Explaining energy changes in reactions

Breaking and making bonds

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

	Breaking bonds	Forming bonds
Type of process	Endothermic	Exothermic
Heat energy transferred	Taken in	Given out

	Type of process
Breaking bonds	Endothermic
Forming bonds	Exothermic

	Heat energy transferred
Breaking bonds	Taken in
Forming bonds	Given 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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Energy changes in chemistry - (CCEA)Explaining energy changes in reactions