How do we show energy changes on a reaction profile, and what makes a reaction exothermic or endothermic?
Describe exothermic and endothermic reactions, draw and interpret reaction profiles, and identify activation energy and overall energy change.
A focused answer to WJEC GCSE Chemistry topic 2.4, covering exothermic and endothermic reactions, drawing and interpreting reaction profile (energy level) diagrams, identifying activation energy and overall energy change, and everyday uses such as hand warmers and cold packs.
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What this topic is asking
WJEC topic 2.4 wants you to classify reactions as exothermic or endothermic by the direction of heat transfer, to draw and interpret reaction profiles (energy level diagrams), and to identify the activation energy and the overall energy change on those profiles. You should also know everyday uses such as hand warmers and cold packs.
Exothermic and endothermic reactions
You can tell them apart in the lab with a thermometer: a temperature rise means exothermic; a temperature fall means endothermic. Examples of exothermic reactions are combustion, neutralisation and many oxidation reactions. Examples of endothermic reactions are thermal decomposition (such as heating calcium carbonate) and some dissolving processes.
Reaction profiles
A reaction profile (energy level diagram) shows how the energy changes as reactants turn into products.
Activation energy on the profile
The peak of the curve represents the energy barrier that must be overcome for the reaction to happen.
The overall energy change
The overall energy change is the vertical gap between the reactant and product energy levels.
- In an exothermic reaction the products are lower, so energy is given out (the overall change is negative).
- In an endothermic reaction the products are higher, so energy is taken in (the overall change is positive).
Everyday uses
Energy changes have practical uses. Self-heating cans and hand warmers use exothermic reactions to release heat. Cold packs for sports injuries use endothermic reactions (or endothermic dissolving) to absorb heat and feel cold. Choosing the right type of reaction lets engineers design a product that warms or cools on demand.
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 sample4 marksDraw and label a reaction profile for an exothermic reaction. Show the reactants, products, activation energy and overall energy change.Show worked answer →
A Unit 2.4 diagram question. Reward: an exothermic profile shows the reactants at a higher energy than the products (products lower). The curve rises to a peak before falling, and the activation energy is the height from the reactants up to the peak. The overall energy change is the (downward) gap between the reactants' and products' energy levels, showing energy released. Markers credit reactants higher than products, the activation energy from reactants to the peak, and the overall energy change as the difference between reactant and product levels. A common error is to draw the products higher than the reactants (that would be endothermic).
WJEC sample3 marksA cold pack uses a reaction that takes in heat. State whether this reaction is exothermic or endothermic and explain how a cold pack works.Show worked answer →
A Unit 2.4 application question. Reward: the reaction is endothermic because it takes in heat from the surroundings. When the pack is activated, the endothermic reaction absorbs heat energy from its surroundings (including the skin), so the surroundings cool down and the pack feels cold. Markers credit endothermic, taking in heat from the surroundings, and the resulting cooling. A common slip is to call it exothermic because the pack "does something", without checking the direction of heat flow.
Related dot points
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Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)