The effect of concentration
4 marks · NormalThe equilibrium below is present in a sealed container: A student adds more oxygen, , to the vessel.
State the expected effect on the equilibrium position. (1)
Explain your answer to part (a), using ideas about concentration and Le Chatelier’s principle. (2)
What will happen to the concentration of SO3 at equilibrium? (1)
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(a) The equilibrium shifts to the right (towards SO3).
(b) Adding more O2 increases its concentration. The system responds by shifting the equilibrium to the right, using up the extra O2 to form more SO3, according to Le Chatelier’s principle.
(c) The concentration of SO3 increases.
Common mistakes: No shift described; Incorrect direction of shift
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Marking points
B1 (a) · 1 mark · States that the equilibrium shifts to the right/towards SO3.
M1 (b) · 1 mark · Refers to the increase in concentration of O2.
M2 (b) · 1 mark · States that the system responds by using up O2 to form more SO3 (Le Chatelier’s principle).
A1 (c) · 1 mark · States that the concentration of SO3 increases.
Worked solution
(a) The equilibrium shifts to the right, towards SO3.
(b) Adding more O2 increases its concentration. According to Le Chatelier's principle, the system opposes the increase by reacting O2 with SO2 to make more SO3, shifting the equilibrium to the right.
(c) The concentration of SO3 at equilibrium increases.