What is the difference between exothermic and endothermic reactions?
Exothermic and endothermic reactions; energy transfer to and from the surroundings; everyday examples; and the required practical on temperature changes.
A focused answer to AQA GCSE Chemistry 4.5.1, covering exothermic and endothermic reactions, how energy is transferred to or from the surroundings, everyday examples such as hand warmers and sports packs, and the required practical on temperature changes.
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What this dot point is asking
AQA wants you to define exothermic and endothermic reactions in terms of energy transfer with the surroundings, give everyday examples of each, and describe the required practical investigating temperature changes in reactions. The crucial idea is the direction of energy flow: in an exothermic reaction energy moves out of the chemicals into the surroundings, and in an endothermic reaction energy moves the other way.
Exothermic reactions
Everyday examples include combustion (burning fuels), neutralisation of acids by alkalis, many oxidation reactions, and self-heating hand warmers (which often use the oxidation of iron). At the molecular level, an exothermic reaction releases more energy when new bonds form in the products than is needed to break the bonds in the reactants.
Endothermic reactions
Everyday examples include thermal decomposition (such as heating metal carbonates), the reaction of citric acid with sodium hydrogen carbonate (used in some instant cold packs and in baking), and sports injury cold packs. Here the energy needed to break the reactant bonds is greater than the energy released forming product bonds, so the mixture draws energy in and feels cold.
The required practical
You can compare different reactions, or vary one factor (such as the mass of metal added, the concentration of acid, or the volumes used) to see its effect on the temperature change, keeping all other variables constant. Typical reactions studied include neutralisation, displacement of one metal by another, and dissolving salts.
Try this
Q1. Define an endothermic reaction. [1 mark]
- Cue. A reaction that takes in energy from the surroundings, so the temperature falls.
Q2. Give one everyday example of an exothermic reaction. [1 mark]
- Cue. Combustion (or neutralisation, or a hand warmer).
Q3. Explain why a polystyrene cup is used instead of a glass beaker in the temperature-change practical. [2 marks]
- Cue. Polystyrene is a good insulator, so it reduces energy transfer to the surroundings, giving a more accurate temperature change.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20184 marksA student investigates the temperature change when sodium hydroxide solution is neutralised by hydrochloric acid. Describe how the student should carry out this investigation to obtain accurate results, and predict the type of temperature change they will observe.Show worked answer →
A 4-mark Paper 1 question on the required practical, marked on method and prediction.
Method (3 marks): measure a fixed volume of acid into a polystyrene cup with a lid (insulation reduces heat loss); record the starting temperature; add a measured volume of sodium hydroxide; stir and record the highest temperature reached; the change is the difference. Use a polystyrene cup rather than glass because it is a good insulator.
Prediction (1 mark): the temperature rises because neutralisation is exothermic, transferring energy to the surroundings.
Markers reward the insulation point and the link between temperature rise and exothermic.
AQA 20213 marksSelf-heating cans use an exothermic reaction, while instant cold packs for sports injuries use an endothermic process. Explain, in terms of energy transfer with the surroundings, why one warms up and the other cools down. Give one further everyday example of each type.Show worked answer →
A 3-mark question testing understanding of energy transfer plus examples.
Exothermic (self-heating can): the reaction transfers energy to the surroundings, so the temperature of the can and its contents rises (1 mark). Endothermic (cold pack): the process takes in energy from the surroundings, so the temperature falls and the pack feels cold (1 mark). Further examples (1 mark): combustion or neutralisation (exothermic); thermal decomposition or citric acid with sodium hydrogen carbonate (endothermic).
Markers want the direction of energy transfer stated explicitly, not just "gives out heat".
Related dot points
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)