Explain why breaking bonds requires energy and forming bonds releases energy, and use this idea to explain how you can determine whether an overall chemical reaction is exothermic or endothermic.

Breaking bonds requires energy because the atoms must be separated against the attractive forces that hold them together – energy is absorbed from the surroundings.

Forming bonds releases energy because the atoms are brought together and the attractive forces are established – energy is given out to the surroundings.

To decide if a reaction is exothermic or endothermic, compare the energy needed to break all bonds in the reactants with the energy released when all bonds in the products are formed.

• If the energy required to break bonds is greater than the energy released, the reaction is endothermic – overall energy is absorbed.

• If the energy released is greater than the energy required, the reaction is exothermic – overall energy is released.

The difference between the two gives the overall ΔE (or ΔH) of the reaction.