Physical Chemistry
7 marks · EasyA student investigates the effect of a possible catalyst on the reversible reaction shown below, carried out in a sealed container:
She wishes to know whether the substance speeds up the reaction, and how changes in pressure affect the amount of SO3 at equilibrium.
Describe a method you would use to show that a substance is a catalyst in this reaction.
Explain what is meant by 'the position of equilibrium' for this reaction.
Predict and explain the effect of increasing pressure on the position of equilibrium of this reaction.
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) To show a substance is a catalyst, carry out the reaction twice: once with and once without the substance under identical conditions. Measure the rate of formation of SO3 in both cases. If the reaction is faster with the substance and the mass of the catalyst remains unchanged after collection and weighing, the substance is a catalyst. Alternatively, filtering, drying and weighing the substance afterwards should show no mass change. · must show compare rates with/without substance, check substance not used up after reaction
(b) The position of equilibrium describes the relative amounts of reactants and products present in the sealed container once the rates of forward and reverse reactions are equal and their concentrations remain constant. It tells us how much SO3 (and SO2 and O2) is present at equilibrium. · must show relative amounts of substances, rates equal, concentrations constant
(c) Increasing pressure shifts the equilibrium towards the side with fewer moles of gas. Here, there are 3 moles of gas on the left (2SO2 + O2) and 2 on the right (2SO3), so increasing pressure will shift the equilibrium to the right and produce more SO3. · must show fewer moles of gas, right-hand side favoured
Common mistakes: Saying catalyst changes equilibrium position; Not comparing moles of gas when discussing pressure effect
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Suggests comparing the rate with/without added substance
M2 (a) · 1 mark · Describes checking mass or properties of substance after reaction to show it is unchanged
M3 (a) · 1 mark · Mentions repeating experiment or measuring rate quantitatively
E1 (b) · 1 mark · Defines position of equilibrium as relative amounts of substances at equilibrium
E2 (b) · 1 mark · Mentions rates of forward and reverse reactions being equal/concentrations constant
R1 (c) · 1 mark · States effect of pressure favours fewer moles of gas
R2 (c) · 1 mark · Correctly predicts that more SO3 forms (position shifts right)
Worked solution
a. To show a substance is a catalyst: Set up the reaction with and without the substance. Measure the time taken for equal amounts of SO3 to form in each case. If the reaction is faster with the substance, test if the mass of the substance is unchanged after collecting it at the end (by filtration and drying). If so, it is acting as a catalyst.
b. The position of equilibrium is the balance point in the reversible reaction where the concentrations of reactants and products remain constant because the forward and reverse reactions happen at the same rate. For this reaction, it tells the relative amounts of SO2, O2 and SO3 at equilibrium.
c. Increasing the pressure increases the yield of SO3 because the forward reaction produces fewer moles of gas (3 on the left, 2 on the right). So, raising the pressure shifts equilibrium to the right and favours SO3 formation.