CLASS 11–12 CHEMISTRY · EQUILIBRIUM

Equilibrium constant Kc

Kc = product of [products]^coefficients ÷ product of [reactants]^coefficients, using equilibrium concentrations in mol/L.

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Worked examples

Example 1 · level 1

For the reaction CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g), the equilibrium concentrations (mol/L) are [CO] = 0.2, [H₂O] = 0.4, [CO₂] = 0.2, [H₂] = 0.25. Calculate Kc, correct to 2 decimal places (ignore units).

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Kc = [CO₂][H₂] / ([CO][H₂O]).
Kc = (0.2 × 0.25) ÷ (0.2 × 0.4).
Kc = 0.625 ≈ 0.63.

Answer: 0.63

Example 2 · level 2

For the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), the equilibrium concentrations (mol/L) are [SO₂] = 0.25, [O₂] = 0.2, [SO₃] = 1.5. Calculate Kc, correct to 2 decimal places (ignore units).

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Kc = [SO₃]² / ([SO₂]²[O₂]).
Kc = (1.5²) ÷ (0.25² × 0.2).
Kc = 180 ≈ 180.

Answer: 180

Example 3 · level 3

0.5 mol/L of CO and 0.5 mol/L of steam react in a closed vessel: CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g). At equilibrium, [CO₂] = 0.25 mol/L. Calculate Kc, correct to 2 decimal places (ignore units).

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CO₂ formed = H₂ formed = x = 0.25 mol/L.
[CO] = 0.5 − 0.25 = 0.25; [H₂O] = 0.5 − 0.25 = 0.25.
Kc = [CO₂][H₂] / ([CO][H₂O]) = (0.25 × 0.25) ÷ (0.25 × 0.25) = 1.

Answer: 1