Adding a catalyst
2 marks · HardA chemist adds a catalyst to a reversible reaction mixture at equilibrium. The student expects the concentration of products to increase once the catalyst is added.
Explain, with a reason, whether the student's expectation is correct.
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The student's expectation is incorrect. Adding a catalyst does not change the position of equilibrium, so the concentration of products at equilibrium remains the same. The catalyst only speeds up the rate at which equilibrium is reached by increasing both the forward and reverse reaction rates equally.
Also accept: No, adding a catalyst does not increase the concentration of products at equilibrium. It only helps the system reach equilibrium faster but does not alter the balance between reactants and products.
Common mistakes: Saying a catalyst increases yield at equilibrium.; Claiming position of equilibrium changes.
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Marking points
B1 · 1 mark · States that adding a catalyst does NOT change the position of equilibrium or does NOT increase the concentration of products at equilibrium.
B2 · 1 mark · Gives a correct reason: the catalyst speeds up both forward and reverse reactions equally OR only increases the rate at which equilibrium is reached.
Worked solution
The student's expectation is not correct because adding a catalyst to a reaction at equilibrium does not increase the concentration of products. This is because a catalyst increases the rates of both the forward and reverse reactions equally, so the position of equilibrium does not change. The system will reach equilibrium faster, but the final composition of reactants and products stays the same.