Monday, May 20, 2019

Energy of Chemical Bonds

Lead Author: Dr. Anne Gull

Chemical energy is a type of energy that is stored in the bonds of compounds. When compounds are formed, some energy is required to "shove" the atoms together. When those bonds are broken, that energy is released.

In typical chemical reaction, some bonds break (and give off energy) and other bonds form (taking in energy. If the total energy given off exceeds the energy need, the excess energy is given off as heat and/or light. When a reaction gives off energy, it is exothermic. Energy exits the reaction. When a reaction takes in energy, it is endothermic. Energy enters the reaction.

Every combination of atoms has a specific bond energy. Knowing the bond energy allows the calculation of energy given off.

For example, the bond between carbon and hydrogen stores 413,000 joules for every mole. That means that one mole of those bonds will give off that much energy.

Chemical energy is stored in bonds, so it’s a type of potential energy that can be released when a chemical reaction occurs. Each type of bond has a unique amount of energy. Some of the values are shown in the chart below. These energy amounts are measured for 1 mole.

The figure below shows the energy stored in several common bonds:

413000 J
347000 J
358000 J
799000 J
467000 J
146000 J
* for CO2
For other C=O bonds, see the link at bottom of page.

In order to figure out how much potential energy is inside a chemical substance, you need to look at the structure to see the bonds (the lines between elements represent the bonds). 

The dash or single line (-) is a single bond. The double line (=) is a different kind of bond, a double bond. There is also triple bonds (≡) three lines.

The energies for the different bonds are different. It is important to recognize the difference and make sure you are using the correct bond energy from the table.

More Bond Energies

For more information and the energies of other types of bonds, see this link:

Click Here

NOTE: The energies on the linked page are given in kilojoules, so multiply by 1000 to get joules per mole.

Example #1 Methane

In the molecule above, there are 4 C-H bonds, so the potential energy stored in this molecule is 4 times the value of each C-H (413000). So there is 4(413000) = 1652000 J of potential energy in one mole of this type of molecule.There are 3 C-H bonds, 1 C-O bond and 1 O-H bond so the potential energy stored in each bond would be added up.

Example #2 Methanol

There are 3 C-H bonds, 1 C-O bond and 1 O-H bond so the potential energy stored in each bond would be added up.

3 C-H 3(413000) J
1 C-O 1(358000) J
1 O-H 1(467000) J
Total 2,064,000 J

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