Reversible Reactions (AQA GCSE Combined Science: Trilogy) : Revision Note

Stewart Hird

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Reversible reactions

What are reversible reactions?

  • Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when all of the reactants are used up

A + B → C + D

  • 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)

 A + B → C + D

  • The reverse direction (which forms the reactants)

C + D → A + B

  • When writing chemical equations for reversible reactions, two arrows are used to indicate the forward and reverse reactions

  • Each one is drawn with just half an arrowhead – the top one points to the right, and the bottom one points to the left: ⇌

A + B bold rightwards harpoon over leftwards harpoon C + D

  • The direction a reversible reaction takes can be changed by changing the reaction conditions

Reversible reaction example

  • Heating ammonium chloride produces ammonia and hydrogen chloride gases

    • This is an endothermic process as energy is provided for the reaction

NH4Cl (s) → NH3 (g) + HCl (g) 

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

    • This is an exothermic process

NH3 (g) + HCl (g) → NH4Cl (s)  

  • So, the reversible reaction is represented like this:

NH4Cl (s) ⇌ NH3 (g) + HCl (g)

Examiner Tips and Tricks

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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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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