Is Energy Released When Chemical Bonds Are Broken or Formed?

One of the biggest challenges for chemistry students is understanding whether energy is required or released when chemical bonds are broken and formed. One reason this can be confusing is that a complete chemical reaction can go either way.

Exothermic reactions release energy in the form of heat, so the sum of the energy released exceeds the amount required. Endothermic reactions absorb energy, so the sum of the energy required exceeds the amount that is released. In all types of chemical reactions, bonds are broken and reassembled to form new products. However, in exothermic, endothermic, and all chemical reactions, it takes energy to break the existing chemical bonds and energy is released when the new bonds form.

Breaking Bonds → Energy Absorbed

Forming Bonds → Energy Released

Breaking Bonds Requires Energy

You have to put energy into a molecule to break its chemical bonds. The amount needed is called the bond energy. After all, molecules don't spontaneously break. For example, when is the last time you saw a pile of wood spontaneously burst into flames or a bucket of water turn into hydrogen and oxygen? Energy must be applied for these reactions to occur.

Forming Bonds Releases Energy

Energy is released when bonds form. Bond formation represents a stable configuration for atoms, sort of like relaxing into a comfy chair. You release all your extra energy when you sink into the chair and it takes more energy to get you back up again.