GCSE Chemistry · AQA 8462 · Topic 4.5.1

Exothermic and endothermic reactions

Key points

In an exothermic reaction energy is transferred to the surroundings, so the temperature goes up. Uses include self-heating cans and hand warmers.
In an endothermic reaction energy is taken from the surroundings, so the temperature goes down. Uses include some sports injury cold packs.
A reaction profile shows reactants and products on an energy scale. In an exothermic profile the products are lower than the reactants; in an endothermic profile they are higher.
The activation energy is the minimum energy colliding particles need to react; it is the 'hump' on the profile.
(Higher) Breaking bonds is endothermic; making bonds is exothermic. Use bond energies: $\Delta H$ = bonds broken − bonds made. A negative answer means the reaction is exothermic.
Temperature changes can be measured in a polystyrene cup with a lid and thermometer to reduce energy losses.

Some reactions get hot and others get cold. Whether a reaction gives out or takes in energy depends on the balance between breaking and making bonds.

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