Exothermic and endothermic reactions · Year 9
Every chemical reaction transfers energy to or from the surroundings. An exothermic reaction gives out energy, so the surroundings get warmer: burning, respiration and neutralisation are exothermic. An endothermic reaction takes in energy, so the surroundings get colder (or energy must be supplied): thermal decomposition and photosynthesis are endothermic. A reaction profile shows the energy of the reactants and products and the activation energy needed to start the reaction.
Key facts
- Exothermic: energy is given out to the surroundings and the temperature rises. Examples: combustion, respiration, neutralisation, most oxidation reactions.
- Endothermic: energy is taken in from the surroundings and the temperature falls. Examples: thermal decomposition, photosynthesis, citric acid with sodium hydrogencarbonate.
- Energy is conserved: whatever the reaction gives out, the surroundings gain.
- The activation energy is the minimum energy particles need to collide and react. A catalyst lowers it.
- On a reaction profile the products are lower than the reactants for an exothermic reaction and higher for an endothermic one.
- The energy change is ΔH = energy of products − energy of reactants: negative for exothermic, positive for endothermic.
- Uses: hand warmers and self-heating cans (exothermic); instant cold packs for sports injuries (endothermic).