O-LevelChemistryRates of Reaction5070/21 · Oct/Nov 2016 · Q810 Marks

Ethanol can be manufactured by reacting ethene with steam in a closed reaction vessel. C2H4(g) + H2O(g) ⇌ C2H5OH(g) The reaction is exothermic.

(a)State two conditions for this reaction.[2]
(b)(i)The graph shows the percentage yield of ethanol at different temperatures. [Figure with graph showing percentage yield of ethanol vs temperature] Describe how, and explain why, the percentage yield changes with temperature.[3]
(b)(ii)Suggest why the reaction is carried out at 300°C and not at 200°C.[2]
(c)Describe how, and explain why, the position of equilibrium changes when the pressure is increased.[2]
(d)Ethanol reacts with methanoic acid, HCO2H, to form ethyl methanoate and water. Construct the equation for this reaction.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2016). The mark or mark type is shown in brackets.

[1] One mark each for any two of : • catalyst/phosphoric acid • high temperature/heat • high pressure
[1] % yield decreases as temperature increases (1)
[1] reaction is exothermic/reaction releases energy (1)
[1] (equilibrium) moves to the left (1)
[1] at 200 °C rate of reaction will be low/slow reaction at 200°C/300°C rate of reaction will be high/fast reaction at 300°C (1)
[1] idea of compromise temperature/idea of balance between lower yield and faster rate (1)
[1] position of equilibrium shifts to the right/equilibrium moves towards the product side (1)
[1] more molecules on left than right/more moles of on left than on right (1)
[1] HCO2H + C2H5OH → HCO2C2H5 + H2O (1)

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