Reversible Reaction – IGCSE Chemistry Definition

A reversible reaction is one that can proceed in both directions — products can be converted back to reactants. This concept is fundamental to the equilibrium section of the 0620 syllabus. You must recognise the reversible arrow symbol and understand that both forward and reverse reactions occur simultaneously when the system reaches equilibrium.

The 0620 definition

A reversible reaction is a reaction that can proceed in both the forward and reverse directions. It is represented by the equilibrium sign: a double-headed arrow with one arrow pointing right and one pointing left (often written as a fish-tail arrow or the symbol used in the equation below).

The reversible arrow

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The ⇌ symbol means:

• Forward reaction: N₂ + 3H₂ → 2NH₃
• Reverse reaction: 2NH₃ → N₂ + 3H₂
• Both reactions can and do occur

Key examples on the 0620 syllabus

Reaction	Forward	Reverse
Haber process	N₂ + 3H₂ → 2NH₃ (exothermic)	2NH₃ → N₂ + 3H₂ (endothermic)
Hydration of copper sulfate	CuSO₄ + 5H₂O → CuSO₄·5H₂O	CuSO₄·5H₂O → CuSO₄ + 5H₂O
Ammonium chloride	NH₃ + HCl → NH₄Cl	NH₄Cl → NH₃ + HCl
Contact process	2SO₂ + O₂ ⇌ 2SO₃	Reverse occurs at high temperature

Copper sulfate hydration — a classic example

• Anhydrous copper sulfate (CuSO₄) is a white powder
• Add water: it turns blue as hydrated copper sulfate (CuSO₄·5H₂O) forms — exothermic
• Heat the blue crystals: they lose water and turn white again — endothermic
• This demonstrates the reaction can go in both directions

If the forward reaction is exothermic, the reverse is always endothermic (by the same amount of energy).

Reversible reactions and equilibrium

In a closed system, a reversible reaction will reach equilibrium — the point where the rate of the forward reaction equals the rate of the reverse reaction. At equilibrium, both reactions continue (dynamic) but concentrations remain constant.

Worked exam question

When blue cobalt(II) chloride paper is exposed to water, it turns pink. When the pink paper is heated, it turns blue again. (a) What type of reaction is this? (1) (b) Draw the appropriate arrow to represent this type of reaction. (1) (c) Is the forward reaction (blue to pink) exothermic or endothermic? Explain. (1)

Mark scheme

(a) Reversible reaction [1]

(b) The equilibrium arrow ⇌ (two half-arrows, one in each direction) [1]

(c) Exothermic [1]; because the reverse reaction requires heating (is endothermic), so the forward reaction must be exothermic

Common exam mistakes

• Using a single arrow (→) instead of the reversible arrow (⇌) for a reversible reaction. The single arrow implies the reaction goes to completion in one direction.
• Forgetting that the forward and reverse reactions have opposite energy changes. If forward is exothermic, reverse is endothermic.
• Confusing reversible reactions with reactions that can be reversed by different methods. A truly reversible reaction reaches equilibrium on its own in a closed system.