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Arthur's notes for Fire (Principles)

Chemical Reactions (Introduced)

Fire is a chemical reaction. Life is a chemical reaction.
Chemical reactions (or simply "reactions") change chemicals, making a new chemical from another chemical (a synthesis). Chemical reactions can turn atoms into molecules (making bonds), molecules into atoms (breaking bonds) or molecules into other kinds of molecules (breaking old bonds and making new ones). Even changes in weak forces are chemical reactions.

All this bond making and breaking in chemical reactions involves energy. Energy is "the ability to do work", and includes heat, light, sound and electricity. Some chemical reactions produce energy (exergonic) and others absorb energy (endergonic). Some chemical reactions produce heat (exothermic) and others absorb heat (endothermic). Heat is only one form of energy, but it is the most obvious one.

DO NOT confuse the energy change caused by the chemical reaction with the conditions that started it. A piece of wood does not "spontaneously ignite", it must first be heated to very high temperatures before it begins to burn. Once it is burning it produces heat of its own from the chemical reaction (chemical changes). It is this "self generated heat" which then feeds energy to the rest of the wood causing it to burn. This is why fires, once started, can get out of control.

A fire (candle flame) produces heat and light. It is usually a slow, steady reaction.
An explosion (dynamite) is a very fast reaction in which the energy is given off very quickly as heat, light, and sound. (The "sound" produces a shock wave that carries away the energy in a destructive "blast".)
This difference is due to the rate of chemical change. Some chemical changes occur very rapidly and the release of energy is fast and impressive (like a firecracker). Others are slow and are hardly noticed from one day to the next (like rust).

The material at the start of the chemical reaction is the reactant and is placed on the left side of the equation.
An arrow points the way to the final products.
REACTANTS ------> PRODUCTS
Some chemical reactions are written with an arrow pointing both ways like this
REACTANTS <------> PRODUCTS
The double arrow is meant to remind you the reaction is "reversible". Most simple chemical reactions are reversible if you change their conditions. However, most of the time Alchemists try to make their equations run as they are written, from left (reactants) to right (products).
Which way the reaction goes depends upon the reaction conditions (temperature, pressure, etc.) because these affect the changes in energy.

2Cu + O2-----> 2CuO is a complete, balanced equation. It has all the ingredients needed to make the products (complete) and in the right numbers (balanced).
Alchemists synthesize (make) molecules using synthesis techniques ("recipes").

All chemical reactions can be classified as one (or several) of these three types.
1. Combination joins two (or more) substances to create a larger substance.
Example: metals combining with oxygen (forming a rust or tarnish).
2Cu + O2 ------> 2CuO
2. Decomposition breaks a substance into two (or more) smaller substances.
Example: hydrogen peroxide breaking down into simple oxygen and water.
2H2O2 ------> O2 + 2H2O
3. Replacement substitutes one element or part of a molecule for another.
Example: copper reacts with sulfuric acid, substituting copper atoms for hydrogen atoms to make copper sulfate.
Cu + H2SO4 ------> CuSO4 + H2

Standard conditions are a kind of baseline (a standard) for Alchemy. One atmosphere (1 atmos) of pressure and 0 degree Centigrade (0o is the freezing point of water) are called the standard conditions.
(Any other temperature or pressure is "non-standard" condition.)
Note (and warning): All Alchemists agree on the pressure. Unfortunately, some Alchemists insist that 25oC is the standard temperature. It depends on how they were raised (as Alchemy students). This is another example of the unfortunate lack of agreement even on such things as "standard conditions"! I point this out, not to confuse you (although it may have), but so you will be aware that it is sometimes important to know which "standard conditions" you are working with. I like to use 0oC and suggest that you do too.

Breaking and Making Bonds

When bonds are broken electrons are rearranged and the atoms with them. New bonds are then formed. Exactly how and why the bonds break and reform is an important subject of Alchemy.

H2 + F2 ------> 2 HF
The bond joining two atoms (H-H) is like a vibrating spring. The "average bond length" is the average distance between the two atoms, but it varies slightly as they wiggle (vibrate) back and forth.
If you were to push atoms too close together you would have to exert energy against the electrostatic repulsion of their (outer) electron shells.
On the other hand, if you were to try to pull the atoms away from each other you would have to exert energy to overcome the attraction caused by the bond itself.
A small amount of energy added to the bond will cause it to stretch, but the bond will spring back to its normal "average bond length" if the bond can withstand the energy. However, a large amount of energy will stretch the bond to the breaking point, causing the atoms to separate completely. The energy needed to break the bond is the bond dissociation energy.

H2 (that is H-H) + bond dissociation energy ------> 2H (that is H and H as separate atoms)

In this example, the high electronegativity of the fluorine atoms provides the bond dissociation energy. The fluorines tug the H-H bond apart due to the difference in electronegativity.
Once the H-H bond has been broken, the atoms will rearrange their electrons to form new bonds with the "best" combination(s) of atoms. H-F bonds are preferred to H-H bonds and F-F bonds, because of fluorine's high electronegativity (among other things).

We can combine the energy diagrams of the old bond broken and the new bond formed to make an energy diagram for the entire reaction.
The path from reactants to products is the reaction co-ordinate and the energy needed to move between the two (reactants to products) represents the energy path to get there. Along the reaction co-ordinate is the point of maximum energy, called the transition state, where the reaction could go either way. At this position a kind of "hybrid" state exists where you could say that all the atoms are bonded together but none of them well bonded. This group of "confused" atoms are called an activated complex. The difference in energy between the reactant and the transition state is called the activation energy. This is the energy required to make the reaction "go".

If the reaction path carries the reactants up to the transition state, the activation energy is really an energy barrier that must first be overcome in order to make the reaction "go". This energy must be added to "push the reaction over the barrier". Some reaction paths have no energy barrier, their path being entirely "downhill". These reactions are "spontaneous" and will occur without doing anything.
Reactions in which the total energy of the products is less than the total energy of the reactants will release energy and are called exergonic reactions. Since most of these release their energy as heat they are usually exothermic reactions too.

Reactions in which the total energy of the products is more than the total energy of the reactants will require energy to be added and are called endergonic reactions. Since most of these absorb their energy as heat they are usually endothermic reactions too.

Fires (and some other chemical reactions) produce more energy than the activation energy. That means the energy from one reaction can provide the energy for another. And so on, and so on, in a series of chemical reactions. Because they continue to produce all the energy they need, fires (and chemical reactions like it) run "on their own" once started. But first they must be started by providing the activation energy for the first reaction. Fires and similar chemical reactions that produce more energy than the activation energy are called chain reactions.

NOTE: although covalent bonds are highlighted in this example, the mechanism is pretty much the same for the breaking and making of all bonds (even the weak ones).

Valency, Oxidation and Reduction

Many chemical reactions involve a change in the valency (or valence) of the atoms. An atom's valence number is the number of electrons it uses to form the compound (not the elemental molecule, because that always has a valency of zero).

Here's a summary of the Valence Rules:
1) Atoms in elemental molecules (atoms not combined with other elements) have a valency number of zero (0) because they are NOT involved (yet) in compounds. For example, molecules of oxygen (O2) and pure metals (Na) have a valency of zero, because they are pure elements.
2) Group I elements have a valency of +1. (Except hydrogen as a hydride. It has a valence of -1.) Group II elements have a valency of +2. Group VII elements have a valency of -1 in the compounds they make. (Because that's the ions they always make.)
3) In electrovalent (ionic) compounds, the valence number of the ion equals the charge (with cations having positive numbers and anions negative numbers).
4) In covalent compounds, the valence number of an atom equals the number of electrons it shares with atoms different from itself (other elements).
5) Oxygen has a valency of -2 in all compounds, except in peroxides (where its valency is -1).

Exceptions to these rules occur, but they are a rare and "weird" area of Alchemy.
Also note, that when atoms are covalently linked in chains, they normally have positive valence numbers.

Elements that have multiple valence numbers are given different endings. The lower valence atom ends in "ous" and the higher valence atom ends in "ic"

Changes In Valency

Chemical reactions can change an atom's valence number. To help keep track of changes in valency, Alchemists often include the valence number in parenthesis. Pure copper has a valency of zero (because metal bonds don't have a real valency). So pure copper (often called "the metal") is sometimes written as copper (0) or Cu (0).

In the past, "oxidation" was the name given to any chemical reaction in which oxygen was combined with a substance.
2Cu + O2 ------> 2CuO ("the copper is oxidized")
As Alchemy matured, folks began to focus on the electrons involved in a reaction. In oxidation, electrons are lost.
2Cu ------> 2Cu+2 + 4e- ("the copper is oxidized")
Notice that the copper metal lost electrons to form copper ions with a +2 charge.
These copper ions are often called "cupric" or written "copper (II)" to highlight the fact that they're missing two electrons.
All reactions in which oxygen is gained involve the loss of electrons, so oxidation is the loss of electrons. Oxidation increases the valence number (in this case, from 0 to +2).

In the past, "reduction" was the name given to any chemical reaction in which oxygen was removed from a substance.
CuO + H2 ------> Cu + H2O ("the copper is reduced")
If you look at the electrons transferred in the reaction, you see that electrons are gained by the copper.
Cu+2 + 2e- ------> Cu ("the copper is reduced")
Notice the copper ion, copper (II), gained two electrons to return to the pure metal.
All reactions in which oxygen is lost involve the gain of electrons, so reduction is the gain of electrons. Reduction decreases the valence number (in this case from +2 to 0).

Loss of Electron(s) is Oxidation (LEO). Gain of Electron(s) is Reduction (GER). "Leo says ger."

The substance that loses electron(s) is oxidized but it is called a reducing agent because it is the agent responsible for reducing the other substance. Reducing agents lose electrons (so they are oxidized).
The substance that gains electron(s) is reduced but is called an oxidizing agent because it is the agent responsible for oxidizing the other substance. Oxidizing agents gain electrons (so they are reduced).

In many reactions, one substance is reduced (loses electrons) while the other is oxidized (gains electrons). The oxidation of one substance (the reducing agent) provides the electrons for the reduction of the other.
Because these two reactions must both occur for the complete reaction to "go", these are often called "redox" reactions (shortened from "reduction and oxidation").

Balancing Equations

A balanced chemical equation has equal numbers of each atom (including each type, or element) and equal numbers of electrons on both sides of the equation. "It's the law!" and it makes sense because you can't create new (or different) atoms from nothing. You can't create electrons from nothing either.
Balancing equations can get tricky and it helps to work slowly, carefully and keep track of everything.
There are no (easy) steps to follow, but there is one important rule - the only way to balance a chemical equation is by changing the number of molecules. You cannot change the molecules themselves (you cannot change the subscripts on the molecules) so you must change the number of molecules (by changing the coefficient, the number in front of the molecules).

Simple Changes in Valency often go "unnoticed" and are very easy to balance.
Some chemical reactions involve a simple rearrangement of the atoms with a minor change in their valency. This unbalanced equation changes two elemental molecules into a compound.
H2 + F2 ------> HF
Start with the molecules on the left side. Each hydrogen's valency is zero. Notice each hydrogen makes one covalent bond to the other, BUT H2 is an elemental compound so both atoms in the molecule have a valency of zero. Each fluorine's valency is also zero (for the same reason).
Now look at the molecule formed on the right side. Both the atoms in the products have a valency of 1 (because they both form one bond to the other element).
It's a simple matter to balance these types of reactions by counting the number of atoms. The atoms going into the equation MUST equal the atoms coming out. In any chemical equation the atoms on the right must equal the atoms on the left.
H2 + F2 will provide enough H and F to make two molecules of HF. Placing a 2 in front of the molecule means that two molecules of HF are made.
H2 + F2 ------> 2HF
This equation is now balanced. There are two hydrogens and two fluorines on both sides of the equation. Notice that the hydrogens have lost their electrons, changing their charge from zero to +1. Both of the hydrogens in the H2 lose their electrons, so they are oxidized ("leo"). Also notice the two fluorine's have each gained an electron, changing their charge from zero to -1. Both the fluorine's in the F2 gained an electron, so they are reduced ("ger").

Often, you must "go back" and change other coefficients (the numbers in front of the molecules) again and again until all the atoms balance.

Complex Changes in Valency are more difficult to balance.
When there is a major change in valence, balancing the equation becomes more complex. The total number of electrons gained (by the oxidizing agent) MUST equal the number of electrons lost (by the reducing agent). "It's the law!" and is the base on which we balance chemical equations involving a change in valency.
Also remember, the atoms that go into the equation must also equal the number and kinds of atoms that come out of it. (But that is obvious!)

All chemical reactions involving a change in valence can be broken down into two reactions - oxidation and reduction. The oxidation of copper can be pictured as a two processes occurring together.
Oxidation is Cu ------> Cu+2 + 2e-
The copper metal changes from a neutral metal (metal bonded) to a cation by oxidation.
Reduction is O2 + 4e- ------> 2O-2
The oxygen changes from a covalent molecule (sharing electrons) to a pair of anions by reduction.

These are called "half reactions" (or "half equations") and together they form a redox "couple".

They occur at the same time (simultaneously) but it is good to think of them as two separate reactions (equations). Notice that the oxygen molecule needs 4 electrons but the copper atom only produces 2 electrons. If you were to combine these two equations you will see that there are not enough electrons to balance the equation. To produce a balanced equation you MUST have equal numbers of electrons transferred. To produce the 4 electrons needed to reduce the oxygen molecule, you must reduce TWO atoms of copper (both producing 2 electrons for a total of 4 electrons). By doubling the amount of copper you double the amount of electrons given up by the reactants.

Doubling the amount of the reducing agent (the copper loses electrons so it is the reducing agent) gives this half reaction
2Cu ------> 2Cu+2 + 4e-
Now, the number of electrons produced by the oxidation of two copper atoms equals the number of electrons needed to reduce the oxygen molecule.

We can now add the two half reactions like this
Reduction is O2 + 4e- ------> 2O-2
Oxidation is 2Cu ---------> 2Cu+2 + 4e-
---------------------------------------------------------
Total Redox is 2Cu + O2 ------> 2Cu+2 + 2O-2

There are 4 electrons on the left and 4 electrons on the right side of these two half equations, so the number of electrons lost (by the copper atoms) EQUALS the number gained (by the oxygen molecule).You can think of these 4 electrons on opposite sides of the equation as "canceling out" each other.
The two ions created in this redox (the Cu+2 and the O-2) are drawn together by electrostatic attraction and held together by ionic bonds.
2Cu+2 + 2O-2 ------> 2CuO.
The reaction is balanced because the net charges (electrons transferred) AND the number of atoms is equal on both sides of the equations.
2Cu + O2 ------ > (2Cu+2 + 2O-2) ------> 2CuO

NOTE: you may have been able to arrive at the same balanced equation by just balancing the equation
Cu + O2 ------> CuO, using the atoms.
But if there is a change in valency you might want to check to see that the electrons "add up" too. In this example, the balancing might appear to be obvious, but some equations involving changes in valency can get tricky.

Very Complex Changes in Valency are very difficult to balance (but you can do it)!
Generally speaking you start with a rough equation, adjust it to balance the atoms and then adjust it again to balance the electrons (if they need balancing).

For example, iron (III) chloride (FeCl3) and sodium sulfite (Na2SO3) in water form iron (II) chloride (FeCl2) and sodium sulfate (Na2SO4) plus hydrochloric acid (HCl).
Write a rough equation to start with
FeCl3 + Na2SO3 -----> FeCl2 + Na2SO4 + HCl
Notice there are no hydrogens on the left side! But this reaction occurs in water (!) and that is where the hydrogens come from, so add the water to give
FeCl3 + Na2SO3 + H2O -----> FeCl2 + Na2SO4 + HCl
Note: You can add water to the right side too but you will need to balance it and cancel it later. Since most reactions occurring in water use only a tiny portion of the water we are not concerned with its production or destruction. We just want to use it to balance the equation!

You have a balanced number of iron (1), chlorine (3), sodium (2), sulfur (1) and oxygen atoms (4). BUT you have two hydrogens on the left (in the water) and only one on the right (in the HCl).
To correct the imbalance of hydrogens, just add an extra molecule of HCl to give
FeCl3 + Na2SO3 + H2O -----> FeCl2 + Na2SO4 + 2HCl
So the hydrogens are balanced but now there are four chlorines on the right (two in the one molecule of FeCl2 and one in each HCl molecule.). So now you must balance the chlorines. This is NOT easy because the only way to change the number of chlorines is to also change the number of iron atoms!
But you must, so add another molecule of FeCl3 to the left side to give you
2FeCl3 + Na2SO3 + H2O -----> FeCl2 + Na2SO4 + 2HCl
But, now notice you have far too many chlorines on the left (6 of them) and the irons are not balanced either! However, by adding an extra molecule of FeCl2 to the right side, suddenly things balance!
2FeCl3 + Na2SO3 + H2O -----> 2FeCl2 + Na2SO4 + 2HCl

Now you have the same atoms on the left as on the right. The atoms balance. But do the electrons?

Some complex redox is going on here. Notice that the iron changes its valency from +3 to +2. We call the iron (III) ferric and the iron (II) ferrous. In this reaction the iron goes from +3 to +2 by gaining an electron. (The electron's negative charge causes the charge on the cations to drop by one.) Because the iron gains an electron, it is reduced. Something else must be oxidized.

If you look carefully you see the sulfur changes its valency (along with the number of oxygens bonded to it). The sulfur-oxygen anion on the left side of the equation, with less oxygens (in the molecule Na2SO3) is called sulfite (SO3-2). The sulfur-oxygen anion on the right side, with more oxygens (SO4-2) is called sulfate. Oxygen doesn't change its valency (oxygen is always -2 unless it is a peroxide, which this isn't) so it must be the sulfurs changing valency. Use this fact (about unchanging oxygens) to figure out the change in the sulfur's valency. On the right, the sulfur has a valency of 4 but on the left it has a valency of 6! To go from +4 to +6 the sulfur must lose two electrons, so the sulfur in this reaction is oxidized.

Summary: the iron is reduced from the ferrous (+3) to the ferric (+2) while the sulfur is oxidized from the sulfite (+4) to the sulfate (+6). We have now identified the atoms with changing valences and thus identified the atoms responsible for the transfer of electrons. No other atoms in this equation change their valency, so all we have to do is be sure the electrons balance by checking that the electrons gained by the iron equal the electrons lost by the sulfurs.

Looking again at the final equation (figured out above) we see:
2FeCl3 + Na2SO3 + H2O -----> 2FeCl2 + Na2SO4 + 2HCl
The two ferric atoms (2Fe+3) on the left each takes on an extra electron to become two ferrous irons (2Fe+2), reducing them. The reduction of the irons needs two electrons.
The single sulfur atom in the sulfite (SO3-2) goes from a valency of +4 to +6 by giving up two electrons, so it is oxidized.
In total, two electrons are transferred and the equation is already balanced!

You could have balanced this equation by FIRST balancing the electrons and THEN balancing the atoms. It doesn't matter which you do first, but it's smart to check them both ways.

Here's the same problem but solved the other way around:
Write a rough equation
FeCl3 + Na2SO3 -----> FeCl2 + Na2SO4 + HCl
Notice that you must oxidize the sulfur to change it from a valency of +4 (in the sulfite) to +6 (in the sulfate) and it must release two electrons to do that. But the iron atom accepts only one electron as it is reduced from the ferric (+3) to the ferrous (+2), so it will take 2 iron atoms to accept those two electrons (one electron to each iron atom). That gives this equation
2FeCl3 + Na2SO3 ------> 2FeCl2 + Na2SO4 + HCl
(Notice you have to add an extra iron to both sides to keep it balanced).

Now you have a balanced equation for the electrons, but the atoms are not at all balanced. As a matter of fact, you still have to bring in some hydrogens to the left side, by adding water. So,
2FeCl3 + Na2SO3 + H2O ------> 2FeCl2 + Na2SO4 + HCl
But you still have an unbalanced equation because you have two hydrogens on the left, but only one on the right. And you have six chlorines on the left but only five on the right! This is easily solved by adding an extra molecule of HCl.
This gives
2FeCl3 + Na2SO3 + H2O ------> 2FeCl2 + Na2SO4 + 2HCl
This second step of balancing (the atoms) did not involve the two elements undergoing redox (the iron and the sulfur) so we don't have to go back to them. This equation is balanced. The atoms and electrons on the right equal the atoms and electrons on the right.

Balancing equations is a lot like doing math puzzles. Some are simple and "obvious". There's only one correct answer but there may be more than one way to get to the same answer. As with math problems, the important point is to work slowly, carefully and be sure you know what you are doing. Once you get the answer, check it to see if it is right, by checking to see that the electrons and atoms balance.

Radicals are a cluster of atoms held together with covalent bonds but they contain an excess or deficiency of electrons, so they are ions.
Note: When determining the valence of atoms in radicals you must remember to take into account the charge on the ion.
Ammonium (NH4)+
Acetate (C2H3O2)-
Carbonate (CO3)-2
Bicarbonate (HCO3)-
Chlorate (ClO3)-
Cyanide (CN)-
Nitrite (NO2)-
Nitrate (NO3)-
Phosphate (PO4)-3
Sulfite (SO3)-2
Sulfate (SO2)-2

Rates of Chemical Reactions

The rate of the chemical reaction is the speed of it. Some reactions are fast while others are slow. Alchemists study the rates of chemical reactions in order to find the fastest, yet safest, path to the product. The rate (the speed) at which the products are formed depends upon the rate at which the reactants collide, interact and are transformed into products.

Consider this equation. (The letters represent molecules.)
A + B ---(---> ABCD---)--->C + D
Reactants---(--->Activated Complex----)----> Products
Chemical reactions require collisions among the reactant molecules, A and B. Therefore, to increase the rate of a chemical reaction you should try to increase the collisions among the reactants.

There are many ways to increase the rates of collisions:
1) mix the reactants together as completely as possible.
2) grind the reactants into tiny particles (if the reactants are solids) because the smaller particles have a greater total surface area, so there are more particles for collisions.
3) increase the concentration of the reactants. This is especially good advice if the chemical reaction must occur in liquids. (Most do.) Some of the fastest reactions in water happen when the reactants are near saturation levels (about to precipitate out of solution, but not quite).
4) increase the pressure in the reaction vessel. This will increase the rate at which the reactant molecules (A's and B's) collide because at higher pressures there are more reactants in a given volume.
5) increase the temperature in the reaction vessel. At higher temperatures all molecules move faster and this increases the chance of a collision..

Reactants must collide in spite of the electrostatic repulsion caused by their electron shells. Remember, even the cations have electron shells (except H+). The best way to overcome this repulsion is to have lots of reactants (high concentration) in a tiny space (high pressure) moving at high speeds (high temperatures).

Note (warning): Sometimes changing the conditions too far (for example, too much heat) will HARM the production of products, because the new conditions may create other kinds of products you did not want or destroy the reactants or products.

After the collision (or during it) the activation complex is formed.
A + B ---(---> ABCD---)--->C + D
Reactants---(--->Activated Complex----)----> Products
This often requires activation energy to overcome a transition barrier (often caused by those repulsive electron shells). An increase in temperature is a very good way to help the molecules into the activated complex. The activated complex may break up to form either the reactants (moves back to the left from which it came) or it may break up to form the products (which is what we want). By maintaining high temperatures and other helpful conditions we keep throwing reactants back into the activated complex and helping the reaction move (eventually) to the right. (Which is what we want.)

As the activated complex breaks up into products it may release more energy than was needed to form the activation complex, in which case it is an exergonic reaction. If that energy is released as heat (most are) it is an exothermic reaction and its temperature increases.
Alternatively, the reaction may release less energy than it absorbed, in which case it is an endergonic reaction. Endergonic reactions need additional energy to be added for them to go. If that extra energy is supplied as heat (most are) it is an endothermic reaction and its temperature decreases as heat is absorbed into the reaction from the environment.

A catalyst ("cat-A-list") is a substance that increases the rate of a chemical reaction, but it is not changed or used up in the reaction. Catalysts work by making a special activation complex with a lower transition state. They do this by grabbing each reactant and holding them together, often in specific positions that help the old bond(s) break and the new bond(s) form. Once the products are released, the catalyst returns unchanged and is used again to make more products. Over and over, again and again.
Industries often use simple catalysts (metals, ions, etc.) to make industrial products.
Life uses enzymes ("N-zimes"), biological catalysts made of large protein molecules, to quickly turn reactants into products.

Equilibrium

When a chemical reaction begins, it turns reactants into products, moving the equation from left to right, because there are only reactants to start with and no products yet formed.
A + B ------> C + D (the forward reaction)
But as reactants are used up and more products are made, the reaction often starts to go in reverse (!) running from right to left.
A + B <------ C + D (the reverse reaction)
This seems odd, but chemical reactions are reversible and thus able to go in the opposite direction (at least to some extent).
These two reactions (forward and reverse) continue to occur at the same time. Eventually (actually, often very quickly) the forward and reverse rates settle down to the same rate. At this point the overall chemical reaction (both forward and reverse) is said to be in a state of equilibrium, meaning opposite reactions (in opposite directions) are going on at the same rate. At this point (equilibrium) products are made as fast as they are destroyed!

It would APPEAR as if the reaction has stopped, but in fact, equilibrium reactions (most reactions) are "dynamic", meaning there is a continuous interchange of atoms between reactants and products.
So we often write the reactions with a double arrow like this
A + B < ------- > C + D

The yield of a reaction is the amount of product produced and it depends on the "position" of the equilibrium. This equilibrium position is the point at which the forward reaction and the reverse reaction have "canceled each other out", producing a "constant" yield. It is WRONG To assume that ALL the product returns to reactant (otherwise there would be no Alchemy). Instead, a set amount of product (the yield) is produced at equilibrium.

Consider this reaction:
100A + 100B ----->
It starts with all reactant and no product (like most reactions). The forward reaction starts and produces something like this:
20A + 20B --------> 80C + 80D
At this point, the large amount of C and D causes the reverse reaction to start. (I'll switch the directions here to make it more clear.)
80C + 80D -----> 79C + 79D + 1A + 1B (plus the 20A and 20B remaining).
BUT, the forward reaction is still occurring, so those newly formed A and B can go into the forward reaction and return the products!
21A + 21B ------> 1C + 1D (plus the 79C and 79D from before)
When the RATES of the forward reaction and reverse reactions are EQUAL, equilibrium has been reached. This usually occurs a wee bit past the point at which the reverse reaction begins. Putting the forward and reverse reactions together could give an equilibrium something like this
25A + 25B <------> 75C + 75D
Because the forward and reverse rates are equal (at equilibrium) the amounts of reactant and product will not change. The reaction will appear to have stopped (or "gone to completion" as Alchemists say). In fact, the reactions continue back-and-forth without changing the overall numbers. At this point the yield will not change. (In this example, you get a yield of 75%.)

Note: a catalyst causes the system to reach equilibrium more quickly but it will not change the position of the equilibrium. Catalysts speed up the reaction but they do not increase the yield, at least not directly.

You can change the position of the equilibrium, and thus the yield of product, by changing the conditions of the reaction. (That's Le Chatelier's principle). A change in temperature, pressure or concentration will upset the equilibrium ("stresses the system") shifting it to a new equilibrium position. By carefully adjusting the conditions of the reaction an Alchemist can (often) increase the yield.

An understanding of what influences the rate of chemical reactions and equilibrium is useful in planning experiments and producing molecules, but in the end, it all comes down to giving it a try!

Enthalpy, Entropy, Thermodynamics and Gibbs Energy

Enthalpy ("N-thal-P") is the energy stored in a compound. It is the total bond energy of the molecules. You can't measure enthalpy directly but you can measure the enthalpy change of chemical reactions. The difference between the enthalpy of the reactants and products is the heat given off or absorbed by the reaction.

The enthalpy change (heat given off or absorbed) = enthalpy of the products - enthalpy of the reactants.

Exothermic reactions release heat because the products formed have less enthalpy than the reactants (enthalpy change is negative). Endothermic reactions absorb heat from the environment to make the higher enthalpy products from lower enthalpy reactants (enthalpy change is positive).

Hess's Law explains that the total enthalpy change (total energy released or absorbed) as the reactions move from reactants to products, is "path independent". This means that the activation energy or any other energy is not included in the final enthalpy change. Only the initial and final enthalpies "count". Hess showed that enthalpy changes (like chemical changes) can be added and subtracted to calculate changes in enthalpy.

There are many different kinds of enthalpy changes (ways to make or absorb energy). Some examples are:
1) Ionization energy is the energy required to remove an electron (endothermic) to form the cation or the energy released (exothermic) when an electron is returned to the cation.
2) Electron affinity is the energy given off (exothermic) when an electron is added to a neutral atom that has a high electron affinity. But atoms with low electron affinity will not readily accept an electron unless energy is provided to the reaction (endothermic).
3) Heat of fusion is the heat absorbed (endothermic) to cause a solid (at the melting point) to turn into a liquid. Heat is given off (exothermic) when the liquid (at the freezing point) becomes a solid.
4) Heat of vaporization is the heat absorbed (endothermic) to cause a liquid (at the boiling point) to turn into a gas. Heat is given off (exothermic) when the gas (at the boiling point) turns into the liquid.

A "system" is a reaction, its container or anything you want to define as being "in the system". It's just a way to think about bits of the universe. A system is often defined as containing certain materials and certain energies. Everything outside the system is "the rest of the universe". An "isolated system" cannot exchange heat (or anything else) with its surroundings.

Entropy ("N-tro-P") is the amount of disorder in a system. Generally speaking, as you put heat into a substance its entropy increases. Solids, especially crystals, are highly ordered and thus have very low entropy. When the solid melts, its entropy increases because it becomes more disordered. Gas is the state of matter with the highest entropy because it has no order to it. The gas molecules just fly around randomly. Entropy is a function of both strong and weak bonds, it's just easier to imagine it with the weak forces responsible for the states of matter.

Large complex molecules (with lots of bonds between lots of atoms) have highly ordered bonds and therefore have less entropy than small molecules. Decomposition reactions increase entropy because they make smaller (disordered) molecules from larger (ordered) ones. Combination reactions decrease entropy because they turn (disordered) small molecules into (more ordered) large molecules
Life is very low in entropy because life contains billions of highly ordered molecules.
Among pure chemicals, crystals have the lowest entropies but they still have a tiny bit of wiggle among the molecules. To get their entropies to absolute zero, you must stop them wiggling by removing all the enthalpy (heat).

"The entropy of the universe tends toward the maximum", meaning the whole universe is always moving toward the most disorder. Only tiny parts of the universe (the sun, the earth, you, me, etc.) enjoy low entropy (order) and that cannot last forever!
Any spontaneous ("natural") reaction that occurs in an isolated system, increases the entropy. (The entropy change is positive.)

Thermodynamics explains how energy moves. Its laws are the very foundation of the way the universe works! Enthalpy and entropy are the two thermodynamic energies that determine whether a chemical reaction will or will not occur at a certain temperature.
Josiah Willard Gibbs applied the laws of thermodynamics to chemical equilibrium and revolutionized Alchemy. Gibbs produced a mathematical equation that allowed him to predict whether a chemical reaction would go forward or reverse and by how much.

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 highest Gibbs energy to lowest. That is, any REAL chemical reaction MUST lose Gibbs energy because the reactants (always the substances with higher Gibbs energy) turn into products (always the substances with lower Gibbs energy). Gibbs energy (like all energy) "runs downhill".

The thermodynamics (enthalpy, entropy and temperature) behind Gibbs energy dictate the direction of a chemical reaction.
Because chemical reactions always run from high Gibbs energy to low Gibbs energy there is always a release of Gibbs energy from any reaction. Another way of thinking about that is that all reactions that "go" (as is and without input from outside the system) give off Gibbs energy and are, therefore, exergonic.
Any reaction that tries to increase the Gibbs energy in the products will NOT "go" unless supplied with Gibbs energy (enthalpy, entropy or heat) from outside the system. This would be an endergonic reaction and it would only "go" in the opposite direction! (Unless supplied with extra energy from elsewhere.)

The amount of Gibbs energy given off in a reaction also determines how far in that direction the chemical equilibrium will lie.
A reaction with a large loss of Gibbs energy (a large negative Gibbs change) will have a higher yield than a reaction with a small loss of Gibbs energy. (And, of course, if the Gibbs energy change is positive the reaction will go in reverse.)


This work was created by Dr Jamie Love and licensed under a Creative Commons Attribution-ShareAlike 4.0 International License Creative Commons Licence.