What are Reversible Reactions? (Edexcel IGCSE Science (Double Award) (Modular)): Revision Note

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

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• 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 salt 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

The dehydration of hydrated salts is often a reversible reaction