Explaining energy changes in reactions - Higher Tier

Breaking and making bonds

Energy is transferred when chemical bonds are broken or are formed.

Breaking bondsForming bonds
Type of processEndothermicExothermic
Heat energy transferredTaken inGiven out

During a chemical reaction:

  • bonds in the reactants are broken
  • new bonds are made in the products

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 the bonds in the products, than is absorbed when breaking bonds in the reactants

A reaction is endothermic if less heat energy is released in making bonds in the products than is absorbed when breaking bonds in the reactants.