What is the effect of increasing pressure on the position of N₂ + 3H₂ ⇌ 2NH₃?

The position moves right, towards the side with fewer moles of gas.

What does a catalyst do to the equilibrium composition?

Nothing: it speeds up forward and reverse reactions equally.

Does adding more reactant change K?

No: the position shifts to restore the same value of K.

What happens to K for an exothermic reaction when temperature rises?

K decreases and the equilibrium moves towards the reactants.

What happens to K for an endothermic reaction when temperature rises?

K increases and the equilibrium moves towards the products.

Write the relationship between ΔStotal and K.

ΔStotal = R ln K, with R = 8.31 J K⁻¹ mol⁻¹.

Why does K fall for an exothermic reaction as T rises?

ΔSsurroundings = −ΔH/T becomes smaller, so ΔStotal falls and so does K.

What does a very large K (e.g. 10⁶) show?

The reaction goes almost to completion, with mainly products.

What does a very small K (e.g. 10⁻⁶) show?

Hardly any reaction occurs, with mainly reactants.

Why are compromise conditions used in industry?

Low temperature favours the yield of an exothermic reaction but gives a slow rate; high pressure costs more.

A heterogeneous equilibrium has a pure solid added. What happens to the position?

Nothing, as a pure solid is not in the K expression.

Factors affecting equilibrium and KEdexcel International A Level Chemistry: Flashcards

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State Le Chatelier's principle.

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State Le Chatelier's principle.: If a system at equilibrium is disturbed, the position of equilibrium moves to oppose the change.
What is the effect of increasing pressure on the position of N₂ + 3H₂ ⇌ 2NH₃?: The position moves right, towards the side with fewer moles of gas.
What does a catalyst do to the equilibrium composition?: Nothing: it speeds up forward and reverse reactions equally.
What is the only factor that changes the value of K?: Temperature.
Does adding more reactant change K?: No: the position shifts to restore the same value of K.
What happens to K for an exothermic reaction when temperature rises?: K decreases and the equilibrium moves towards the reactants.
What happens to K for an endothermic reaction when temperature rises?: K increases and the equilibrium moves towards the products.
Write the relationship between ΔStotal and K.: ΔStotal = R ln K, with R = 8.31 J K⁻¹ mol⁻¹.
Why does K fall for an exothermic reaction as T rises?: ΔSsurroundings = −ΔH/T becomes smaller, so ΔStotal falls and so does K.
What does a very large K (e.g. 10⁶) show?: The reaction goes almost to completion, with mainly products.
What does a very small K (e.g. 10⁻⁶) show?: Hardly any reaction occurs, with mainly reactants.
What is the effect of pressure on a system with equal moles of gas on each side?: No effect on composition.
Why are compromise conditions used in industry?: Low temperature favours the yield of an exothermic reaction but gives a slow rate; high pressure costs more.
A heterogeneous equilibrium has a pure solid added. What happens to the position?: Nothing, as a pure solid is not in the K expression.

Exam questions on Factors affecting equilibrium and K

Ammonia is manufactured by the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. The reaction is carried out at about 450 °C and 200 atm using an iron catalyst.
Explain why using the iron catalyst does not change the equilibrium yield of ammonia.2 marks
Limestone is heated in a sealed kiln in which this equilibrium is established: CaCO₃(s) ⇌ CaO(s) + CO₂(g), ΔH = +178 kJ mol⁻¹. For this equilibrium Kp = p(CO₂).
The volume of the sealed kiln is halved at constant temperature. Explain why the equilibrium pressure of carbon dioxide returns to its original value.2 marks
The Contact process makes sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹. The entropy change of the system is −188 J K⁻¹ mol⁻¹, and both this and ΔH may be assumed constant with temperature. Use R = 8.31 J K⁻¹ mol⁻¹.
Calculate ΔStotal at 700 K and hence the value of K at 700 K.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).