How do reactions release or take in energy, and how do we measure it?
Exothermic and endothermic reactions, energy level diagrams, and measuring temperature changes using calorimetry to compare the energy released by fuels.
A CCEA GCSE Chemistry answer on energy changes, covering exothermic and endothermic reactions, energy level diagrams and activation energy, and how calorimetry is used to measure and compare the energy released when fuels burn.
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What this dot point is asking
CCEA wants you to define exothermic and endothermic reactions, draw and interpret energy level diagrams, and use calorimetry to measure and compare the energy released by burning fuels.
Exothermic and endothermic reactions
Examples of exothermic reactions are combustion, neutralisation and most oxidation reactions. Examples of endothermic reactions are thermal decomposition (such as heating a carbonate) and the reaction in some sports cold packs.
Energy level diagrams
A catalyst lowers the height of the activation energy hump, but does not change the start and end levels, so it does not change whether the reaction is exothermic or endothermic.
Bond energy idea
Energy is needed to break bonds (endothermic) and released when bonds form (exothermic). If more energy is released forming new bonds than is taken in breaking the old ones, the reaction is overall exothermic; if less, it is endothermic. This is why combustion, which forms strong bonds in carbon dioxide and water, releases so much energy.
Calorimetry
To compare fuels fairly, use the same mass of water and measure the energy released per gram of fuel. Heat loss to the surroundings is the main source of error, so the value found is usually lower than the true energy content.
Worked example
Examples in context
Example 1. Hand warmers and cold packs. Reusable hand warmers use an exothermic reaction or crystallisation to release heat, while instant cold packs use an endothermic process that absorbs heat. Both are everyday devices built directly on the difference between exothermic and endothermic changes.
Example 2. Comparing fuels. Calorimetry lets scientists compare how much energy different fuels release per gram, which informs choices about which fuel to use for heating or transport. The same calculation used in the lab underlies these real energy comparisons.
Try this
Q1. State whether combustion is exothermic or endothermic. [1 mark]
- Cue. Exothermic (it releases energy).
Q2. Write the equation used to calculate the energy transferred in calorimetry. [1 mark]
- Cue. .
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20184 marksDefine exothermic and endothermic reactions, and sketch the energy level diagram for an exothermic reaction, labelling the activation energy and the energy change.Show worked answer →
Markers want both definitions and a correctly labelled diagram.
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings rises (the products have less energy than the reactants).
An endothermic reaction takes in energy from the surroundings, so the temperature falls (the products have more energy than the reactants).
The energy level diagram for an exothermic reaction shows the reactants higher than the products. The curve rises to a peak (the top is the activation energy measured from the reactants) then falls to the lower product level. The energy change is the drop from reactant level to product level (negative, energy released).
Markers reward both definitions, reactants above products, the activation energy hump, and the downward energy change.
CCEA 20214 marksA spirit burner of ethanol heats 100 g of water, raising its temperature by 20 degrees C. Calculate the energy transferred to the water. (Specific heat capacity of water = 4.2 J per g per degree C)Show worked answer →
The marks are for the correct equation and substitution.
Use the calorimetry equation , where is the mass of water, the specific heat capacity and the temperature rise.
Working it out: , then .
So the energy transferred to the water is 8400 J (8.4 kJ).
Markers reward the equation Q equals m c delta T, correct substitution, and the answer 8400 J (or 8.4 kJ).
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)