Exothermic and endothermic reactions
Energy in reactions
Energy is conserved in chemical reactions: the total amount of energy is the same at the end as at the start. But energy is usually transferred between the reacting chemicals and the .
- An reaction transfers energy to the surroundings, so the temperature of the surroundings increases.
- An reaction takes in energy from the surroundings, so the temperature of the surroundings decreases.
Key terms
Everyday examples
| Examples | Uses | |
|---|---|---|
| Combustion (burning fuels), respiration, neutralisation, many oxidation reactions | Hand warmers, self-heating cans | |
| Thermal decomposition, the reaction of citric acid with sodium hydrogencarbonate, photosynthesis | Instant cold packs for sports injuries |
Dissolving some salts, such as ammonium nitrate, also takes in energy and feels cold.
Required practical: temperature changes
This investigates how a variable, such as the volume of alkali added, affects the temperature change.
- Put a polystyrene cup inside a beaker so it can't tip over.
- Measure 30 cm³ of dilute hydrochloric acid into the cup and record its temperature.
- Add 5 cm³ of sodium hydroxide solution, stir, and record the highest temperature reached.
- Keep adding 5 cm³ portions, recording the highest temperature each time, up to 40 cm³.
- Repeat the whole experiment, then plot temperature against volume of alkali added.
Hazard: acids and alkalis are irritants — wear eye protection.
Better accuracy: put a lid on the cup to cut heat loss; polystyrene is a good insulator.