Reactions and energy
Every chemical reaction involves a change in energy, usually seen as a change in temperature. Thermochemistry is the study of these heat changes. Reactions are grouped into two types depending on whether they give out heat or take it in.
Key idea
An exothermic reaction releases heat to the surroundings, so the temperature rises. An endothermic reaction absorbs heat from the surroundings, so the temperature falls.
Exothermic reactions
In an exothermic reaction, energy is given out and the mixture and its surroundings get hotter. Familiar examples include the combustion of fuels, respiration in cells, and the reaction of a metal with acid. The heat you feel from a burning candle or a hand-warmer comes from exothermic reactions.
Example
When magnesium burns in air, it releases a large amount of heat and light: magnesium + oxygen → magnesium oxide. This is exothermic, so the temperature rises.
Endothermic reactions
In an endothermic reaction, energy is taken in from the surroundings, so the mixture and its container feel colder. Dissolving certain salts, such as ammonium nitrate in water, and the reaction in some instant cold packs are endothermic. Photosynthesis, which takes in light energy, is also endothermic.
Remember
- Exothermic: gives out heat, temperature rises (e.g. combustion).
- Endothermic: takes in heat, temperature falls (e.g. some salts dissolving).
- Measure the change with a thermometer in °C.