Exothermic and Endothermic Reactions
Practise Exothermic and Endothermic Reactions for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Exothermic reactions transfer energy out and raise the temperature; endothermic reactions take energy in and cool the surroundings.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Energy is conserved in chemical reactions, so the total energy before and after a reaction is the same. What changes is where that energy is stored.
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases. Combustion, oxidation and neutralisation are exothermic. Everyday uses include self-heating cans and hand warmers.
An endothermic reaction takes in energy from the surroundings, so the temperature decreases. Thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate are endothermic. Sports injury packs use this to provide cooling.
Revision notes
Exothermic reactions
Energy is transferred to the surroundings, so the temperature of the surroundings increases.
Examples: combustion, oxidation reactions such as rusting, and neutralisation. Uses include self-heating cans and hand warmers.
Endothermic reactions
Energy is taken in from the surroundings, so the temperature of the surroundings decreases.
Examples: thermal decomposition, and the reaction of citric acid with sodium hydrogencarbonate. Uses include sports injury packs for cooling.
Conservation of energy
Energy is always conserved — it is neither created nor destroyed.
In an exothermic reaction the products store less energy than the reactants, and the difference is released. In an endothermic reaction the products store more, and the difference is taken in.
Key points
- Energy is conserved in all chemical reactions.
- Exothermic reactions transfer energy to the surroundings.
- Exothermic reactions increase the surrounding temperature.
- Combustion and neutralisation are exothermic.
- Endothermic reactions take in energy.
- Thermal decomposition is endothermic.
Worked examples
Example 1
State what happens to the temperature of the surroundings during an exothermic reaction. [1 mark]
Model answer
The temperature of the surroundings increases (1 mark).
Example 2
Give one everyday use of an exothermic reaction and one of an endothermic reaction. [2 marks]
Model answer
Exothermic: self-heating cans or hand warmers (1 mark). Endothermic: sports injury packs used for cooling (1 mark).
Example 3
Explain why energy is conserved in a chemical reaction even though the temperature changes. [2 marks]
Model answer
Energy is not created or destroyed, only transferred between the reacting chemicals and the surroundings (1 mark). In an exothermic reaction the products store less energy than the reactants, and the difference is released to the surroundings (1 mark).
Common mistakes
Saying exothermic reactions get colder.
They release energy, so the surroundings get warmer.
Saying energy is created in an exothermic reaction.
Energy is conserved; it is transferred, not made.
Describing the temperature of the reaction rather than the surroundings.
The definitions refer to energy transferred to or from the surroundings.
Classifying neutralisation as endothermic.
It is exothermic.
Exam tips
- Say transferred to or from the surroundings in your definitions.
- Learn two examples and one use of each type.
- State that energy is conserved when explaining.
- Link the energy change to the temperature change of the surroundings.
Key terms
- Exothermic
- A reaction transferring energy to the surroundings.
- Endothermic
- A reaction taking in energy from the surroundings.
- Conservation of energy
- The principle that energy is neither created nor destroyed.
- Thermal decomposition
- Breaking down a compound by heating, an endothermic process.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.