AQA GCSE Chemistry

Revision Notes

6.2.1 Reversible Reactions

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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
  • For example heating ammonium chloride produces ammonia and hydrogen chloride gases:

NH4Cl (s) → NH3 (g) + HCl (g) 

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

NH3 (g) + HCl (g) → NH4Cl (s)  

  • So, the reversible reaction is represented like this:

NH4Cl (s) NH3 (g) + HCl (g)

Exam Tip

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

A + B C + D

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