5.1.1Energy Changes
Energy Transfer in Reactions — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 2 of 5
Part 1 of 5Energy is conserved
Conservation of energy
- Conservation of energy Energy cannot be created or destroyed in a chemical reaction; it is only transferred.
- The total energy before and after is the same.
Measured by temperature
- Measured by temperature Energy transfer to or from the surroundings is detected as a change in temperature with a thermometer.
Link to products
- Link to products If energy goes to the surroundings, the products store less energy than the reactants did, by the same amount.
Part 2 of 5Exothermic and endothermic reactions
| Feature | Exothermic | Endothermic |
|---|---|---|
| Energy transfer | Released to surroundings | Taken in from surroundings |
| Temperature of surroundings | Rises | Falls |
| Energy of products compared with reactants | Lower | Higher |
| Examples | Combustion, neutralisation, oxidation of metals, many oxidations | Thermal decomposition, electrolysis, first stage of photosynthesis, citric acid with sodium hydrogencarbonate |
Part 3 of 5Everyday applications
Using energy changes
Hand warmers
- Hand warmers Exothermic oxidation of iron or crystallisation of a salt releases heat on demand.
Self-heating cans
- Self-heating cans An exothermic reaction inside warms food or drinks.
Cold packs
- Cold packs An endothermic dissolving or reaction takes in heat from an injury to reduce swelling.
Fuels
- Fuels Combustion gives heat energy for cooking, heating and transport.
Part 4 of 5Observing a temperature change
1
Measure the starting temperature of the reactants with a thermometer.
2
Mix the reactants, stirring gently.
3
Record the highest or lowest temperature reached.
4
Subtract the starting value from the final value to find the temperature change.
5
A rise shows exothermic; a fall shows endothermic.
Part 5 of 5Where the energy goes
1Reactants
- Reactants Particles hold energy in their chemical bonds.
2Reaction
- Reaction Bonds are broken and new bonds formed.
3Energy exchange
- Energy exchange Net energy is released to, or absorbed from, the surroundings.
4Temperature changes
- Temperature changes The surroundings warm up or cool down accordingly.
Key terms
| Term | Meaning |
|---|---|
| Exothermic | A reaction that transfers energy to the surroundings, so the temperature rises. |
| Endothermic | A reaction that takes in energy from the surroundings, so the temperature falls. |
| Surroundings | Everything outside the reacting chemicals, such as the solution, container and air. |
| Conservation of energy | The principle that energy is transferred but never created or destroyed. |
| Thermal decomposition | Breaking down of a compound by heating. |
Avoid these mistakes
1
Saying exothermic reactions 'give out cold' — they heat the surroundings.
2
Describing the temperature of the chemicals instead of the surroundings; the thermometer measures the solution.
3
Stating that energy is made in an exothermic reaction; it is transferred from stored chemical energy.
4
Calling all reactions needing heating endothermic — a reaction may need heat to start yet release more overall.
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