Why do reactions give out or take in heat, 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 focused CCEA GCSE Double Award Science (Chemistry Unit C2) answer on energy changes, covering exothermic and endothermic reactions, energy level diagrams, activation energy, and measuring temperature changes using calorimetry to compare fuels.
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What this dot point is asking
CCEA Double Award wants exothermic and endothermic reactions defined, energy level diagrams drawn and read, and how to measure energy changes by calorimetry to compare fuels. The calorimetry practical and the energy diagrams are the reliable marks.
Exothermic and endothermic reactions
Examples of exothermic reactions are combustion (burning fuels), neutralisation and the reactions in hand warmers. Examples of endothermic changes are thermal decomposition (such as breaking down a carbonate) and the reactions in some cold packs.
Energy level diagrams
An energy level diagram plots the energy of the reactants and products:
- In an exothermic reaction, the products are at a lower energy than the reactants, so energy is released (the difference is given out).
- In an endothermic reaction, the products are at a higher energy than the reactants, so energy is taken in.
The activation energy is the height of the hump between reactants and products - the minimum energy needed to start the reaction. A catalyst lowers this hump.
Calorimetry
Heat is lost to the surroundings, so calorimetry usually underestimates the true energy. Reducing draughts and insulating the can improves the result.
Energy changes happen because bonds are broken and made during a reaction. Breaking bonds takes in energy (endothermic) and making bonds gives out energy (exothermic). If more energy is released making the new bonds than is taken in breaking the old ones, the reaction is exothermic overall; if less, it is endothermic. This is why combustion, which makes strong bonds in carbon dioxide and water, releases so much energy.
Examples in context
Example 1. Self-heating cans. A self-heating food can uses an exothermic reaction (such as calcium oxide and water) to release heat and warm the food. This is an exothermic reaction put to practical use, with the temperature rise being the useful output.
Example 2. Sports cold packs. A cold pack uses an endothermic process that takes in heat from the surroundings, so it feels cold and can ease an injury. This shows an endothermic change lowering the temperature on purpose.
Try this
Q1. Is combustion exothermic or endothermic? [1 mark]
- Cue. Exothermic.
Q2. In an exothermic reaction, are the products higher or lower in energy than the reactants? [1 mark]
- Cue. Lower.
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-style3 marksDefine an exothermic reaction and give two everyday examples.Show worked answer →
Definition plus examples for three marks.
An exothermic reaction transfers energy to the surroundings, usually as heat, so the temperature of the surroundings rises.
Examples include combustion (burning fuels), neutralisation, and the reactions in hand warmers or respiration.
Markers reward energy released to the surroundings and two valid examples.
CCEA-style4 marksDescribe how you would use calorimetry to compare the energy released by two fuels.Show worked answer →
Describe a fair test for four marks.
Burn each fuel under a metal can holding a measured volume of water.
Measure the temperature rise of the water for the same mass of fuel burned.
Keep variables the same: the volume of water, the distance from the flame, and the can.
The fuel that gives the bigger temperature rise releases more energy. Markers reward measuring the temperature rise, the same volume of water, and a fair comparison.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)