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Energy changes in chemistry - (CCEA)Explaining energy changes in reactions

When a chemical reaction occurs, energy is transferred to, or from, the surroundings. An exothermic reaction releases heat to the surroundings and an endothermic reaction absorbs heat from the surroundings.

Part of Chemistry (Single Science)Further chemical reactions, rates and equilibrium, calculations and organic chemistry

Explaining energy changes in reactions

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
Type of process
Breaking bondsEndothermic
Forming bondsExothermic
Heat energy transferred
Breaking bondsTaken in
Forming bondsGiven 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.

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