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H2 equilibrium: Kc expressions, reaction tables and changing conditions

At dynamic equilibrium the forward and reverse reactions continue at equal rates in a closed system. Concentrations remain constant; they need not be equal. Distinguish a change in equilibrium composition from a change in the equilibrium constant.

Choose the method

Write Kc from the balanced equation, raising each concentration to its coefficient. Use equilibrium concentrations, not the amounts initially mixed.

Build an initial-change-equilibrium table. Changes in amounts follow the equation coefficients; convert amounts to concentrations using the vessel volume before substitution.

For a specified reaction, K changes with temperature. At fixed temperature a concentration change or pressure change may shift the composition, while a catalyst reaches the same equilibrium faster.

Worked examples

Example 1

For H₂(g) + I₂(g) ⇌ 2HI(g), equilibrium concentrations are 0.20, 0.20 and 0.80 mol/dm³ respectively. Calculate Kc.

Kc = [HI]²/([H₂][I₂]).
Kc = 0.80²/(0.20 × 0.20) = 16.
The concentration powers cancel here, so this school-level expression has no concentration units.

Example 2

A 2.0 dm³ vessel initially contains 1.0 mol H₂ and 1.0 mol I₂, with no HI. At equilibrium it contains 1.2 mol HI. Find all equilibrium concentrations.

Producing 1.2 mol HI consumes 0.60 mol of each reactant.
Amounts at equilibrium: H₂ = 0.40 mol, I₂ = 0.40 mol, HI = 1.2 mol.
Divide each by 2.0 dm³: [H₂] = [I₂] = 0.20 mol/dm³; [HI] = 0.60 mol/dm³.

Try it yourself

For an exothermic forward reaction at equilibrium, predict the effect of increasing temperature on K.

Show the worked answer

Added thermal energy favours the endothermic reverse direction.
The equilibrium product-to-reactant ratio in the K expression decreases, so K decreases. A catalyst would not have this effect.

Common mistakes

Equal rates do not mean equal concentrations.
Increasing pressure by reducing volume matters for gaseous equilibrium when the two sides have different total gaseous coefficients; count them first.

Sources and review

SEAB 2026 syllabus: H2 Chemistry 9476: chemical equilibria.

Original Rae practice, prepared with AI assistance. Selected numerical results and their displayed working are automatically checked at publication; this does not verify every explanation. Curriculum references checked on 5 September 2026. No teacher review or SEAB endorsement is claimed.

What the syllabus expects

Using the forward and backward reaction rates, explain what a reversible reaction and a dynamic equilibrium are.
State Le Chatelier's Principle and use it to reason out, from suitable information, how a change in concentration, pressure or temperature disturbs a system at equilibrium.
Decide which of concentration, pressure, temperature or a catalyst actually alters a reaction's equilibrium constant.
Write the equilibrium-constant expressions, Kc from concentrations and Kp from partial pressures.
Scope: the link between Kp and Kc is not required
From appropriate data, compute equilibrium constants using either concentrations or partial pressures.
Given the data, work out how much of each species is present at equilibrium.
Scope: no quadratic equations will be involved
Describe and justify the conditions chosen for the Haber process, showing why a grasp of equilibrium matters to industry.

How it's examined

Questions on this topic most often ask you to describe, evaluate, explain. About 4% of the past-paper style questions in Rae's bank for this subject sit in this topic.

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