Adding a catalyst
4 marks · NormalThe graph below shows the concentration of reactant A and product B in a reversible reaction over time.
What evidence from the graph shows that the reaction has reached equilibrium?
Describe what happens to the concentrations of A and B immediately after the catalyst is added at t = 5 min.
Explain why the equilibrium concentrations do not change after the catalyst is added.
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(a) The concentrations of both A and B become constant / do not change with time after a certain point.
(b) After the catalyst is added, the rates of both the forward and reverse reactions increase, so equilibrium is reached faster. The concentrations approach their final values more quickly but do not change further once equilibrium is reached.
(c) Because a catalyst affects the rate, not the position, so equilibrium concentrations remain the same.
Common mistakes: Saying concentrations change at equilibrium after catalyst; Confusing rate and position of equilibrium
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies that equilibrium is when concentrations remain constant / stop changing.
M1 (b) · 1 mark · Describes both rates increase or reaction proceeds faster after catalyst is added.
A1 (b) · 1 mark · States equilibrium is reached more quickly and/or final concentrations do not change.
B2 (c) · 1 mark · Explains that a catalyst only affects rate, not position of equilibrium.
Worked solution
a) The graph shows both A and B concentrations become constant after t = 8 min; this indicates equilibrium.
b) Immediately after the catalyst is added at t = 5 min, both the forward and reverse reaction rates increase, so the concentrations move more rapidly towards their equilibrium values. The final concentrations remain unchanged but are reached sooner.
c) The equilibrium concentrations stay the same because a catalyst only speeds up the rates equally; it does not affect the position of equilibrium.