What are Reversible Reactions? (Edexcel IGCSE Chemistry (Modular))

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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 all of the reactants are used up

  • 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 forming the products

    • The reverse reaction forming 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) → 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)

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

  • One example is copper(II) sulfate:

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

CuSO4•5H2O ⇌ CuSO4 + 5H2O

  • The hydrated salt is copper(II) sulfate pentahydrate, CuSO4•5H2O

    • These are usually seen as blue crystals

    • The hydrated calt can be heated / dehydrated to form anhydrous copper(II) sulfate, CuSO4 

    • This reaction is endothermic as energy is taken in to remove the water

  • The anhydrous salt is copper(II) sulfate

    • This is usually seen as white crystals / powder

    • Adding water to the anhydrous salt forms the hydrated copper(II) sulfate pentahydrate, CuSO4•5H2O

    • This reaction is highly exothermic

The dehydration of hydrated copper(II) sulfate

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

The dehydration of hydrated salts is often a reversible reaction

Examiner 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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