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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Explain why the mass of the container remains constant even though the solid changes colour.
[1 mark]
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Explain what happens to the gas produced when the sealed container is cooled, and why the grey solid turns white again.
[2 marks]
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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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Describe what is meant by a reversible reaction in the context of the Haber process.
[1 mark]
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Describe why increasing the pressure favours the forward reaction in the Haber process.
[1 mark]
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Describe how the position of equilibrium changes when temperature is increased, and explain why this creates a dilemma for industrial ammonia production.
[1 mark]
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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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(01.1) Calculate the total number of moles of gas in the container at equilibrium.
[1 mark]
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(01.2) Calculate the percentage of sulfur trioxide in the mixture at equilibrium. Give your answer to the nearest whole number.
[1 mark]
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- (01.1) 120 (moles)
- (01.2) 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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(01.1) Describe what is meant by a reversible reaction.
[1 mark]
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(01.2) Describe what happens when the reaction reaches equilibrium.
[1 mark]
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- (01.1) A reaction that can go forwards and backwards / reaction can go in both directions
- (01.2) The rate of the forward reaction equals the rate of the reverse reaction
- (01.2) 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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(01.1) 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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(01.2) 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]
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- (01.1) rate of forward reaction equals rate of reverse reaction
- (01.2) 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)
- (01.2) high pressure requires expensive equipment/thick-walled vessels OR creates safety risks OR is costly to maintain
GCSE Perfect