Reversible Reactions — Reversible Reactions and Equilibria | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable

Low

Stable6%

Haber process conditions and Le Chatelier's principle regularly examined.

A reversible reaction can proceed in both directions: the products can themselves combine, or break apart, to give the original reactants back again
The forward reaction turns reactants into products
The reverse (backward) reaction turns products back into reactants
A reversible equation uses a special two-headed arrow, ⇌, instead of the single arrow → used for one-way reactions
- The top half-arrow points right (forward); the bottom half-arrow points left (reverse)
Whether the reaction settles toward the products side or toward the reactants side depends on the conditions — temperature, pressure, concentration

Example — thermal decomposition of ammonium chloride

NH₄Cl(s) → NH₃(g) + HCl(g)

Lift the test tube away from the heat and the gases meet cooler glass higher up; the two recombine and a white solid of ammonium chloride condenses on the wall:

NH₃(g) + HCl(g) → NH₄Cl(s)

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

The forward step is endothermic (heating drives it); the reverse step is exothermic (cooling drives it)

Example — hydration of copper(II) sulfate

Many ionic salts crystallise with water locked into their crystal lattice — this water is called water of crystallisation, and the salt is described as hydrated
Blue copper(II) sulfate crystals are the pentahydrate, CuSO₄·5H₂O
Heat the blue crystals strongly and the water is driven off, leaving the white solid called anhydrous copper(II) sulfate, CuSO₄
Add water back to the white anhydrous powder and the blue hydrate forms again, with a noticeable release of heat
Written as a reversible reaction:

CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5 H₂O(l)

The forward step (dehydration) is endothermic; the reverse step (rehydration) is strongly exothermic — which is why anhydrous copper(II) sulfate is also the chemical test for the presence of water (see topic 17 section 5)