AQA GCSE Chemistry 4.5 Energy changes: a complete overview

What topic 4.5 actually demands

Energy changes explains why some reactions warm their surroundings and others cool them, and lets you predict and calculate the energy involved. Topic 4.5 rewards precise use of the words exothermic and endothermic, accurate reaction profiles, and careful bond energy arithmetic at Higher tier. It links closely to rates (activation energy) and to quantitative chemistry.

This guide walks through all three dot points of the topic in specification order, then sets out the exam patterns AQA repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Exothermic and endothermic reactions

An exothermic reaction transfers energy to the surroundings, so the temperature rises (combustion, neutralisation, hand warmers). An endothermic reaction takes in energy from the surroundings, so the temperature falls (thermal decomposition, citric acid with sodium hydrogen carbonate, cold packs). The required practical measures these changes by recording the temperature of a reaction mixture in an insulated container to reduce heat loss.

Reaction profiles and activation energy

A reaction profile plots energy against the progress of the reaction. In an exothermic reaction the products are lower in energy than the reactants; in an endothermic reaction they are higher. The height of the hump above the reactants is the activation energy, the minimum energy colliding particles need to react. A catalyst lowers the activation energy by providing an alternative pathway, shown as a lower hump, without changing the overall energy change.

Bond energy calculations

At Higher tier, the overall energy change can be calculated from bond energies. Breaking bonds is endothermic and making bonds is exothermic, so:

A negative result is exothermic and a positive result is endothermic.

How topic 4.5 is examined

A typical AQA profile for this topic:

• Classification. Identifying exothermic and endothermic reactions and their examples.
• Diagrams. Drawing and labelling reaction profiles, including activation energy and the catalysed pathway.
• Practical. The temperature-change required practical, including variables and how insulation improves accuracy.
• Calculation. Bond energy calculations to find and interpret the overall energy change.

Check your knowledge

A mix of recall and calculation questions covering topic 4.5. Attempt them under timed conditions, then check against the solutions.

1. Define an exothermic reaction. (1 mark)
2. Give one everyday example of an endothermic reaction. (1 mark)
3. On a reaction profile, describe where the products lie for an endothermic reaction. (1 mark)
4. Define activation energy. (1 mark)
5. Explain how a catalyst affects a reaction profile. (2 marks)
6. State whether breaking bonds is exothermic or endothermic, and explain. (2 marks)
7. A reaction breaks bonds requiring $2500$ kJ and makes bonds releasing $2700$ kJ. Calculate the energy change and state the type of reaction. (2 marks)
8. Explain why an insulated cup gives more accurate results in the temperature-change practical. (1 mark)