16.1 - Exothermic & Endothermic Reactions
Exothermic and endothermic reactions
- In an exothermic reaction, heat is given out to the surroundings, so the temperature of the surroundings rises.
- In an endothermic reaction, heat is taken in from the surroundings, so the temperature of the surroundings falls.
Energy is conserved
- Energy can't be created or destroyed, only transferred.
- In an exothermic reaction, the products hold less energy than the reactants because energy was released to the surroundings.
- In an endothermic reaction, the products hold more energy than the reactants because energy was taken in from the surroundings.
Sign of the energy change
- Negative (−) - Energy given out (exothermic).
- Positive (+) - Energy taken in (endothermic).
| Exothermic reaction | Endothermic reaction | |
|---|---|---|
| Energy is | transferred to the surroundings | taken in from the surroundings |
| Temperature of the surroundings | rises | falls |
| Compared with reactants, products | hold less energy | hold more energy |
Neutralisation is exothermic
When an acid reacts with a base or an alkali to produce a salt and water, heat is given out to the surroundings.
Activation energy
- For particles to react, they must collide with sufficient energy.
- The activation energy is the minimum amount of energy colliding particles must have for a reaction to occur. In the diagrams below, it's the height of the peak above the reactants' level.
Why there's a minimum energy
Chemical reactions require bonds in the reactants to break, which takes energy.
Size of activation energy
- Small activation energy - Reactions start easily at room temperature because many collisions already carry enough energy.
- Large activation energy - Reactions require energy from outside (usually by heating) so particles collide with enough energy to react.
TermDefinition
Exothermic reactionA reaction in which heat is given out to the surroundings, so the temperature of the surroundings rises.
Endothermic reactionA reaction in which heat is taken in from the surroundings, so the temperature of the surroundings falls.
Activation energyThe minimum amount of energy that colliding particles must have to react.