What is the difference between exothermic and endothermic reactions, and how do reaction profiles show it?
Exothermic and endothermic reactions: the energy transfer to or from the surroundings, examples and temperature changes, reaction profiles and activation energy, and the core practical on temperature change.
A focused answer to Edexcel GCSE Chemistry topic 7, covering the difference between exothermic and endothermic reactions, examples and the temperature changes they cause, how reaction profiles show the energy change and the activation energy, and the core practical investigating temperature changes in reactions.
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What this dot point is asking
Edexcel wants you to distinguish exothermic and endothermic reactions by their energy transfer and temperature change, give examples of each, draw and interpret reaction profiles showing the activation energy and overall energy change, and describe the core practical investigating temperature changes. Reaction profiles and identifying the type from a temperature change are the key marks.
Exothermic and endothermic reactions
You can tell which is which from the temperature change: a rise means exothermic (energy released), a fall means endothermic (energy absorbed).
Examples
Reaction profiles
A reaction profile plots energy (vertical axis) against the progress of the reaction (horizontal axis):
- The reactants start at one energy level and the products end at another.
- In an exothermic reaction the products are at a lower energy than the reactants (energy is released), so the line ends lower than it started.
- In an endothermic reaction the products are at a higher energy than the reactants (energy is absorbed), so the line ends higher.
- The curve always rises to a peak between reactants and products; the height of this peak above the reactants is the activation energy.
A catalyst lowers the height of this peak (the activation energy) but does not change the energy levels of the reactants or products.
The core practical: temperature change
The Edexcel core practical investigates the temperature change in reactions, such as displacement, neutralisation or dissolving.
- Measure the starting temperature of the solution with a thermometer.
- Add the second reactant, stir, and record the highest or lowest temperature reached.
- Calculate the temperature change: a rise indicates exothermic, a fall indicates endothermic.
- Use a polystyrene cup with a lid to reduce energy loss to the surroundings for a more accurate result.
Try this
Q1. State whether neutralisation is exothermic or endothermic. [1 mark]
- Cue. Exothermic.
Q2. On a reaction profile, where are the products relative to the reactants for an endothermic reaction? [1 mark]
- Cue. Higher in energy than the reactants.
Q3. A reaction causes the temperature to fall. State the type of reaction and explain why the temperature falls. [2 marks]
- Cue. Endothermic; it takes in energy from the surroundings, cooling them.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20184 marksA reaction is exothermic. Sketch and label a reaction profile for this reaction, marking the reactants, products, the activation energy and the overall energy change. Explain how the profile shows the reaction is exothermic.Show worked answer →
A 4-mark reaction-profile question.
The profile should show the reactants at a higher energy level than the products (1 mark), with a curve that rises to a peak (the activation energy, measured from the reactants to the top) before falling to the products (1 mark for the activation energy arrow). The overall energy change is the difference between the reactant and product energy levels (1 mark). The reaction is exothermic because the products are at a lower energy than the reactants, so energy is released to the surroundings (1 mark).
Markers reward products below reactants, the activation energy from reactants to the peak, and the link to energy released.
Edexcel 20213 marksA student mixes two solutions and the temperature falls from to . State whether the reaction is exothermic or endothermic, calculate the temperature change, and explain your reasoning in terms of energy transfer.Show worked answer →
A 3-mark energy-change interpretation question.
The reaction is endothermic (1 mark). The temperature change is (a fall of ) (1 mark). It is endothermic because the temperature falls, which means the reaction has taken in (absorbed) energy from the surroundings, cooling them down (1 mark).
Markers reward identifying endothermic from the temperature fall and linking it to energy taken in from the surroundings.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)