Reversible Reactions (Edexcel IGCSE Chemistry: Double Science)

• 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 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)

The dehydration of hydrated salts is often a reversible reaction