5.2 - Bond Energy Calculations

Bond breaking and bond making

Every chemical reaction breaks bonds in the reactants and makes new bonds in the products.

Breaking bonds is endothermic

Breaking bonds is an endothermic process - it takes energy in from the surroundings.

Making bonds is exothermic

Making bonds is an exothermic process - it gives energy out to the surroundings.

The overall energy change is the balance

Whether a reaction gives out or takes in energy overall depends on the balance:

• If more energy is given out making new bonds than is taken in breaking old ones, the reaction is exothermic.
• If more energy is taken in breaking old bonds than is given out making new ones, the reaction is endothermic.