AQA GCSE Chemistry

Revision Notes

6.2.1 Reversible Reactions

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 occuring 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

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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