Explain — 3 marks
A student heats calcium carbonate in a sealed container. The white solid gradually turns grey, and a gas is produced. After a while, if the container is cooled, the grey solid slowly turns white again. The student notices that the mass of the container stays constant throughout.
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(a) Explain why the mass of the container remains constant even though the solid changes colour.
[1 mark]
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(b) Explain what happens to the gas produced when the sealed container is cooled, and why the grey solid turns white again.
[2 marks]
Show mark scheme
- (a) The reaction is reversible/the gas produced remains in the sealed container and does not escape
- (b) When cooled, the gas molecules have less energy/move more slowly and combine/react with the grey solid
- (b) The reverse reaction occurs, converting the grey solid (calcium oxide) back to white solid (calcium carbonate)
Describe — 3 marks
A chemical plant produces ammonia using the Haber process. The reaction is: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The plant operates at high temperature and high pressure. Workers notice that increasing the pressure increases ammonia yield, but increasing the temperature decreases it, even though the reaction rate increases.
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(a) Describe what is meant by a reversible reaction in the context of the Haber process.
[1 mark]
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(b) Describe why increasing the pressure favours the forward reaction in the Haber process.
[1 mark]
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(c) Describe how the position of equilibrium changes when temperature is increased, and explain why this creates a dilemma for industrial ammonia production.
[1 mark]
Show mark scheme
- (a) A reaction where reactants can form products AND products can react to form reactants simultaneously
- (b) Increasing pressure shifts equilibrium to the side with fewer moles of gas / shifts equilibrium to the right / favours the forward reaction because there are 4 moles of gas on the left and 2 moles on the right
- (c) Increasing temperature shifts equilibrium to the left / favours the reverse reaction / decreases ammonia yield because the forward reaction is exothermic. The dilemma is that higher temperature increases reaction rate (producing ammonia faster) but shifts equilibrium left (producing less ammonia at equilibrium)
Calculate — 2 marks
The Contact process is used in industry to manufacture sulfur trioxide for making sulfuric acid. Sulfur dioxide and oxygen react reversibly in a sealed container. At equilibrium, the mixture contains 40 moles of sulfur dioxide, 20 moles of oxygen, and 60 moles of sulfur trioxide.
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(a) Calculate the total number of moles of gas in the container at equilibrium.
[1 mark]
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(b) Calculate the percentage of sulfur trioxide in the mixture at equilibrium. Give your answer to the nearest whole number.
[1 mark]
Show mark scheme
- (a) 120 (moles)
- (b) 50% (accept 50)
Describe — 2 marks
Ammonia is produced industrially by the Haber process. In this process, nitrogen gas from the air reacts with hydrogen gas to form ammonia. The reaction is reversible and reaches a state of equilibrium. The equation for the reaction is: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
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(a) Describe what is meant by a reversible reaction.
[1 mark]
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(b) Describe what happens when the reaction reaches equilibrium.
[1 mark]
Show mark scheme
- (a) A reaction that can go forwards and backwards / reaction can go in both directions
- (b) The rate of the forward reaction equals the rate of the reverse reaction
- (b) Concentrations of reactants and products remain constant
Evaluate — 3 marks
Ethanol can be produced industrially by the hydration of ethene. Ethene gas reacts with steam in a reversible reaction according to the equation: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g). The forward reaction is exothermic. When carried out in a closed system, the reaction reaches equilibrium.
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(a) State what happens to the rate of the forward reaction and the rate of the reverse reaction when this reversible reaction reaches equilibrium.
[1 mark]
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(b) In the industrial process, a high pressure of 60-70 atmospheres is maintained. Evaluate the use of high pressure in this process to maximise the yield of ethanol.
[2 marks]
Show mark scheme
- (a) rate of forward reaction equals rate of reverse reaction
- (b) high pressure shifts the position of equilibrium to the right/towards ethanol because there are fewer moles of gas on the product side (2 moles of gas → 1 mole of gas)
- (b) high pressure requires expensive equipment/thick-walled vessels OR creates safety risks OR is costly to maintain
GCSE Perfect