Energy Changes & Reversible Reactions (AQA GCSE Chemistry: Combined Science)

• 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
  • 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

Reversible reaction example

• A common example is copper(II) sulfate which crystallises forming the salt copper(II) sulfate pentahydrate, CuSO₄.5H₂O
• If hydrated copper(II) sulfate crystals are heated, the blue crystals turn into a white powder and a clear, colourless liquid (water) collects at the top of the test tube 
  • This forward reaction is endothermic
• When anhydrous copper(II) sulfate crystals are then added to water, the crystals turns blue and heat is given off 
  • This backward / reverse reaction is exothermic 

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

The forward reaction is exothermic and the reverse reaction is endothermic