Reversible Reactions


  • Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when the reactants have been exhausted
  • In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again
  • It is said that the reaction can occur in both directions: the forward reaction (which forms the products) and the reverse direction (which forms the reactants)
  • When writing chemical equations for reversible reactions, two opposing arrows are used to indicate the forward and reverse reactions occurring at the same time
  • Each one is drawn with just half an arrowhead – the top one points to the right, and the bottom one points to the left
  • The direction a reversible reaction takes can be changed by changing the reaction conditions




Thermal Decomposition of Ammonium Chloride

  • Heating ammonium chloride produces ammonia and hydrogen chloride gases:


NH4Cl (s) → NH(g) + HCl (g)

  • As the hot gases cool down they recombine to form solid ammonium chloride


NH(g) + HCl (g) → NH4Cl (s)  


  • So, the reversible reaction is represented like this:


NH4Cl (s) ⇌ NH(g) + HCl (g)




Dehydration of Hydrated Copper(II) Sulfate

  • 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
  • The equation for the reaction is:


CuSO4.5H2O (s) ⇌ CuSO4 (s)  +  5H2O (l) 





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


The dehydration of hydrated salts is often a reversible reaction



Exam Tip

The reverse reaction may also be called the backwards reaction. A generic reversible reaction is shown as


A + B ⇌ C + D