What are exothermic and endothermic reactions, and how are they shown on a reaction profile?
Exothermic and endothermic reactions, examples of each, and reaction profile diagrams showing activation energy.
A focused answer to the WJEC GCSE Science Double Award Unit 5 topic on energy changes, covering exothermic and endothermic reactions with examples, and reaction profile diagrams showing the energy change and activation energy.
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What this dot point is asking
WJEC Double Award Unit 5 wants you to describe exothermic and endothermic reactions with examples, and interpret reaction profile diagrams including activation energy.
Exothermic and endothermic reactions
- Exothermic examples: combustion (burning), neutralisation, and most oxidation reactions. Hand warmers use exothermic reactions.
- Endothermic examples: thermal decomposition (e.g. heating limestone), and some dissolving. Sports cold packs use endothermic processes.
You can tell which type a reaction is by measuring the temperature before and after: a rise means exothermic, a fall means endothermic.
Reaction profiles
Activation energy
A reaction with a high activation energy is harder to start; a catalyst lowers the activation energy, providing an easier route, which is why catalysts speed reactions up. This links the energy change to rates of reaction.
Bonds breaking and making
Energy changes can be explained by bond breaking and making. Breaking bonds in the reactants takes in energy (endothermic), while making new bonds in the products gives out energy (exothermic). If more energy is released making bonds than is taken in breaking them, the reaction is exothermic overall; if more is taken in than released, it is endothermic. This explains why the overall reaction can be exothermic or endothermic, and it is the reason a reaction profile shows the products higher or lower than the reactants.
Measuring an energy change
A simple practical measures the energy change of a reaction in solution. You record the temperature of the reactants, mix them in an insulated cup, and record the highest or lowest temperature reached. A rise shows an exothermic reaction; a fall shows an endothermic one, and the size of the temperature change shows how much energy was transferred. Using an insulated container reduces heat loss to the surroundings, making the measurement more accurate. Describing this method and interpreting the temperature change is a common practical question.
Try this
Q1. A reaction makes the surroundings warmer. Is it exothermic or endothermic? [1 mark]
- Cue. Exothermic.
Q2. What does a catalyst do to the activation energy? [1 mark]
- Cue. It lowers it (providing an easier route).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style3 marksExplain the difference between an exothermic and an endothermic reaction, giving one example of each.Show worked answer →
A Unit 5 explain question. Reward: an exothermic reaction gives out energy to the surroundings, so the temperature rises, for example combustion or neutralisation (1, +1 for example); an endothermic reaction takes in energy from the surroundings, so the temperature falls, for example thermal decomposition or some dissolving (1). Markers credit gives out/temperature rises for exothermic and takes in/temperature falls for endothermic, with examples. A common error is to mix up which way the temperature changes.
WJEC style4 marksDescribe what a reaction profile shows for an exothermic reaction, including the activation energy.Show worked answer →
A Unit 5 explain question worth 4 marks. Reward: the profile shows the energy of the reactants and products (1); for an exothermic reaction, the products are lower in energy than the reactants (energy is released) (1); the activation energy is the energy hump the reactants must get over to react, shown as the rise to the peak (1); the difference between reactants and products is the overall energy change (1). Markers credit products lower than reactants, the activation energy hump, and the energy change. A common error is to draw the products higher (that is endothermic).
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