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Energy Changes & Reactions (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Exam code: 8465

Stewart Hird

Written by: Stewart Hird

Updated on

AQA Combined Science: Synergy: Physical SciencesAQA Combined Science: Trilogy: ChemistryAQA Chemistry

Heat exchange in reactions

  • The changes in heat content can be determined and measured with a thermometer

  • Note that the overall amount of energy does not change as energy is conserved in reactions

  • This is known as the law of conservation of energy

  • This means that it cannot be created or destroyed but it can be transferred

  • So, if energy is transferred to the surroundings during a chemical reaction, then the products formed must have less energy than the reactants by the same amount as that transferred

Examiner Tips and Tricks

Physical processes can also involve heat exchange. Examples include freezing or melting which involve a change in state.

You may be asked to deduce whether a reaction is exothermic or endothermic given experimental results involving temperature changes. This may be for a reaction or experiment that you are unfamiliar with but the principle is still the same. Use the temperature change of the surroundings to decide whether it is exothermic or endothermic.

What is an exothermic reaction?

  • An exothermic reaction releases heat energy

    • This means that the temperature increases

Diagram showing heat energy being transferred outward from an exothermic reaction to the surroundings
Diagram showing the transfer of heat energy outwards from an exothermic reaction

  • Examples of exothermic reactions include:

    • Neutralisation

    • Combustion

What is an endothermic reaction?

  • An endothermic reaction takes heat energy in

    • This means that the temperature decreases

Diagram showing heat energy being transferred from the surroundings into an endothermic reaction
Diagram showing the transfer of heat energy from the surroundings into an endothermic reaction

  • Examples of endothermic reactions include:

    • Electrolysis

    • Thermal decomposition reactions

    • The first stages of photosynthesis

    • The reaction between citric acid and sodium hydrogencarbonate

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.