Reversible reactions occur when the products of a reaction can react with each other or decompose to form the original reactants. This means that the reaction can go both ways, as represented by the symbol ⇌, which is made up of two half arrows pointing in opposite directions.
For example, the reaction A + B ⇌ C + D can go in two directions. The forward reaction goes to the right, meaning that the reactants A and B form the products C and D:
A + B → C + D
The backward reaction goes to the left, meaning that the products C and D form the reactants A and B:
C + D → A + B
When we heat ammonium chloride (NH4Cl), it reacts to form ammonia (NH3) and hydrogen chloride (HCl).
NH4Cl (s) → NH3 (g) + HCl (g)
When the products (ammonia and hydrogen chloride) are cool enough, they react together to form ammonium chloride.
NH3 (g) + HCl (g) → NH4Cl (s)
So, the reversible reaction is:
Ammonium chloride ⇌ Ammonia + Hydrogen chloride
NH4Cl (s) ⇌ NH3 (g) + HCl (g)
Reversible reactions can change direction depending on the conditions. If a reaction is exothermic in one direction, it will be endothermic in the opposite direction. However, the amount of energy transferred in both the forward and reverse reactions is the same.
For example, when we heat blue hydrated copper sulfate, it reacts to form anhydrous copper sulfate (a white powder) and water.
Hydrated copper sulfate → Anhydrous copper sulfate + Water
CuSO4.5H2O (s) → CuSO4 (s) + 5H2O (l)
As we are heating the hydrated copper sulfate, this reaction is endothermic. However, we can add water back to the anhydrous copper sulfate to reverse the reaction, forming hydrated copper sulfate again.
Anhydrous copper sulfate + Water → Hydrated copper sulfate
CuSO4 (s) + 5H2O (l) → CuSO4.5H2O (s)
In the reverse reaction, energy is released, which gives out heat, making it exothermic.
Therefore, the reversible reaction is:
Hydrated copper sulfate ⇌ Anhydrous copper sulfate + Water
CuSO4.5H2O (s) ⇌ CuSO4 (s) + 5H2O (l)