The position of equilibrium

5 marks · Hard

In a sealed container at equilibrium, the following reaction occurs:

The forward reaction is exothermic.

(a)

State how an increase in pressure will affect the position of equilibrium. Explain your answer.

[2 marks]

(b)

Describe what happens to the position of equilibrium if the temperature is decreased. Explain your answer.

[2 marks]

(c)

If some AB(g) is removed from the container, what will happen to the amounts of A2(g), B2(g), and AB(g) as the system re-establishes equilibrium? Explain.

[1 mark]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

(a) Equilibrium shifts to the right/towards products/more AB(g) is formed, because there are fewer moles of gas on the right. · must show States shift to right/products, Correct reference to fewer moles of gas

(b) Equilibrium shifts to the right/towards products because the forward reaction is exothermic and is favoured by lower temperature. · must show Shift to products/right, Exothermic favoured at low temp

(c) Amounts of A2(g) and B2(g) decrease, AB(g) increases, as system shifts to the right to replace AB(g) that was removed. · must show A2 and B2 decrease, AB increases, Explanation that system shifts to replace removed AB

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · States equilibrium shifts to right/towards products (or similar).

B2 (a) · 1 mark · Explains this is because there are fewer moles of gas on right, so the system reduces pressure.

B3 (b) · 1 mark · States equilibrium shifts to right/towards products.

B4 (b) · 1 mark · Explains this is because exothermic forward reaction is favoured when temperature decreases.

B5 (c) · 1 mark · States more AB(g) is made, A2(g) and B2(g) decrease, as the equilibrium shifts right to replace AB removed.

Worked solution

(a) There are 2 moles of gas on the left and 2 moles of gas on the right (left: 1 A2 + 1 B2 = 2; right: 2 AB = 2), so an increase in pressure will not affect the position of equilibrium.

However, if the equation had more moles of gas on one side, increasing pressure would favour the side with fewer moles.

(b) Lowering temperature favours the exothermic forward reaction, so the system shifts towards the products (to the right), making more AB(g).

(c) Removing AB(g) means the system is no longer at equilibrium. The reaction shifts right to make more AB(g), so amounts of A2(g) and B2(g) decrease and AB(g) increases until equilibrium is restored.