogen (N2) is a redox reaction. You balanced the atoms, so the electrons will balance. But check it to see if that is right. Identify the redox pair and prove the electrons are balanced. You'll learn a little more about valency. Also, compare this reaction with the oxidation (combustion) of methane. What is different about it? Could those differences be the reason why nitrogen oxidation is endothermic but methane oxidation is exothermic? Explain. (Give it some thought. Think about bonds, etc.) |
Nitrogen dioxide (NO2), produced by the oxidation of N2 inside the carburetor, is pushed out the car's exhaust and escapes into the atmosphere, creating "smog". Eventually the nitrogen dioxide reacts with water in the air to form nitric acid (HNO3) and nitric oxide (NO). Write a balanced equation for this reaction. Note this is NOT combustion! |
Let's have a closer look at that new molecule, nitric acid (HNO3).
When nitric acid is added to water it ionizes into its two ions. No surprise here. It behaves like most ionic molecules. Table salt (NaCl) does the same thing. But here the cation is H+ (just a simple proton!) and the anion is the radical nitrate (NO3-). Write an equation for the ionization of nitric acid (as it dissolves in water). |
Think back to what you know about water. Most water molecules are best imagined as (polarized) covalent molecules. But a few are ionized. Write an equation for the ionization of water. (Easy!) |
What would happen to a drop (or lake) of water if nitric acid fell into it? Think about the ions. |
Recall that equation (again) for the reaction of nitrogen dioxide
with water.
What is the valency of the nitrogen in nitric oxide? (NO)? What is the valency of the nitrogen in nitrogen dioxide (NO2)? What is the valency of the nitrogen in nitric acid (HNO3)? Also, identify the redox "pair" in the equation. |
If you were surprised by nitrogen's many possible valences, here's more to surprise you.
Nitrous oxide is not involved in the chemical reaction we have
been looking at. But it is worth mentioning.
Hint: Recall the formula of nitric oxide (NO). Recall how cations have different names based upon valence number. What is the difference between nitric oxide (NO) and nitrous oxide. First find the valence number of the nitrogen atom in nitrous oxide and then "design" a NEUTRAL molecule made of it and oxygen. |
Let's move on to other air pollution Alchemy and give you a chance to try your hand at more of this difficult stuff! Many fossil fuels contain sulfur (S) in its pure elemental form. (It can be in any one of many allomorphs, but that doesn't matter for these examples. Just think of it as single atoms of sulfur.) When the fuel is combusted, the sulfur (S) is also oxidized (in a side reaction) to make sulfur dioxide and sulfur trioxide. Write and balance an equation for the oxidation of sulfur (S) into sulfur dioxide. Write and balance an equation for the oxidation of sulfur (S) into sulfur trioxide. Remember, it takes oxygen (O2) to oxidize them and both products are neutral molecules. |
Determine the valence numbers for the sulfurs in sulfur dioxide and sulfur trioxide. Also, use your knowledge of the way we name cations to "guess" the other names for those two molecules. (Like "something-oxide".) |
Magnesium (Mg) is a metal from the Group II elements. What is magnesium's valence number in any compound it forms? How many oxygens can combine with magnesium (to produce a neutral compound)? How many sulfates (SO4-2) can combine with magnesium (to produce a neutral compound)? |
Magnesium oxide is used in "scrubbers" to remove the sulfur dioxide from the smoke of coal-burning plants. The magnesium oxide combines with the sulfur dioxide and oxygen (O2) to turn them all into harmless magnesium sulfate. Write a skeleton equation for what goes on inside the "scrubbers" and then balance it. |
The sulfur dioxide and oxygen are gases but the magnesium dioxide is a solid. What might you do to increase the efficiency of this "scrubber" reaction? (Don't think about catalysts. Think about collisions.) |
Here's the chemical equation for the ionization of table salt
in water.
NaCl(s) + H2O(l) ------> Na+(aq) + Cl-(aq) + H2O(l) What do you think (l) and (aq) stand for? |
Write an equation, including subscripts for states, for the ionization of water. |
I've made a serious mistake in ALL the equations I've been writing in my questions and answers! It has nothing to do with the valences, balancing or even forgetting to include the subscript states. What is wrong with all those equations? |
What is Le Chatelier's principle? |
What is enthalpy? Does it play a part in determining a reaction's direction? Does it affect exothermic and endothermic reactions? |
Consider these imaginary reactions and their changes in enthalpy: A------> B (200 units) meaning it needs 200 units of enthalpy. B------> C (-50 units) meaning it gives off 50 units of enthalpy. C------> D (100 units) meaning it needs 100 units of enthalpy. D ------> E (-1000 units) meaning it gives off 1000 units of enthalpy. Determine whether each of these reactions is exothermic or endothermic. |
How many units of enthalpy would you need (or be given off) to turn A turn into E? Is it (the total of all these reactions) endothermic or exothermic? |
Use those same values to figure out how many units of enthalpy would you need (or will be given off) to turn A into C? Is it (the total) endothermic or exothermic? |
Use those same values to figure out how many units of enthalpy would you need (or be given off) to turn B into A? Is that an endothermic or exothermic reaction? |
Use those same values to figure out how many units of enthalpy you would need to make this reaction go from A to E? A------> B ------> B ------> A -------> B ------> A ------> B ------> C ------> D ------> E |
Those previous questions all used the enthalpy CHANGE of each REACTION. But there's another way to look at these problems. You can think about the enthalpy IN each MOLECULE. Here's the enthalpy of (in) each molecule: A = 100 units B = 300 units C = 250 units D = 350 units E = -650 units Now use these values for the enthalpy of the molecules to calculate the enthalpy changes that occur in each reaction. Explain why each is exothermic or endothermic. You should arrive at the same values as you were given in question 40, so use it (Q 40) to check your answers. Hint: I will get you started with this clue. To turn molecule A (which has 100 units) into molecule B (with 300 units), what must be added? Does that look familiar? (Look at the value for that reaction in Q 40.) |
The gas nitrogen dioxide (NO2) can be made from the gases nitrogen (N2) and oxygen (O2) in this reaction 2O2(g) + N2(g) ------> 2NO2(g) but it takes 66.4 units of enthalpy to make this reaction "go".
What is the enthalpy (of formation) of NO2? (Be sure to get the sign right too.) Hint: Think about what you know about enthalpy and what you just learned about the enthalpy of formation. Warning: Notice it takes 66.4 units to make TWO molecules of NO2, but the question asks for the enthalpy of just one molecule. |
Imagine somebody told you he could turn O2 and N2 into NO2 (all gases), but it would "cost" only 10 units to make each NO2 molecule. No other reactants are needed and no other products are made. He says he uses 6 different steps to get there, but only 10 units are used up all together! Can he be right? Could his secret be a catalyst? Explain. |
Here's another one. 2H2(g) + O2(g) ------> 2H2O(g) and it produces 484 units of enthalpy (heat) in the process. What is the enthalpy (of formation) of H2O? (Be sure to get the sign right too.) |
An Alchemist says she has measured the enthalpy of liquid water,
H2O(l), to be -285.
Can she be right? Explain. Warning: Give this a wee bit of thought. Look at what she has measured and what you have measured (calculated). |
Use the information from the last two questions to find the heat of fusion of water. |
If a powerful electric current is passed through water, the water molecule breaks up into hydrogen and oxygen gas. This is the reverse of the reaction described by that female Alchemist. Assuming it is at standard conditions, you should be able to tell how much enthalpy is needed to produce these two gases from water. |
What is entropy and why is it important? |
Imagine you tossed ten coins into the air. Roughly, how many would land "heads" and how many "tails"? What would you have to do to get all of them to be "heads"? What does this have to do with entropy? |
When a natural or spontaneous reaction occurs in an isolated system, what will be the sign of the total entropy change? Explain what that means. Is order or disorder created? What must be done to cause a reaction to go from disorder to order? |
What is Gibbs energy? What three things are included in Gibbs energy? Why is Gibbs energy important? |
Try to use your understanding of Gibbs energy to explain how some reactions do things that seem "unnatural". |
Describe the enthalpy and entropy changes that occur as water freezes. Determine whether the changes in enthalpy and entropy are positive or negative. How might they (enthalpy and entropy) affect each other? Try to guess how temperature might influence the freezing and melting reactions. |
Some people believe that the evolution of life is contrary to
the laws of nature.
Does the long and complex "reaction" called evolution
mean the ideas about Gibbs energy don't work?
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Water and carbon dioxide can be turned into oxygen (O2) and a sugar called glucose (C6H12O6). Write a skeleton equation and then a balanced equation for that chemical reaction. |
Take a good look at the equation you have just balanced.
Glucose is a molecule of VERY high enthalpy. Oxygen
(O2) is too.
Give that some thought. Explain the enthalpy and entropy changes going on in the reaction. Do you think the reaction will "go" in the direction as written? Explain. (Without the Gibbs energy values it is impossible to say for sure, but tell me what you think might or might not happen.) |
You may have been surprised to learn that reaction runs the opposite way. Thermodynamics tells us so. (You didn't find that out exactly. You have a general overview.) What if you REALLY wanted that reaction to go to the right (as written), from carbon dioxide and water to glucose and oxygen? What must you do to make it run that way? What must you add? (Hint: you don't add molecules or atoms because that would change the reaction.) |
You now know that (without sunlight or other sources of outside energy) that reaction should go in this direction. C6H12O6 + O2 ------> 6CO2 + 6H2O (By the way, notice that the glucose is oxidized.) Does this reaction give off energy? |
In the previous question you learned that energy is given off in a hot flame. That's the definition of combustion. You also learned that the activation energy must first be supplied to get it over the transition barrier. Is there a way to make that reaction go without setting it alight? What could lower the transition barrier? |
Use the reactions of photosynthesis and respiration to explain how life on earth "works". Where does the sun fit into all this? How is your own Alchemy affected by the plants and how does your own Alchemy affect the plants? |
Recently, some new ecologies have been discovered around deep sea volcanoes. These ecosystems are far too deep to use the sun. Indeed, it is pitch black down there. Yet, they are run by the production and destruction of glucose! Obviously, respiration (the enzymatic oxidation of glucose to carbon dioxide and water) doesn't need the sun or any other external energy to run. That's because respiration has a negative Gibbs energy change. But the opposite reaction, turning carbon dioxide and water into high enthalpy and low entropy glucose, requires energy. How can these deep sea ecologies make glucose?
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Where can you learn more about Alchemy and the universe? |
Chemical reactions involve changes in how electrons are distributed (among strong bonds) or how electrons interact (among weak bonds). Chemical reactions often rearrange atoms, but they never create new atoms (elements) or electrons.
Nuclear reactions have nothing to do with electrons.
Note.
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Exothermic reactions give off heat as higher enthalpy reactants are turned into lower enthalpy products. Endothermic reactions absorb heat in order to turn lower enthalpy reactants into higher enthalpy products. An exothermic reaction feels warm (perhaps hot and explosive) while an endothermic reaction feels cold. |
Combination joins two (or more) substances to create a larger substance. Decomposition breaks a substance into two (or more) smaller substances. Replacement substitutes one element or part of a molecule for another. |
Alchemists define standard condition as one atmosphere of pressure and 0 degree Centigrade (but some use 25oC). Any other temperature or pressure is a non-standard condition. Alchemists often change conditions in order to make a reaction go in a certain direction or at a certain speed. The standard condition is used as a baseline. The rates or direction of a chemical reaction at a non-standard condition is often compared to the rate or direction at the standard condition. |
The outer shell of atoms keeps atoms apart by electrostatic repulsion. As atoms approach each other their electrons start to push against each other. It is this electrostatic repulsion that you must push against in order to squeeze atoms tighter together. |
It is the bond that keeps two atoms together! That's not meant to be a silly answer, but it explains that atoms are held together by bonds and bonds can be made in different ways.
Two atoms held together by a covalent bond are kept from drifting
apart by each atom's "desire" to reach a noble electronic
structure by sharing electrons between them.
The important point to understand is that ALL atoms are prevented from getting too close together by the electrostatic repulsion of their outer shells, but there are a variety of different processes keeping atoms together. |
Electrostatic REPULSION keeps ALL atoms from getting too close, even those atoms drawn to each other by electrostatic ATTRACTION! That may seem a wee bit strange, but it is true. Even the sodium cation, with its overall positive charge, has shells of electrons that will keep the anion from colliding with its nucleus. The sodium cation's overall positive charge "peeks out" to attract the anion, but the cation's shell keeps them apart. |
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If the energy output (from an exergonic reaction) is more than
the activation energy, a chain reaction can be occur - a
series of chemical reactions that can run on their own. But even
these require some activation energy to get them started. (Otherwise,
they would already have "gone"! Right?)
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1. Elemental molecules have a valency of zero (0).
2. Atoms in ionic molecules:
3. Atoms in covalent molecules:
4. Oxygen has a valency of -2 except peroxides, which are -1. Try to figure out the valence number of other elements by using the valence number of the atoms and ions it bonds to. |
Reduction is the gain of electrons (GER).
Oxidation is the loss of electrons (LEO).
Oxidizing agents cause the other substance to oxidize by
stealing away electrons.
Reducing agents give the other substance an electron, reducing
it.
Redox is short for "reduction-oxidation".
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First start with a rough equation, describing the reaction in
words.
The next step is the real balancing act. Look at both sides of
the equation. Count up the atoms.
Approach all balanced equations as a puzzle to be solved. Looking
over the unbalanced equation you see that hydrogens and oxygens
are the trouble. Perhaps that gave you a clue. Water is made of
both these elements and water is produced in this reaction. By
adding an extra molecule of water (adding a coefficient) you get
the hydrogens balanced.
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Here's that balanced equation again: CH4 + 2O2 ------> CO2 + 2H2O
To really understand a redox reaction you have to break it up
into a pair of redox equations (sometimes called half-equations
because each represents only half the "action".) But
that's not always easy to do.
Notice that the carbon from the methane is oxidized (in both meanings of the word, but here I mean "it combines with oxygen"). But the hydrogens are oxidized too! BOTH of the elements in the methane molecule are picking up oxygens, so they are BOTH oxidized. The entire molecule of methane (CH4) oxidized!
That means the oxygen molecule (O2) must be the one reduced! That
may or may not have come as a surprise. Think back to your definitions
of reduction and it becomes clear. Reduction once meant that an
atom loses its oxygen atom(s). That is exactly what happens to
EACH oxygen atom in the oxygen molecule. When any atom, even an
oxygen, loses an oxygen atom it is reduced.
Now let's summarize the redox "pair".
If you had trouble with that, don't be disappointed. It isn't obvious what is going on. You have to look at the "details". Read through the explanation again to make sure you "get it" before doing the next question. |
Here's that balanced equation again: CH4 + 2O2 ------> CO2 + 2H2O
Let's start with the reduction. The two oxygen molecules are made
up of four oxygen atoms all with a valency of zero. They have
no charge. They are turned into four oxygen anions, each with
a charge of -2. (You can also think of those 4 oxygen atoms as
simply part of compound elements, in which case they have a valency
of -2. It's the same thing.) That means each oxygen atom must
get 2 electrons. So the half-reaction of reduction must involve
oxygen and the correct number of electrons to do the job.
Methane is oxidized. The full-equation is balanced so that means
all eight of the electrons collected by the oxygens must come
from the one molecule of methane! And that's exactly what happens.
Summarizing the redox equations we get
You may have noticed we are not completely done. The reaction doesn't end here. All those ions on the right side must go to form the final products. Imagine all those positive and negative ions mixed together. They will quickly be attracted by their opposite charges and form new bonds. C+4 + 4H+ + 4O-2 ------> CO2 + 2H2O
Notice that they are drawn to each other by electrostatic attraction,
but the bonds they eventually make are covalent (for the most part). The carbon cation
(C+4) is "happy" to attract a couple oxygen anions.
Those oxygen anions neutralize the charges AND provide the electrons
needed for the covalent bonds in the carbon dioxide! The same
thing happens with the hydrogens (naked protons). They attract the oxygen anions
and these anions provide ALL the electrons needed to make the covalent
bonds in the new water molecules.
This has been a VERY long and difficult question to answer. If you had all that figured out, you'll make a better wizard than me! It took me a long time to understand this reaction the first time I saw it. That's because it is complex and involves many "in between" steps which do not come immediately to mind. However, it shows you that you can understand chemical reactions by working slowing and carefully and keeping in mind the basic laws of Alchemy. Here you have seen valency changes, electrostatics, and the formation of covalent bonds from "scratch". Alchemists call all these detailed steps a reaction mechanism. Sometimes you have to go into the details of a reaction mechanism in order to understand the reaction.
Some Alchemy students are tempted to ignore this equation (and
ones like it) because it's hard. But that is exactly why you should
NOT ignore it. The reaction you have been looking at and working
so hard on, is a very important part of Alchemy. So important
that Alchemists give it a special name - combustion. Combustion
is the RAPID oxidation of a substance to its oxides (in this case,
carbon dioxide and water) with the production of a lot of heat
and light. Most (but not all) combustion involves the oxidation
of a hydrocarbon (as in our example, here). This double oxidation
(double combustion) is a very FAST reaction and it produces a
LOT of energy (as heat and light). "Fire" is combustion.
All the energy of this combustion comes about from the C-H bonds
destroyed. The products (carbon dioxide and water) are much lower
in enthalpy than the reactants, so the reaction is (VERY) exothermic.
Read these last few answers again (about methane's combustion) and be sure you understand them before going on to the next question. |
Start with a skeleton equation. Hopefully, you understood that an equation involving the combustion of octane (or any hydrocarbon) requires oxygen (O2) and produces carbon dioxide and water. So the skeleton equation is C8H18 + O2 ------> CO2 + H2O Now you must balance the atoms. You can start with any element. Most experienced Alchemists leave the oxygen for last. They know that a combustion reaction will use up lots of oxygen so they tackle the other elements first. But you can do them in whatever order you like. Here's how I did it.
There are 8 carbons in the octane and all carbons end up as CO2
in the products. So each molecule of octane provides enough carbon
for 8 molecules of carbon dioxide. You will also notice that the
one molecule of octane has enough hydrogen to produce 9 molecules
of water (18 hydrogen can make 9 H2O). Therefore one molecule
of octane has enough carbon for 8 carbon dioxide molecules and
enough hydrogen for 9 water molecules.
Now the carbons and hydrogens balance, but the oxygens don't.
You must place a coefficient in front of the oxygen molecules
to give a balanced number of oxygens. This is not easy. How many
oxygen atoms are in the product (in total)? There are 16 oxygens
in the carbon dioxide molecules and 9 oxygens in the water molecules.
That makes 25 oxygen atoms in the products so you must have 25
oxygen atoms in the reactants.
It looks as if you need twelve and a half (12 1/2) O2 to make
the equation balance. That would provide 25 oxygen atoms. But
there is no such thing as a half-molecule.
2C8H18 + 25O2 ------> 16CO2 + 18H2O
Now you have 16 carbons, 32 hydrogens and 50 oxygen atoms on both
sides. It's balanced.
You may have arrived at the same answer by a slightly different route, but you had to deal with the odd number of oxygens somehow. Half-molecules don't really exist but you will often see Alchemist using them in "balanced" equations. Sometimes they do that because they want to avoid doubling everything. (They like to work with only one molecule of octane in their thoughts.) Balanced chemical equations are really just a way to calculate ratios (proportions) among the products and reactants. Using 1/2 as a coefficient isn't really wrong, but it does present a wrong image because you can't have half-molecules in real Alchemy. |
If there were not enough oxygen, combustion would be very poor, maybe none at all. The exact effect depends upon exactly how little oxygen there is and for how long.
With no oxygen, there would be no combustion, no energy given
off and no products produced.
When there is some oxygen but not enough, things get complicated. Recall that equations represent the reaction at the scale of a few molecules at a time. Indeed, the best balanced equations have the minimum amount of molecules (coefficients). It makes the equation simpler to understand. But a combustion chamber in a car's engine will have many billions of molecules. If there are not enough oxygen molecules to go around, all the oxygen will be used up but not all the fuel. That effects both the power output (energy released) and products (molecules) made. If there is not enough oxygen you will get "incomplete combustion". The power will be less than "perfect". Indeed it can be very bad. More importantly, the products created are often not the ones you expect. The octane may not get broken down completely into carbon dioxide. Instead you may find small hydrocarbons in the product. There are other incomplete combustion products to worry about. They are explained in the next questions. |
Carbon monoxide (CO), instead of carbon dioxide, is produced from
incomplete combustion of hydrocarbons. The skeleton equation for
the incomplete combustion of octane is
Notice that the reactants are exactly the same as for complete combustion. You may wonder (then) how does the reaction "know" it is to be complete or incomplete in the combustion? You will arrive at the answer here.
Balance the equation.
Now the oxygens have to be balanced. There are only two on the
left but 17 on the right. Once again, we have that weird half-molecule
problem. It will take 8 1/2 molecules of O2 to give you the 17
oxygen atoms required. That makes
But half-molecules aren't real so multiply everything by two
to give
Now you have whole-molecules and a balanced equation (16 carbons, 26 hydrogens and 34 oxygens) Compare the balanced equations for complete combustion 2C8H18 + 25O2 ------> 16CO2 + 18H2O incomplete combustion 2C8H18 + 17O2 ------> 16CO + 18H2O
There are only two differences between the two equations.
Inside an "internal combustion chamber" there are billions of oxygen molecules and billions of octane molecules. Most of them are converted to carbon dioxide (and water). But some octane molecules find themselves in a wee part of the chamber without enough oxygen and are incompletely combusted to carbon monoxide (and water). All "internal combustion engines" should be run in places with lots of air. As carbon monoxide diffuses into the air it picks up another oxygen atom to become carbon dioxide. If you try to draw the Lewis structure of CO you will find it difficult. You need that extra oxygen atom to complete carbon's outer shell. It is this desire to complete its shell that causes the carbon in CO to grab another oxygen atom, but it takes some time for that to happen. It takes a lot more carbon dioxide than carbon monoxide to kill you. As a matter of fact, you make carbon dioxide and your body has evolved chemical pathways to get rid of it. (Of course, eventually, even these pathways can get over loaded.) Carbon monoxide is a real killer. Avoid it! |
Carbon monoxide (CO) is converted into carbon dioxide (CO2) by
oxygen (O2). That can be written as
The carbons balance but the oxygens don't. With three oxygen atoms on the left and two on the right, it is not obvious how to balance it.
Perhaps you thought "Hmmm. Three oxygens on one side and
two on the other. Maybe I need a number of oxygens which can be
divided by both three and two." So you put a 3 in front of
the carbon dioxide to give you six oxygens. But then you had three
carbons, so you put a 3 in front of the carbon monoxide. You might
have had an equation like this
You may have discovered the "trick" is to add another
molecule of carbon monoxide to give you
That still isn't balanced but look at the ratio of oxygen atoms. You now have four on the left and two on the right. That ratio helps you deal with the oxygens. Now all you have to do is add another carbon dioxide molecule to the product-side and you have a balanced equation. 2CO + O2 ------> 2CO2
Three tips: 1) If you get stuck on a "puzzle", start over again with a different idea to try. That idea could be as "crazy" as changing the number of already balanced atoms (carbons in this case). It is not an easy step to take, but tackling a problem from a different "angle" might set you on the correct path. You learn nothing by giving up. You learn a lot by trying again. 2) Don't start with oxygen (unless you have too). I find it best to leave oxygens for last whenever possible. 3) Diatomic molecules like O2 (and N2, F2 and so on) can be trouble if you have to balance them among molecules with odd numbers of that element. It often helps to establish a "half-balanced equation" where the ratios of that element on the two sides of the equation is two to one (or one to two). Try doubling the number of molecules that have the odd number (usually 1) of that element. Then you are dealing with groups of molecules that can be balanced by even number (diatomic) molecules. Like all "tips", these are not rules to follow. They are suggestions to try. |
No, this (2CO + O2 ------> 2CO2) is not combustion. Combustion
is a special kind of oxidation. Combustion is rapid and produces
a lot of heat and light. But the oxidation of carbon monoxide
is slow. If it were fast, the CO would not last long enough to
worry about.
You now know the two kinds of oxidation:
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A catalyst is a substance that increases the rate of a chemical reaction without being used up or changed by the reaction. Catalysts don't make a reaction "go". They just make them go FASTER.
Catalysts provide an alternative pathway for the chemical reaction,
in which the activation energy is lower. This increases the rate
at which the reaction comes to equilibrium.
If you were not sure how to fit the catalyst into the equation, don't feel bad. Different Alchemists have different opinions!
2CO + O2 ------> 2CO2 is the balanced equation for the oxidation
of carbon monoxide to carbon dioxide. The catalyst is not used
up by the reaction but it must be present. Therefore, we can add
it to the equation by placing it on both sides of the equation.
Because the catalyst is not changed, it is on both sides of the equation. The equation remains balanced. However, most Alchemists do not like the idea of putting the catalyst in the equation like this. They insist that the catalyst, like the electrons in redox reactions, should be "canceled out". But other Alchemists argue that to leave out the catalyst would cause other Alchemists to be ignorant of the "magic ingredient"! Alchemists have agreed a compromise. They put the catalyst above (or in) the line that connects the reactants to the products. In that way it shows that the catalyst is involved in the reaction but not as reactants or products. 2CO + O2 ---Pt---> 2CO2 is the correct way to show the catalyst in a reaction. You may be asking yourself, "Is this now a combustion reaction?" Well, the answer is no. True, the catalyst has speeded up the reaction, but only a small amount of heat is produced and no light. So it is NOT combustion. It is a catalyzed slow oxidation. That seems weird because we have here a fast "slow oxidation"! However, it is the catalyst that speeds it up, not the reactants. (Besides, there's no light and only a little heat.) |
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The oxidation of molecular nitrogen (N2) to produce nitrogen dioxide
(NO2) looks like this
The oxygens balance but the nitrogens don't. By adding another
molecule of nitrogen dioxide to the products we get the nitrogens
balanced.
But now the oxygens are not balanced! There are two on the left
and four on the right. That's easy to fix. Just add another molecule
of oxygen (O2) to the reactants.
Now all the atoms are balanced. By the way, you may be wondering, "Is this combustion or slow oxidation?" To know the answer to that you would have to know if heat and light are produced. You can't tell from this equation, but you might have a clue to the energy change going on. Recall that the oxidation of nitrogen occurs inside the hot chamber of the combustion engine. This side reaction is caused by the heat from the combustion of the octane. Nitrogen isn't oxidized outside in the "normal air" (otherwise nitrogen dioxide would be all around us, even before cars were invented). To put it another way, this reaction won't occur under standard conditions. From that clue you may realize that this reaction requires heat to make it "go". It is an endothermic reaction, not exothermic. It produces no heat. It needs heat! So it can't be combustion. It must be a "slow oxidation" even though it happens rapidly inside the engine. It is unfortunate that the term "slow oxidation" is used to describe reactions that may be very fast! But it is the RAPID RELEASE of LOTS of energy (production of lots of heat and light) which helps you to identify a combustion reaction. Any other oxidation is a "slow" oxidation. Combustion of octane and slow oxidation of nitrogen occur in the chamber at the same time. The heat of combustion provides the heat need by the side reaction. If we could make an internal combustion engine that worked at a lower temperature we could avoid the side reaction and not produce the nitrogen dioxide (pollution). Unfortunately, the heat of combustion makes that impossible. |
N2 + 2O2 ------> 2NO2 is the equation you balanced before. The atoms balance. Now you have a chance to check the electrons. Hopefully, you recognized some of the features of oxidation to help you. The nitrogen atoms pick up oxygens so they are oxidized and the oxygen atoms each lose their oxygen (partner), so they are reduced. At least that 's the old way of looking at it. You can use that old definition to help you along in working out the electrons. Now take a look at it from the point of view of valency changes. Both the reactants are elemental molecules, so all the atoms start with a valency of zero. The product (NO2) is a compound molecule, so the atoms must have changed their valency. That may have worried you, but a little bit of thought would clear it up. As the reactants become products they change their valency from zero to something else. One element is reduced while the other is oxidized!
When beginning a redox problem it is useful to start with the
oxygens (the opposite advice if you were just trying to balance
the atoms). Oxygens in all compounds can have a valency of either
-2 (usually) or -1 (if a peroxide).
The 2 molecules of oxygen require 8 electrons. All eight of those
electrons must be from the nitrogen atoms. N2 is the only other
molecule involved. The two nitrogen atoms must each give up 4
electrons. Both nitrogen atoms will become powerful cations (N+4).
Write that as a half-reaction for the oxidation (lose of electrons)
of nitrogen.
Now summarize the two half-reactions by adding them together.
The ions are attracted to each other to form two molecules of NO2.
You will have noticed some similarities and some differences with
the oxidation of N2 and the combustion of methane.
The oxidation of nitrogen does not produce energy because of these
differences. It takes a lot of energy to break N2's triple bond.
It's easier to break up the C-H bonds because they aren't as strong.
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Nitrogen dioxide and water go to nitric acid and nitric oxide.
The skeleton equation is
Let's leave the oxygens for last. There's one nitrogen on the
left but two on the right (one in each molecule). Let's fix that
by putting a 2 in front of the nitrogen dioxide
Now the nitrogens balance. Good.
Ah, oh! Look what has happened. By doubling the nitric acid you
have doubled not only the hydrogens but also added more nitrogen.
There are now three nitrogens on the right but only two on the
left. So add another nitrogen dioxide to the left side. That gives
you
And even the oxygens balance! Now you have a balanced equation - 3 nitrogens, 7 oxygens and 2 hydrogens.
You may have gotten the correct answer by a different route. For
example, if you had started with the hydrogens your thoughts would
go like this.
That needs an extra hydrogen on the right, so add an extra molecule
of nitric acid to the right to give
The hydrogens are balanced but the nitrogens aren't. You have
three nitrogen atoms on the right but only one on the left. You
can correct the imbalance by having a total of three nitrogen
dioxide molecules on the left, so add two more.
Now the nitrogens balance (3 on each side), the hydrogens balance (2 on each side) and even the oxygens are balanced (7 on both sides). It doesn't matter how you balance chemical equations just so long as you get it right! As a matter of fact, if you get "stuck" while trying to balance a chemical equation, it often helps to start over with a different element. Sometimes it is easier working the same problem in different way. |
Nitric acid ionizes to a proton and nitrate. HNO3 ------> H+ + NO3-
This equation says it all. Notice that the equation describing
the ionization of a substance dissolved in water is "automatically
balanced". (Unless you made a mistake writing it!) If you
like, you can include the water too. It doesn't really change
anything because there's no exchange of atoms with the water.
Water doesn't enter the reaction. But these ions enter the water and affect it. The nitrate is usually gobbled up by algae in the water. But the H+ ion hangs around. |
If you had
Water ionizes like this
To remove the other hydrogen from hydroxide (OH-) takes a lot of energy so it doesn't happen. |
Two ionizations are going on. Consider them together.
H2O ------> H+ + OH-
Perhaps you noticed the increase in the H+. The ionization of nitric acid (HNO3) in water (drops or lakes) increases the total amount of H+ in the water. The more nitric acid you add the more protons (H+) are dissolved in the water. This is true of all acids. If you dissolve them in water they increase the H+ concentration of the water. That's why they are called acids.
Nitric acid falls out of the sky as "acid rain" (Other
acids do too, but HNO3 is a major one.). It increases the number
of protons in the water making the water more acidic. "Acidity"
is a measure of how much H+ is in the water.
The pH scale is a weird mathematical way of counting, which doesn't make sense unless you understand algebra and logarithms. But, just to get your "bearings" on the pH scale, here's a few things every Alchemy student should know.
Normal water has equal amounts of H+ and OH- so it has a pH of 7. (The number seven has to do with the way pH is calculated. Don't worry about that until Advanced Alchemy.)
We call this water "neutral". It is "neutral" because there are EQUAL amount of protons (H+) and hydroxides (OH-).
As you add protons (H+) to water the pH goes DOWN! (Didn't I say
it was a weird mathematical way of counting?)
Water (or solutions made with water) with too many H+ has a pH
below 7 and is described as "acidic".
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Whenever you need to find the valence number of an atom in a molecule, the first thing you should do is look for elements whose valency never (or rarely) change. Ask yourself, "Which of these elements have valences I know?" None of the oxygens in these three molecules are peroxides (otherwise they would be called that) so they must be "normal" oxygen with a valence number of -2. You should also recall that hydrogen has a valency of +1. The next step is to ask yourself, "Is the molecule neutral or an ion?" All three of the molecules here are neutral (otherwise they would be carrying a charge). So the valency of all the atoms in any one molecule must "add up " to zero (no charge).
Now you are ready to determine the valence number of all the atoms
in the molecules. Each molecule is a small puzzle.
Nitric oxide (NO) has one oxygen atom with a valence number of -2. So the valence number of the nitrogen must be +2. If the nitrogen had any other valency, the molecule would have a charge. Nitrogen dioxide (NO2) has two oxygen atoms, both with a valence number of -2. Together, the two oxygen atoms should give the molecule a valence of -4, but the whole molecule is neutral. That means the nitrogen atom must have a valence of +4. That's the only way to make the molecule neutral. Nitric acid (HNO3) is a wee bit more complex, but the tricks are the same. The three oxygens all together produce a valence of -6. The hydrogen has a valence of +1, so it will help to bring the charge closer to neutral. The -6 from the oxygens is turned into a -5 when the hydrogen contributes its +1. That leaves a charge of -5 (the sum of the hydrogen and oxygen valences). The molecule is neutral so this nitrogen must have a valence number of +5.
Identifying the redox "pair" requires you to look at
the changing valences in the equation
The nitrogens in the NO2 molecules have a valence number of +4.
Notice that they MUST all have the same valency because they are
the same "type" of nitrogen, because they are in identical
molecules (of NO2).
The valences of the nitrogen atoms in the products are of two
types.
Summarizing that:
All the redox is caused by the nitrogen atoms! You may be wondering about the balancing of the electrons. The oxidation releases one electron but the reduction requires two electrons. That may not seem right, but if you take into account the coefficients you see that the electrons will balance. You can even do some electron accounting (like before) if you are unconvinced. |
This puzzle requires you to recall how cations with different valence numbers are given different endings. And it also requires that you be able to work backward to figure out how to get a neutral molecule. "Cation-ous" is always of lower valency than a molecule called "cation-ic". (Recall ferrous was Fe+2 but ferric was Fe+3.) Nitric oxide (NO) is the "cation-ic" molecule. You were asked to find the formula for the "cation-ous" molecule. That should have been your clue. The nitrogen in nitrIC oxide is +2. The "-ic" ending tells you there is another possible valence number for nitrogen LOWER than the "-ic" ion. That means the nitrOUS ion has a valency LOWER than +2, so it must be +1. You may have asked, "Why not zero, or -1? They are lower than +2. Lots of numbers are lower than +2." True, but nitrogen with a valence of zero would be molecular nitrogen (N2) and would not be able to bond to any oxygen atoms. And any valence less than zero (like -1) would give you a nitrogen atom with a negative charge. That wouldn't be able to bond to the oxygen either (because oxygen has a negative valency, even if it were a peroxide). You found that the nitrogen in nitrous oxide has a valence number of +1 (N+1). So, what's the formula for nitrous oxide? Well, it must have an oxygen or it wouldn't be an oxide. You may have started with NO. That has both a nitrogen and an oxygen so it is some kind of nitrogen-oxide. But that's nitric oxide, not nitrous oxide. You should be asking yourself, "How can I make a neutral molecule from oxygens (which always have a -2 valency) and nitrogens with a valence number of only +1?" Hopefully, you discovered that there MUST be TWO nitrous (N+1) atoms for each oxygen. Otherwise the molecule would have a charge. N2O would be a neutral nitrous oxide. (So would N4O2 or N6O3, but it is impossible to get the electrons to be shared correctly. It is always best to expect the smallest, simplest molecules.) The correct formula for nitrous oxide is N2O. Some people call it dinitrogen oxide and that is a perfectly good name for it. (But if I had called it that in the first place you would not have discovered how to do these kinds of problems.) A sharp Alchemist must always pay attention to the little details in order to get all the information. N2O is also called "laughing gas" because inhaling small amounts can drug people into a relaxed state, often producing laughter. It is used in minor surgery to decrease pain and produce unconsciousness (sleep). It is also used as a propellant in some foods that squirt out of cans. It takes an awful lot of food cans to get enough laughing gas to "feel it". And it wastes a lot of food! Some people actually make and sell "pure" laughing gas but there can be a hidden danger here. N2O (like NO2) easily picks up water to become nitric acid (HNO3). Doctors (the only people who should "give" N2O) are careful enough to be sure the gas they use is pure and hasn't been converted into the acid. They know, and so should you, that inhaling a little N2O can make you laugh but inhaling a little HNO3 can make you dead! Nitrogen is an extremely versatile element capable of a wide variety of valence numbers. I think these last few problems prove that! Nitrogen has many different valence numbers but only the first two (nitrous and nitric) have special names. |
These questions require you to figure out the chemical formulas of sulfur dioxide and sulfur trioxide before you can write the equations.
Sulfur dioxide has two oxygens. That's why it is DIoxide. So it
is SO2.
Now let's write the equations, starting with the oxidation of
sulfur to sulfur dioxide.
The skeleton equation for the oxidation of sulfur to sulfur trioxide
is
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The sulfur in sulfur dioxide (SO2) has a valence of +4 to counter
the charges from the two oxygens.
The sulfur in sulfur dioxide has the lower valence number so it
can be called sulfurous and the whole molecule can be called sulfurous
oxide.
You maybe wondering why Alchemists bother with two names for the same molecule. It can be a wee bit irritating for an Alchemy student to have to learn these "tricks". But these different names are useful for different things. Sometimes it is more important for an Alchemist to keep track of the number of oxygens. At other times it is more important to keep track of the valence numbers. It is always important to learn how to find the other names and use names to figure out the formulas. It is also important to know how to name chemicals when you just see their chemical formulas. Most of the time, you will see a formula written along with the name, but not always.
Both sulfur dioxide and sulfur trioxide can combine with water
to produce powerful acids.
Both of these ionize in water releasing a lot of protons (H+)
Both of these acids contribute to acid rain (like nitric acid does) lowering the pH of the water (making it acidic). |
Because it is a Group II element, magnesium compounds must have magnesium in the +2 state. That's because the Group II elements like to lose two electrons (to achieve the noble electronic configuration). Oxygen is -2 in all compounds (except peroxides, which this isn't). One atom of magnesium will combine with one atom of oxygen to give magnesium oxide (MgO). This is called "magnesia" by some people and is the common mineral called "periclase". Sulfate (SO4-2) also has a valence of -2 so it will combine with only one atom of magnesium to give magnesium sulfate (MgSO4). This compound quickly surrounds itself with water molecules to become the common mineral "epsomite" (from which we get "Epson salts".) The simple oxygen anion and the large sulfate radical behave the same way, by bonding to only one magnesium atom. That may have surprised you. MgO is neutral and so is MgSO4. Notice that oxygen anion (O-2) and sulfate radical (SO4-2) have the same valency (-2). The sulfur uses its valency to collect 4 oxygen atoms and then, all together, the entire radical has the same valence number as a single oxygen anion. That seems so wrong but is so right! The radical is much larger than the simple anion, but it can substitute for it (in bonding to magnesium) because they have the same valency. |
The skeleton equation is
This is definitely a combination reaction! The magnesium and sulfurs
are already balanced, but you have those pesky oxygens again to
deal with. By adding another molecule of MgO to the left side
you end up with even numbers of oxygens all over the equation.
But, of course, now you have to re-balance the magnesium atoms
by adding another molecule of MgSO4 to the right side.
But now you have to balance the sulfurs! Add another SO2 to the
reactants and everything is balanced.
There's some interesting valency changes going on here and you might want to practice your redox math to figure them out. Sulfur's change in valency causes the sulfate radical to form and allows it to substitute for the oxygen in MgO to form the final product. |
To help a reaction along, it's a good idea to get the reactants to mix well together. Both the gases are good at that but solids need some help. Grind the magnesium oxide into a fine dust or filter the gases through a fine mesh of the solid reactant.. That will increase the surface of MgO available for the reaction. The gases will have better chances to collide with magnesium dioxide as a dust than as chunks of rock.
You may also have suggested increasing the temperature. That's
usually a good idea but in this case the smoke is hot enough.
The physical state of matter is very important to chemical reactions. It is so important that sometimes Alchemists include the state of each molecule in the chemical reaction as subscripts.
In the "scrubbers" equation it may be important to point
out that the MgO is a solid. Then other Alchemists will
understand where the problem may be in the reaction.
The (s) stands for "solid" and the (g) for "gas". |
I bet you figured out right away that (l) stood for "liquid".
But (aq) is something new. It stands for "aqua". "Aqua"
is Latin for "water". (Arthur might have got that one,
but don't feel bad if you didn't get it.) The (aq) subscript means
the material is ionized in water.
You may have noticed that H2O(l) was on both sides of the equation. It didn't really need to be there. I just wanted to give you a clue as to what (l) stands for! |
H2O(l) ------> H+(aq) + OH--(aq) The neutral water molecule (l) has broken up into two ions (aq). |
I've has written all the reactions to be going only one-way!
But reactions are reversible. Instead of
These double arrows show that the reaction is reversible. Most reactions are reversible, at least in theory. Some Alchemists argue that all reactions are reversible, but we just don't know how! This idea of reversibility can be very irritating and confusing. Most of the time we are interested in only one direction. Also (you will recall) the Gibbs energy determines the direction a reaction will "go". If these ideas have left you confused, you are not alone! An explanation is in order.
"All" chemical reactions are reversible. This can be "explained"
in two different ways.
On the other hand, most chemical reactions are obviously not reversible. What conditions of temperature, pressure etc. could cause a candle to "unburn"?! What conditions would reverse death?! Common sense tells us that the idea of reversibility is wrong! Chemical reactions run in the direction that produces excess Gibbs energy. That's the law of the universe - the law of thermodynamics. High energy reactants turn into low energy products. The one way arrow in a chemical reaction is determined by the change in Gibbs energy. The Gibbs energy is calculated to take into account the temperature and other energies involved. If the change in Gibbs energy is very, very negative (that is, if the reactants have much more Gibbs energy than the products) the reaction will "go completely" in that direction (from reactants to products). But equilibrium is always going on. For BIG changes in Gibbs energy the equilibrium is very much in the direction of the products (goes to the right of the equation). A very tiny amount of product "leaks" back into reactants, but this is so small as to be easily ignored. Most simple chemical reactions can be easily reversed by changing the conditions. Most Alchemists work with simple chemical reactions, so they like to say "all" chemical reactions are reversible, and in theory they are right. But when asked to prove it, they (we) resort to math and theory instead of producing a working example. All chemical reactions are reversible in theory, but only a few chemical reactions are reversible in practice. Use a double arrow when you want to highlight or talk about reversibility. Use a one-way arrow when you are clearly interested in the conditions that make the reaction go that way. |
Le Chatelier's principle says that you can change the equilibrium position by changing the conditions of the reaction. Le Chatelier called this "stressing the system". By changing the temperature, pressure or concentration, an Alchemist can shift the equilibrium and (hopefully) increase the yield (amount of product). Le Chatelier's principle is at the heart of the argument about reversibility. If you can change the equilibrium position you can (in theory) make it go the other way. |
Enthalpy is the energy stored in a compound. Along with temperature and entropy, enthalpy determines the direction a reaction goes. The difference in enthalpy between the products and the reactants causes heat to be given off or absorbed.
When the enthalpy of the products is less than the enthalpy of
the reactants, the extra enthalpy is given off as heat - an exothermic
reaction.
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Hopefully, you found this easy (because there are some clues with the words "needs" or "gives off").
A------> B (200 units) meaning it needs 200 units of enthalpy.
B------> C (-50 units) meaning it gives off 50 units of enthalpy.
C------> D (100 units) meaning it needs 100 units of enthalpy.
D ------> E (-1000 units) meaning it needs 1000 units of enthalpy.
Remember: Endothermic reactions have positive enthalpies and require energy (usually heat) to be added. An exothermic reaction would have a negative value and give off the energy. The sign (- or +) tells you if enthalpy is given off or absorbed. The number tells you how much enthalpy is given off or absorbed. |
Hopefully you realized that to turn A into E you had to sum (collect the minus and pluses of) all the enthalpies in all the reactions. That's 200 -50 + 100 - 1000 = -750 units of enthalpy. (It's just simple math!) Because the number is negative this is the amount of enthalpy you get out of the reaction. So this total series of reactions gives off heat. It is an exothermic reaction Note: To calculate the enthalpy changes in a series of reactions, you sum them up (collect all the numbers together, adding the positive values and subtracting the negative values). You may have mistakenly subtracted 1000 units (the last reaction) from 200 units (the first reaction) to arrive at the wrong answer (-800 units). Perhaps you felt that was right, because it is close. But it's wrong. You have misunderstood Hess's Law. We'll come back to that later. Also, notice that even though this TOTAL series of reactions gives off enthalpy, it requires an input of enthalpy to get it over some reactions (the first reaction and the third reaction). Therefore, this series of reactions will not occur without some input of enthalpy to get over those "humps". The first (and third) reactions are like the activation energy of a single reaction. Some Alchemists like to think of activation energy that way - as a step in a series of reactions. |
To turn A into C all you have to do is sum the first two enthalpy changes. 200 - 50 = 150 units of enthalpy. This is an endothermic series of reactions because it "sums" to a positive number. Therefore you must add 150 units to turn A into C. |
Turning B into A is the reverse of turning A into B. Reversing
the reaction reverses the energy.
Notice the change in the sign from positive to negative. This negative value means the reaction is exothermic. It gives off 200 units of enthalpy (probably as heat). Note: The reverse of any exothermic reaction is an endothermic reaction and the reverse of any endothermic reaction is an exothermic reaction. Changing the direction changes the sign. |
All that back and forth between A and B would "cancel out" as if it wasn't even there. The answer is the same as before (in A 41). It is as if you had gone from A to B to C to D to E without all that back and forth so it's -750 units. It's (still) an exothermic reaction (over all). |
Some students find this idea very hard while others catch on right away. So don't feel bad if you couldn't get it. The change in enthalpy that occurs in the reaction is caused by the difference between the reactants and the products. Here's what I hope you found.
A ------> B is A (100 units) turning to B (300 units) and that
requires 200 units to be added.
B ------> C is B (300 units) turning into C (250 units). Because C has less enthalpy than B, 50 units less, this reaction will give off 50 units. It's an exothermic reaction. C ------> D is C (250 units) turning into D (350 units). To climb up to 350 units, an extra 100 units must be added. It's endothermic because it needs the energy to climb up to D. D ------> E causes D (350 units) to become E (-650). That is a big drop in energy. A drop of 1000 units. That is a negative 1000 units because 1000 units of enthalpy is given off in the reaction, so it is exothermic. See how the differences in enthalpy between reactants and products is the enthalpy change of the reactions? If you think about it, that makes sense. This also shows you how to think about enthalpy changes. What is the difference in enthalpy between molecule A (100 units) and molecule E (-650 units). The answer is -750 units. That is the same answer you got in A 41. But in A 41 you went through the entire path. If you had a catalyst to take you from A directly to E, the change in enthalpy would still be the same (-750 units). That is what Hess meant by path independence. No matter how you get from A to E the TOTAL CHANGE in enthalpy is the same. You may be wondering how does one figure out the enthalpy of a molecule. You may recall that I told Arthur that you can't figure out the enthalpy of a molecule directly. That's because the enthalpy of any molecule is hidden in the molecule. It is hidden in the bonds, physical state and other factors that affect the molecule(s).
The values in these examples were made up (so you could work with numbers). But you can see from this way of thinking how the enthalpy of a molecule might be found. We can measure the CHANGES in enthalpy by measuring the heat given off or absorbed in the REACTION as a reactant is turned into product. This is the reverse of the way you have been doing these problems. For example, if you knew that molecule A has 100 units of enthalpy and found (by measuring heat absorbed) that the reaction to make B requires 200 units, you would know that B must have 300 units of enthalpy. But, how did you know that A has 100 units? Well, maybe you know how much energy it took to make A from another molecule. But then, how did you know how many units of enthalpy were in that first molecule?! This could go on forever! What we need is a value for a starting molecule and from that we can figure out how other molecules are made from it. You will recall that there are many different kinds of enthalpies (energies of ionization, fusion etc.). The enthalpies to make a molecule are called the enthalpies of formation. Alchemists have agreed that the most common form of any element will be given the value of zero enthalpy at standard condition (of temperature and pressure, remember?). For example, at standard temperature and pressure (0C and 1 atmosphere) carbon (C) and calcium (Ca) are solids while oxygen (O2), hydrogen (H2) and nitrogen (N2) are gases. But they ALL have a standard enthalpy of zero because they are the common form of the pure element. Keep this in mind as you do the next problems. |
The equation looks like this 2O2(g) + N2(g) ------> 2NO2(g) The enthalpies of the oxygen and the nitrogen are both zero (0). That's because these two gases are the common form of that element at standard conditions. It took 66.4 units to make two molecules of NO2 so each molecule used up 33.2 units. That means both molecules of nitrogen dioxide have 33.2 units of enthalpy each. Because you had to add the energy, this is an endothermic reaction and the NO2 molecule has an enthalpy of formation of (positive) 33.2 units. You may have wondered about the 2 in front of the O2. It means twice the O2 so that is twice the enthalpy of O2. Or simply twice zero (nothing), so it is still zero. |
He's wrong. It can't be done! Recall what Hess told us. The total
change in enthalpy is independent of path. No tricks, not even
a catalyst, can change that. NO2(g) has 33.2 units of enthalpy in
it regardless of how it is made.
A catalyst only reduces the transition barrier of a reaction pathway. But it doesn't (can't) change the enthalpy of the final molecules it makes. That's determined by the molecule itself. The enthalpy of formation is a characteristic of the molecule, just like melting point or mass. Experienced Alchemists are always cautious about claims to make molecules in an easier way. Hess's Law is the foundation of their caution. Any two methods that turn the same reactants into the same products MUST have the same enthalpy change (in total). It's the law of thermodynamics. It's Hess's Law. When Alchemists (or anybody) reports a different value (enthalpy of formation) for the same molecule it usually means a mistake has been made. Perhaps they forgot to include a temperature change or a change in the physical states of the chemicals involved. Maybe some energy leaked in or out. |
This is similar to Q 46. Both the reactants have enthalpies of formation of zero because they are the common form of each element (at standard conditions). That means all 484 units of enthalpy that came out of the reaction came about from the formation of the two water molecules. Each water molecule has an enthalpy of formation of -242 units. There's a lot of energy released when hydrogen gas is oxidized to water. As a matter of fact, this reaction is being explored as a clean way to produce energy. The problem is that it takes a lot of energy to make the hydrogen gas in the first place. |
She' right! But it would be easy to jump to the conclusion that she is wrong. Perhaps you shouted, "No! No way! Hess's Law....". Unfortunately, you forgot to consider the STATES of the water. This is an easy thing to forget. Even the best Alchemists make this mistake.
The enthalpy of the water you calculated was the enthalpy of steam.
Notice it is a gas with the subscript (g). But she has worked
out the enthalpy of liquid water (l). The difference in enthalpy
is because water is less "energetic" than steam. As
steam cools down it gives off heat - the heat of fusion (remember?).
This wasn't a "trick" question. It was meant to point out the most common misunderstanding about enthalpy and enthalpy changes. The enthalpy of formation is one thing. The enthalpy (or heat) of fusion is another. An Alchemist must keep track of any enthalpy changes that occur. This includes changes of state as well as changes in molecules. |
You are asked to find the heat of fusion of water. This is the
chemical reaction
You will recall the enthalpy of formation for liquid water was
determined by that female Alchemist as -285 units for each water
molecule. You figured out the enthalpy of gaseous water to be -242
units. You can write that as
Imagine liquid water (l = -285) is turned to steam (g= -242).
The reactant (liquid water) GAINS 43 units of enthalpy as it turns
to steam.
H2O(l) + 32 enthalpy units ------> H2O(g) The heat of fusion of water is 43 units of enthalpy. It's an endothermic reaction. Thermodynamics is complicated. You have to keep track of everything. All changes must be included in order to get the right answer. Even changes in temperature! For example, the 43 units needed to change water into steam will only work if the water is already at the boiling point. And the steam it produces will also be at the boiling point. You need to keep track of all the changes. Advanced Alchemists are expected to follow enthalpy changes in detail. But I think you've seen why it is an advanced topic! |
The reaction described is called electrolysis, which means
to use electricity to break up molecules. In this case it is
This is the reverse of the reaction described by that female Alchemist
The electrolysis of water is just the reverse, so the enthalpy change will be just the reverse, (positive) 285 units. That means you must use up 285 units in the electrolysis of water. The reverse of any reaction has the enthalpy changes reversed. You may have thought the correct answer is (positive) 242 units. That's close, but you would have to be doing the electrolysis on water steam (g) not water liquid (l). Thermodynamics is not easy. It requires attention to detail. There are other factors which complicate calculations. Things like temperature changes and pressure changes come into play. Ionization energy and electronegativity may be important. It depends on what you are doing and what you are trying to figure out. Chances are, you found these last 10 problems to be difficult, perhaps frustrating! No one finds this stuff easy. And there are parts of the thermodynamics you haven't learned about. Please read over these last 10 problems so you will see why things were right and wrong. Try to see what is important, what can be reversed and what can be ignored. You may feel very confused the first time you went through them. A second reading may help. |
Entropy is the amount of disorder in a system. The universe and everything in it is heading toward a state of maximum disorder, maximum entropy. Entropy, along with enthalpy and temperature, is used to calculate the Gibbs energy of molecules and the direction a reaction goes. |
Roughly (maybe exactly) 5 of the coins would be "heads" and the other 5 would be "tails". To get all of them to come up "heads" you would have to turn the "tails" over! That requires work. Your work. Another possibility would be to toss them again, and again until you eventually got all of them to come up "heads". But that would require a lot of tossing! (Over a thousand tries before you are likely to be so lucky.) Entropy is really a matter of probabilities (an advanced form of math). The random tossing of coins shows that entropy rules! Only an outside force would make the coins come up all "heads" all the time. You would have to cheat at the tossing or change them after they landed. Randomness is the way the universe works! You have to use energy to get anything in order. You have to use energy to place the coins all "heads" up. Entropy (disorder) is natural and free. Order requires effort (work). |
The total entropy change for a spontaneous (natural) isolated reaction must be positive. That means the total entropy of the reactants must be less than the total entropy of the products. That way, more entropy (disorder) is created and the extra entropy is given off as positive entropy (more disorder). The universe LOVES to go toward disorder. That's why a spontaneous, isolated reaction must become more disordered. The universe runs disorder "uphill"! To cause a reaction to go from disorder to order requires energy. That's because it's a negative change in entropy so the energy must be supplied. It will not be spontaneous because it requires energy to do it. Cleaning a room or arranging coins takes energy. Neither of them occurs spontaneously.
Notice that entropy and enthalpy have opposite signs for "downhill".
Remember that it's "natural" for things to go to disorder. That means it's natural for things to give off positive entropy and become more disordered. (It's weird, but it's true!) |
Gibbs energy is the total amount of chemical energy in a system and includes the entropy, enthalpy and temperature of all the chemicals involved. Chemical reactions always run from higher Gibbs energy to lower Gibbs energy. That means all chemical reactions give off Gibbs energy and (thus) have a negative change in Gibbs energy. Gibbs energy determines the direction of a chemical reaction and how far in that direction the chemical equilibrium will lie. |
This may have required a bit of imagination on your part, but it is the key to understanding chemical reactions. All three chemical variables (temperature, enthalpy and entropy) come together to determine the Gibbs energy of a system (the molecules, their state, etc.). To make a reaction "go", the reactants must have more Gibbs energy than the products. That is, the TOTAL GIBBS energy runs downhill from high Gibbs energy to low Gibbs energy. Gibbs energy is given off in all reactions. (Otherwise it goes in the opposite direction to give off the energy.) A reaction may create products with lower enthalpy than the reactants. To do that either the temperature will increase (releasing the enthalpy as heat) or the entropy must decrease (disorder is decrease by using the enthalpy released). Or both can happen! All three parts (enthalpy, temperature and entropy) come together in the Gibbs energy.
Consider the formation of water (steam) from the gases O2 and H2.
But look at the entropy change in this reaction. Three molecules
of reactants (two H2 and one O2) become two molecules (of water).
It is easier for three molecules to be disordered than
for two molecules to be disordered.
You would have to see the exact numbers (including temperatures) in order to calculate the change in Gibbs energy. But you can be sure it would be a negative change in Gibbs energy because the reaction is spontaneous (goes in the direction as written). To reverse the reaction (to get oxygen and hydrogen from water) you have to add extra energy. That's what electrolysis does. To completely understand that would require you to calculate the effects of electricity too! As you might imagine, to fully understand the thermodynamics of chemical reactions and predict their Gibbs energy changes requires a lot of calculations and a deeper understanding of the energies involved. But all of the calculations involve the temperature, enthalpy and entropy changes. |
As water freezes it gives off heat! Liquid water has more enthalpy than
ice, so it releases that enthalpy as heat in an exothermic reaction.
(But once the ice forms it doesn't give off any more heat.) The
enthalpy change is negative (because it's exothermic). The enthalpy
runs downhill, "naturally".
As water freezes it becomes more ordered! Ice crystals are much
more ordered than liquid water. The freezing of water decreases
the entropy (causes more order) . The entropy change is negative,
but that is "unnatural" for entropy. (Positive entropy
changes are "natural").
The enthalpy given off by the freezing of water provides the energy
to decrease the entropy. Just as it takes energy to order a messy room,
the order in ice is created by the energy of enthalpy.
And don't forget that all chemical reactions are influenced by temperature. Imagine a
piece of ice that is just at the melting point. But it doesn't
melt because it needs a wee bit of heat. If it is just a fraction
of one degree below the melting point, the Gibbs energy change
for melting is still positive. It won't happen. But, when you
increase the temperature, you add enough energy to make the Gibbs
energy change for melting negative. It melts.
You've seen in these last dozen (or so) problems that the energy which "powers" a reaction is difficult to calculate. And it is not easy keeping track of all the different things going on. Let's review what you have learned about the thermodynamics of chemical reactions and add some new thoughts.
The changes in Gibbs energy are at the heart of all chemical reactions.
Temperature is the easiest part of the Gibbs energy to understand. You deal with temperatures every day so it is not a new concept.
Enthalpy is the hidden energy of molecules. There are MANY
different kinds of enthalpy; formation, fusion, vaporization,
ionization and so on. There are many more kinds of enthalpy that
we haven't even discussed. In the calculation of Gibbs energy,
enthalpy usually plays the most important part. Perhaps you
had a "hunch" that it was these hidden energies of molecules
that are most responsible for chemical reactions. That "hunch"
is right. All those different enthalpies must be included in any
calculation of Gibbs energy. They can be "summed" just
like any math. Hess's Law helps us to understand how different
paths between the reactants and the products will "sum"
to the same enthalpy.
You probably never heard of entropy before I told you about
it. Entropy is a weird concept. It appears to be "magical"
and "backward", but if you think about entropy (disorder)
and keep track of what is going on, you will begin to see it everywhere.
In the energy diagrams we always talked about "energy". Perhaps you thought that energy was not clearly defined. Let's be clear here. The energy in those reaction diagrams can be either the enthalpy or the Gibbs energy (which includes enthalpy as well as temperature and entropy). You can use energy diagrams either way. But always be sure you understand which kind of energy is used to make the diagrams because it will influence what you learn from them. Don't confuse the two.
Enthalpy diagrams can run downhill (exothermic) or uphill (endothermic). Gibbs energy diagrams can run downhill (exergonic) but not uphill (endergonic). |
NO! (Of course not.) People who argue that the evolution of life violates the laws of nature, thermodynamics or Gibbs energy change have forgotten (or not learned) how these laws really work. Evolution has created a world full of life. Life is very ordered and very diverse. Life is very low entropy. Evolution is a process of creating very low entropy, but creating very low entropy is not against the laws of nature. All it takes to lower entropy is some energy, some "work". Remember, changes in enthalpy can "power" the changes in entropy. Entropy can decrease (order increase) if there is a source of energy (enthalpy) pushing it towards order. Remember how water freezes from liquid to ice. Liquid water has lower entropy than ice. (Because ice is crystals. Remember?) You will recall (from the previous question) how the enthalpy released by freezing provides the energy to put the ice crystals in order. It's natural. It happens all the time. Every snowflake and every iceberg were made by lowering the entropy. What powers the evolution of life? Energy of course! Energy from the sun. For billions of years life on Earth has been living off the sun and using the energy of sunlight to power its ecosystems and evolution. Millions of years before that, small, simple disordered molecules were turned into large, complex order molecules by the energy of sunlight (and volcanoes). Some of those molecules became the building blocks of life (proteins, DNA, fats, etc.). The evolution of life, from its beginning to its present state, is a decrease in entropy caused by the use of the energy from the sun. What would happen without the sun?
(Most) Life on earth would die. (Most) Ecosystems would collapse. (Most) Evolution
would end. (Most) Entropy would stop decreasing.
The Gibbs energy change required for evolution and life is not against the laws of nature. Of course, the order created by life is much more complex than ice crystals! To understand the details of evolution and life you must learn about all the sciences - from Geology to Genetics. But you know enough about thermodynamics and Gibbs energy change to understand that creating order from disorder requires only energy. Life is Alchemy and life does not violate any laws of nature. |
The skeleton equation shows us the basic ingredients in an equation.
Hopefully you wrote this skeleton equation.
Now you have to balance it. Let's start with the carbons. You
have 6 on the right so you need 6 on the left like this
The carbons are now balanced so let's do the hydrogens. There's
2 on the left (in the H2O) but 12 on the right (in the glucose)
so you must have 6 water molecules to supply all the hydrogens
for the reaction.
OK, the hydrogens and carbons balance, so we must now balance
the oxygens. Every molecule in this reaction has oxygen in it
so you really have to do your sums right! On the left (the reactants)
are 12 oxygens in the carbon dioxides and 6 in the waters for
a total of 18 oxygens. On the right (products) there are 6 oxygens
in the glucose and 2 in the oxygen molecule for a total of 8 oxygens.
There are not enough oxygens on the right.
It's balanced! (Check it and be sure.) |
This reaction does NOT "go" in the direction as written. It goes the opposite way! (But you may not have guessed that.)
The enthalpy changes are positive.
The entropy changes are negative. Disorder decreases.
You can't calculate the exact Gibbs energy of the reactants and products (from the little information supplied here), but these changes in enthalpy and entropy should have given you a clue. Enthalpy is increasing, going uphill, and requires energy to do that. (The enthalpy change is positive so it is an endothermic reaction.) Entropy is decreasing, but that means it is becoming more ordered, and that requires energy too. (Note, although the entropy is going "downhill" by decreasing, entropy really likes to roll "uphill". It is natural to make disorder out of order.) When you put those two ideas together you have to ask yourself , "Where do I get the energy to increase the enthalpy of the molecules and decrease the entropy (increase the order)?" Perhaps you thought (hoped) temperature would be the key to making this reaction "go". But it doesn't really help. Like many problems in this course, it is more important to learn from the answers than to get them right each time. I hope you understand that from the above argument you could predict that this reaction is probably endergonic and it will not go in that direction. |
You need ENERGY to make that reaction "go" from CO2
and H2O to glucose and O2.
Plants use sunlight to make this reaction "go".
This extra energy is used to push the low enthalpy reactants uphill to high enthalpy products. It also supplies the energy to turn the disordered (high entropy) reactants into ordered (low entropy) products.
This
By the way, you may have suggested that a catalyst could be used to make the reaction go "uphill". But that's not right. Catalysts only lower the activation energy and help speed up a reaction but they don't (can't) make reactions run against the laws of thermodynamics. It takes energy (not catalysts) to push a reaction "uphill". Also, don't be mistaken by the fact that plants use enzymes (a special kind of bio-catalyst) to do photosynthesis. True, plants use enzymes (catalysts), but that is to speed up the reaction. Enzymes help with the speed of a reaction but NOT the direction it goes. (Many people, including some teachers make this mistake when talking about enzymes.) |
It sure does! C6H12O6 + O2 ------> 6CO2 + 6H2O causes high enthalpy, low entropy molecules to turn into low enthalpy, high entropy molecules. A great deal of energy is released. This is an exothermic reaction and produces disorder. It's very exergonic! A lump of glucose needs only a tiny flame to set it alight and it will burn very hot, bright and fast! That's combustion. The tiny flame to get it started is required to help get the reaction over the activation energy. (Remember activation energy?) Once started this reaction produces enough heat to power itself in a chain reaction. |
A catalyst will lower the transition barrier so you don't need as much (or any) activation energy to make it "go". (NOTICE the catalyst is NOT affecting the direction. It only effects the barrier.) You and your enzymes (bio-catalysts) are doing this all the time. A series of enzymes turn glucose and oxygen into carbon dioxide and water. This is called respiration. (Some folks insist on calling this "oxidation". They are right, but respiration is a very special kind of oxidation of glucose.) Respiration uses enzymes to slowly oxidize glucose. You may be saying, "Hey, enzymes speed up the reaction." And you are right. Without the enzymes, this reaction (the oxidation of glucose) takes a long time. Don't confuse that with combustion. Combustion is MUCH faster than respiration (even with those enzymes!), but it requires a tiny flame to get it going, and it produces a flame.
This reaction
Respiration uses enzymes to speed up the reaction by lowering the transition barrier. The energy produced in respiration is used to power "life" in a series of (millions of ) chemical reactions. It is not combustion because no flame and very little heat are produced. Compared to combustion, respiration is slow. Combustion produces a hot, bright flame. It requires some energy to get it over the energy barrier. Once it gets started, combustion produces lots of energy in a small space and uses it to power the next reaction in a series of chain reactions. |
Photosynthesis and respiration are both chemical reactions that
use enzymes. They "power" most of the earth's ecology. The sun and photosynthesis
begin it all.
This turns high entropy, low enthalpy molecules into low entropy, high enthalpy molecules. Without the sun (or other energy source) this would not happen. Photosynthesis uses sunlight to "pump up" the molecules.
Respiration is the reverse of photosynthesis, but instead of light
being produced, the energy is used to make other bio-molecules
and power "life".
All the atoms and molecules are recycled through the exchange of photosynthesis and respiration. They just go around and around. The effect of all this is the transfer of energy. Sunlight is turned into life energy! We (and all animals as well as many micro-organisms) cannot do photosynthesis. We don't have the enzymes (or cell structures needed). We all depend on plants to make the glucose. Notice that plants also produce oxygen (O2) in photosynthesis. We need that too. It is required for respiration. Note: Plants also respire (undergo respiration). Plants can both respire and photosynthesize! They capture sunlight energy to make their own life energy (and bio-molecules). In the sunlight, there's more photosynthesis than respiration. In the dark, plants can only respire (like an animal). |
Most of life on earth uses the glucose (and oxygen) made by photosynthesis, but some deep sea ecologies don't need the sun. Many of the microbes living around deep sea volcanoes are able to make high energy products using the conditions and (nasty) high energy molecules from the volcanoes. They use high energy molecules and non-standard conditions to get the energy needed to make glucose. The details of their Alchemy can be complex, but you can understand the basic ideas behind them.
The production of glucose by the microbes around these deep sea "vents" has parallels to the production of nitric oxide (which we talked about earlier) produced in a car's combustion chamber. Neither glucose nor nitric oxide can be made from their reactants at standard conditions. Nitric oxide isn't made outside of a combustion chamber because the Gibbs energy change is positive at standard conditions. Glucose isn't made (at standard conditions) without the extra energy of sunlight. (Otherwise, it too would have a positive Gibbs energy change.) The high temperatures and pressures inside the combustion chamber, along with the enthalpy change of the combustion reaction itself, supply the conditions and enthalpy needed to make nitric oxide. Nitric oxide production is a side reaction of the combustion and the chamber provides non-standard conditions. Similarly, "volcanic microbes" make glucose in a kind of side reaction to the sulfur reactions. The high temperatures and pressure around the sea vents provide the non-standard conditions! You may be asking, "Where do these high enthalpy sulfur compounds come from? How does the volcano make high enthalpy materials in the first place?" These are good questions because all high enthalpy products must come from somewhere! Where?
So, high enthalpy sulfur molecules are used to "power" the formation of high enthalpy glucose. Those high enthalpy sulfur molecules come from the volcanoes that make them from the high heat and pressure in the earth. That heat is caused by the decay of radioactive materials! Some biologists believe there may be life on (actually in) the moons of our outer solar system!
Not long ago we believed that ALL life required the sun. More recently, life has been discovered around deep sea "vents" and even inside oil fields and rocks! This has come as a real surprise. We forgot to think about alternative ways of making high enthalpy molecules. Maybe there are other ways we have yet to figure out. (Maybe there is a world where constant electrical storms produce high enthalpy molecules from the lightning and those molecules are used to "power" an alien ecology!) Life can be defined in a variety of ways.
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There are lots of places to learn about Alchemy (chemistry) and the universe. Long ago the only way to learn something was to find it out all by yourself. That's both difficult and dangerous (especially with Alchemy)! Later on, the ancient Greeks discovered the usefulness of teaching and they created the first schools. Schools and teachers are a great source of learning. Eventually books were invented. Books allowed teachers to teach when far away from their students. And books let students learn what they want at their own pace. Libraries are a great source of books. More recently, computers and the Internet have become a great way to learn about anything! Ask your teachers about learning new things. Borrow some books from the library. And, of course, surf the Internet.
You can learn anything you set your mind to learn.
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