Explaining energy changes in reactions - Higher

Breaking and making bonds

Energy is transferred when bonds are broken or are formed.

Breaking bondsForming bonds
Type of processEndothermicExothermic
Heat energy transferredTaken inGiven out

During a chemical reaction:

The difference between the energy needed to break bonds and the energy released when new bonds are made determines the type of reaction.

A reaction is:

  • exothermic if more heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
  • endothermic if less heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants