What is dynamic equilibrium?

The state in a closed system when the forward and reverse reactions occur at equal rates and the concentrations are constant.

Why is equilibrium described as dynamic?

Both the forward and reverse reactions continue to occur.

State Le Chatelier's principle.

If a system at equilibrium is subjected to a change, the position of equilibrium moves to oppose the change.

What is the effect of increasing the temperature on an exothermic reaction at equilibrium?

The equilibrium shifts to the left (the endothermic direction), so the yield of products decreases.

What is the effect of increasing the pressure on a gaseous equilibrium?

It shifts to the side with fewer moles of gas.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g): which way does increasing the pressure shift the equilibrium?

To the right, because there are 2 mol of gas on the right and 4 mol on the left.

What is the effect of increasing the concentration of a reactant?

The equilibrium shifts to the right to use up some of the added reactant.

What effect does a catalyst have on the position of equilibrium?

None. It speeds up the forward and reverse reactions equally.

Why does a catalyst still matter for an equilibrium reaction?

Equilibrium is reached more quickly.

Why is a compromise temperature used for an exothermic industrial reaction?

A low temperature gives a high yield but a slow rate, so a moderate temperature gives an acceptable yield in a reasonable time.

Why is a very high pressure not used even if it raises the yield?

The equipment is expensive, compression needs a lot of energy and there are safety risks.

Why are unreacted gases recycled?

It increases the overall yield and avoids wasting raw materials.

Dynamic equilibrium and Le Chatelier's principleAQA A-Level Chemistry: Flashcards

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What is a reversible reaction?: A reaction in which the products can react to re-form the reactants, shown by ⇌.
What is dynamic equilibrium?: The state in a closed system when the forward and reverse reactions occur at equal rates and the concentrations are constant.
Why is equilibrium described as dynamic?: Both the forward and reverse reactions continue to occur.
State Le Chatelier's principle.: If a system at equilibrium is subjected to a change, the position of equilibrium moves to oppose the change.
What is the effect of increasing the temperature on an exothermic reaction at equilibrium?: The equilibrium shifts to the left (the endothermic direction), so the yield of products decreases.
What is the effect of increasing the pressure on a gaseous equilibrium?: It shifts to the side with fewer moles of gas.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g): which way does increasing the pressure shift the equilibrium?: To the right, because there are 2 mol of gas on the right and 4 mol on the left.
What is the effect of increasing the concentration of a reactant?: The equilibrium shifts to the right to use up some of the added reactant.
What effect does a catalyst have on the position of equilibrium?: None. It speeds up the forward and reverse reactions equally.
Why does a catalyst still matter for an equilibrium reaction?: Equilibrium is reached more quickly.
Why is a compromise temperature used for an exothermic industrial reaction?: A low temperature gives a high yield but a slow rate, so a moderate temperature gives an acceptable yield in a reasonable time.
Why is a very high pressure not used even if it raises the yield?: The equipment is expensive, compression needs a lot of energy and there are safety risks.
Why are unreacted gases recycled?: It increases the overall yield and avoids wasting raw materials.

Exam questions on Dynamic equilibrium and Le Chatelier's principle

Cobalt(II) ions in aqueous solution take part in the equilibrium [Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) + 6H₂O(l). The [Co(H₂O)₆]²⁺ ion is pink and the [CoCl₄]²⁻ ion is blue, and the forward reaction is endothermic. A student prepares a purple mixture of the two ions in a sealed tube and leaves it at constant temperature.
The purple mixture turns blue when it is heated. Explain this observation.2 marks
Ammonia is manufactured by the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. The process uses an iron catalyst at a temperature of about 450 °C and a pressure of about 20 MPa.
Explain why a compromise temperature of about 450 °C is used rather than a much lower temperature.2 marks
In the Contact process, sulfur dioxide is oxidised to sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹. A vanadium(V) oxide catalyst is used at about 450 °C and a pressure of about 200 kPa. Under these conditions about 99% of the sulfur dioxide is converted into sulfur trioxide.
Predict and explain the effect of increasing the temperature on the equilibrium yield of sulfur trioxide.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).