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FIRE |
Good. Imagine a simple molecule. The simplest molecule in the universe, H2. 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 vibrate back and forth. |
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Yeah. Together, in a covalent bond, both hydrogens share their
electrons so they have the electron configuration of helium.
Right again. Both hydrogens would rather be together than apart.
That "desire" to reach the noble state is a powerful
force.
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That's because it's new to you. Don't let that frighten you. We'll go through it one step at a time. I've included the diagram of bond lengths in our drawing to help you understand this graph. We "plot" the position of the two atoms along the horizontal.So the distance between the two atoms is along the "horizon".Aye. In this diagram, I have put one atom at the far left side and won't move it. The other atom is moved horizontally to indicate the distance between them.OK. What this up and down part of the diagram?The up and down part (or vertical part) of the diagram shows how much energy is needed to put the atom in that position. It indicates the amount of energy needed to push or pull the atoms together. |
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Very good, Arthur. By pushing the two atoms together you are using energy to push them "up-hill". When you release the energy the atoms spring back to the average bond length as if they had rolled back down into the "valley". Now tell me what would happen if you pulled the two atoms apart a wee bit?Well, if you pulled them apart you would be increasing the bond distance. But the atoms want to be together to keep a noble electron configuration, so it takes energy to pull them to a longer bond length.Right. What would happen if you released your grip on those two atoms that you had pulled a wee bit apart.They would snap back to their average bond length! They would go back into the "energy valley". |
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Ah, now that is different. The two ions are held together by electrostatic
attraction. You would have to use some energy to pull on
them. If you pulled them a wee bit apart and then released your
grip, they would spring back to their average bond length because
of the attraction of their opposite charges. But they wouldn't
go colliding into each other because their outer shells would
keep them apart.
Very well said. An important point here is that this energy diagram would look much the same for either covalent or ionic bonds. Weak bonds also have similar energy diagrams. There is always a minimum energy point at an average bond length. The minimum bond length is caused by electrostatic repulsion. The maximum bond length is caused by the actually mechanism that causes the bonds to form anyway. That is true of all bonds including the weak ones. Do you see what I mean? |
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Do all bonds have these properties?
Yes. Here I've added another energy diagram. This one is for HF.It looks a lot like the one for H2. But the valley for HF is lower than the valley for H2. |
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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.It looks like the path of someone walking over a hill and into a deeper valley. |
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Once it gets to the transition state it may or may not go to the
right, to make a product. It could go back where it came to (re)make
the reactants.
Right. This might go on thousands of times each second, so eventually most of them go to the right. Once they get to the right side of the hill, they are products and are not as likely to return to reactants (on the left side of the hill) because that's a higher hill to climb.Is there a name for the energy needed to climb up this hill?Yes. The difference in energy between the reactant and the transition state is called the activation energy. |
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It does require a bit of thinking. But you'll get it. Let's see
what else you can learn from energy diagrams.
Hmmmm,
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Do all reactions give off energy?
No. Some reactions need more energy than they produce. 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.I see. "Endergonic" means "below the energy" so they need energy to go. Endergonic reactions need energy to go up the energy hill all the way to products.Right.So it is the total energy in or out of the reaction that determines if it is exergonic (releasing extra energy) or endergonic (needing more energy). But a reactions may give off heat (be exothermic) or take up heat (endothermic) from its environment regardless of whether it is an exergonic or endergonic reaction. |
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Right. Once it is over the energy barrier fire releases more energy than you put into it. That "new" energy can be used to power another reaction. |
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Hmmm.
The middle of the Table has a great deal going on in it. Electrons in the outer shell might form ionic bonds with each other, but many of them form covalent bonds too. And metal ones. The elements from Groups III to Group VI are involved in the "staircase" aren't they? Yes, they are. Do you recall what forms that "staircase"?Metals on the bottom, semi-metals over them and nonmetals on top. It all has to do with the metal properties and that has do with how well they conduct electricity. Going down a Group adds extra shells which are more likely to get involved in "super sharing" their electrons (in metal bonds and to carry electrical currents). But going across a Period shrinks the outer shell inward so the electrons aren't as likely to be "super shared". |
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PRINCIPLES OF ALCHEMY
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This work was created by Dr Jamie Love and licensed under a Creative Commons Attribution-ShareAlike 4.0 International License
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