AQA GCSE Chemistry

Revision Notes

6.2.2 Energy Changes & Reversible Reactions

Energy Changes & Reversible Reactions

  • Energy changes also accompany chemical changes and energy can be given out (exothermic) or taken in (endothermic)
  • The majority of chemical reactions are exothermic with only a small number being endothermic
  • For a reversible reaction, if it is exothermic in one direction then it must be endothermic in the opposite direction
  • The amount of energy transferred in either direction is the same
  • Reversible reactions can be seen in some hydrated salts
  • These are salts that contain water of crystallisation which affects their shape and colour
  • Water of crystallisation is the water that is included in the structure of some salts during the crystallisation process
  • A common example is copper(II) sulfate which crystallises forming the salt copper(II) sulfate pentahydrate, CuSO4.5H2O
  • Water of crystallisation is indicated with a dot written in between the salt and the surrounding water molecules
  •  Anhydrous salts are those that have lost their water of crystallisation, usually by heating, in which the salt becomes dehydrated
  • When anhydrous copper(II) sulfate is added to water, it turns blue and heat is given off so the reaction is exothermic
  • When hydrated copper(II) sulfate crystals are heated in a test tube, the blue crystals turn into a white powder and a clear, colourless liquid (water) collects at the top of the test tube

Energy Changes & Reversible Reactions, downloadable IGCSE & GCSE Chemistry revision notes

Energy changes & reversible reactions example

Exam Tip

Make sure you know the terms anhydrous, hydrated and water of crystallisation.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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