Bond Energy: Definition & Equation

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  • 0:02 What Holds Atoms Together?
  • 1:13 Understanding Bond Energy
  • 2:52 Calculating Energy Change
  • 5:58 Bond Length and Bond Energy
  • 6:55 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

Chemical reactions involve the breaking and forming of chemical bonds. In this video lesson, we will learn about bond energy and how we can use it to measure the overall energy change of a chemical reaction.

What Holds Atoms Together?

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.

Understanding Bond Energy

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:

Hydrogen and oxygen molecules breaking apart to form water molecules

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:

Exothermic reaction
energy diagram of exothermic reaction

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:

Endothermic reaction
energy diagram of endothermic reaction

Calculating Energy Change

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:

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