The effect of concentration
3 marks · EasyThe equilibrium below is set up in a sealed vessel:
The graph shows how the concentration of AB(g) changes after some B2(g) is added:
Describe what happens to [AB] after B2(g) is added.
Predict and explain the direction in which the position of equilibrium shifts after B2(g) is added.
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) The concentration of AB increases until it levels off.
(b) The equilibrium shifts to the left (towards the formation of AB), because adding B2 increases a product's concentration, so the system makes more AB to use up the added B2.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that [AB] increases after B2 is added (from graph).
B2 (b) · 1 mark · States that the equilibrium shifts left/towards AB formation.
B3 (b) · 1 mark · Explanation: The system responds to increased B2 by using up the excess to produce more AB (Le Chatelier's principle).
Worked solution
a) [AB] increases steadily after B2 is added and then levels off (graph shows rise from 6 to 8 mol/dm³).
b) The equilibrium shifts to the left (towards formation of AB) because adding B2 (a product) causes the system to use up B2 by reacting it with A2 to make AB, according to Le Chatelier's principle.