Reversible Reactions (AQA GCSE Chemistry: Combined Science)

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  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)