Endothermic & Exothermic

  • 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 molecular shape and colour.
  • Water of crystallisation is the water that is stoichiometrically 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 molecule 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 crystals are added to water, they turn 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.

Endothermic & Exothermic Table, IGCSE & GCSE Chemistry revision notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.