Explaining heat energy changes

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 form, determines the type of reaction. A reaction is:

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

An energy level diagram shows these differences more clearly.

Energy level diagrams

An energy level diagram shows the energy in the reactants and products, and the difference in energy between them.

Exothermic reaction

The energy level decreases in an exothermic reaction. This is because energy is given out to the surroundings.

Graph of energy against progress of reaction. Horizontal line represents reactants energy. Drops vertically as energy released. Horizontal line at a lower level represents products' energy.A downwards arrow shows that energy is given out

Endothermic reaction

The energy level increases in an endothermic reaction. This is because energy is taken in from the surroundings.

Graph of energy against progress of reaction. A horizontal line represents the reactants' energy. It rises vertically as energy is absorbed. A horizontal line at a higher level then represents the products' energy.An upwards arrow shows that energy is taken in