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Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.
If you're making a model plane and you want to stick some parts together, what do you normally use to do this? Of course, the answer is you would use some type of glue. You also know there are different types of glue, and they can vary in how well they stick; in other words, their strength. The stronger the glue, the better it sticks.
Now if you want to take something apart again, you better not use superglue! It is almost impossible to break apart something glued together with super glue because it bonds so strongly. You can pull and pull, and you probably won't get it apart. However, a different type of glue, like a craft glue stick, bonds less strongly and can be easily pulled apart again. Only a small amount of effort is needed.
This is also true of chemical bonds. Chemical bonds can be thought of as the glue that holds atoms together. Just like real glue, different bonds can vary in bond strength. Bond energy is the amount of energy needed to break apart a specific chemical bond.
When a chemical reaction occurs, molecular bonds are broken and other bonds are formed to make different molecules. For example, here the bonds of two hydrogen molecules and one oxygen molecule are broken to form 2 molecules of water:
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Bonds do not break and form spontaneously; an energy change is required. We have learned that this energy is known as bond energy. A little later we will look at some values for average bond energies and use them to calculate an overall energy change of a reaction.
First, let's return to what happens during a chemical reaction. The atoms in the reactants rearrange their chemical bonds to form products. The new arrangement of bonds does not have the same total energy as the bonds in the reactants. When chemical reactions happen, there is always an accompanying energy change.
In many chemical reactions, the energy of the products ends up lower than the energy of the reactants. These reactions are known as exothermic, and energy is given off, usually as heat. You can see from the diagram that energy is lower at the end and energy is given off:
 |
Chemical reactions where the products have a higher energy than the reactants are called endothermic. The reactants must absorb energy from their environment to react. This time the diagram shows that energy is absorbed and that energy is higher at the end:
 |
Now let's return to bond energy values and look at a table of average bond energies for different bonds.
| Bond | Bond Energy (kJ/mol) | Bond | Bond Energy (kJ/mol) | Bond | Bond Energy |
|---|---|---|---|---|---|
| H-H | 436 | H-F | 565 | H-Cl | 427 |
| O-H | 467 | N-H | 391 | C-H | 413 |
| C-C | 347 | C=C | 614 | C ≡ C | 839 |
| C-O | 358 | C=O | 745 | C ≡ O | 1072 |
| C-N | 305 | C=N | 615 | C ≡ N | 891 |
| N-N | 160 | N=N | 418 | N ≡ N | 941 |
| O-O | 146 | O=O | 498 |
Bond-breaking requires energy -- it is an endothermic process -- so bond energies are always reported as positive numbers. You can see that the values vary tremendously depending on the atoms and also on the number of bonds between the atoms. The larger the average bond energy, the stronger the bond. You will also see that molecules with multiple bonds have much higher values than those with just a single bond. It takes more energy to break multiple bonds.
For example, you will see that the triple-bonded carbon is much stronger at 839 kJ/mole, compared to the double-bonded carbon at 614 kJ/mole, and then compared to the single-bonded carbon at 347 kJ/mol. This is like comparing super glue to hot glue to craft glue stick.
Okay, using these average bond energy values, we can now calculate the overall energy change for the formation of 2 moles of water. Let us take a closer look at what we have going on here:
| Molecule | Bonds per Molecule | Moles in Reaction | Moles of Bonds | Process | Energy per Bond | Total Energy |
|---|---|---|---|---|---|---|
| H-H | 1 | 2 | 1 x 2 = 2 | breaking | +436 kJ | 2 x (+436) = +872 kJ |
| O=O | 1 | 1 | 1 x 1 = 1 | breaking | +498 kJ | 1 x (+498) = +498 kJ |
| H-O-H | 2 | 2 | 2 x 2 = 4 | forming | -467 kJ | 4 x (-467) = -1868 kJ |
We have one single bond on the hydrogen molecule breaking, and there are two moles of hydrogen. Each bond has a bond energy of +436 kJ. So 2 x 436 = +872 kJ. We also have an oxygen double bond breaking. We only have one mole of oxygen. This double bond has a bond energy of +498 kJ. Now we are forming 2 moles of water, each one containing 2 O-H bonds. We have 4 OH bonds in total, each with a bond energy of 467 kJ. But notice the different sign.
Unlike bond breaking, which needs energy, bond formation gives out energy. In other words, it is an exothermic process. So when using bond energies for forming bonds, we change the values to a negative number.
So our reaction results in a net energy change of +872 + 498 - 1868 = -498 kJ. This negative value tells us that this reaction is an exothermic reaction -- it gives off energy.
Finally, it turns out that there is another quantity related to bond energy: bond length. Bond length is the distance between the nuclei of two bonded atoms. In general, we find that the shorter the bond length, the higher the bond energy. We need more energy to break a short bond compared to a longer one.
This may not make that much sense, so let's think about it this way. Say you have a thin stick that's 12 inches long. It's easy to snap the long stick to start with. But as you keep snapping the stick into smaller and smaller pieces, the effort it takes gets harder and harder. Eventually you get to a point where no amount of effort, or energy, is going to break it.
The short stick has a higher 'bond energy' and is less likely to break.
In this lesson, you have learned that chemical bonds are the glue that hold atoms together. Different chemical bonds have different bond strength. Bond energy is the amount of energy needed to break apart a specific chemical bond. The larger the bond energy, the stronger the bond.
During a chemical reaction, there is always an accompanying energy change. We can use average bond energies to calculate the net overall energy change of a reaction. An exothermic reaction results in products at lower energy than reactants and gives out energy. An endothermic reaction results in products at higher energy than reactants and absorbs energy. Finally, the shorter the bond, the stronger the bond.
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