O-LevelChemistryRates of Reaction5070/22 · Oct/Nov 2014 · Q810 Marks

The ester, ethyl ethanoate, reacts with hydroxide ions to form ethanoate ions and ethanol. CH3COOC2H5 + OH¯ → CH3COO¯ + C2H5OH ethyl ethanoate ethanoate ions

(a)The graph shows how the concentration of ethanoate ions, CH3COO¯, changes as the reaction proceeds. [Figure showing a graph of concentration of CH3COO- ions in mol/dm-3 vs time/s]
(a)(i)Use the information in the graph to deduce the mass of ethanoate ions in 200 cm³ of solution when the reaction is complete. mass = ................................. g[2]
(a)(ii)Use the information in the graph to calculate the average rate of reaction, in mol/dm³/s, during the first 300 seconds. average rate of reaction ................................. mol/dm³/s[1]
(a)(iii)Describe and explain, using the kinetic particle theory, the change in the rate of reaction with time.[3]
(b)Aqueous sodium hydroxide reacts with aqueous iron(II) sulfate, FeSO4. Construct the ionic equation, with state symbols, for this reaction.[2]
(c)Iron(II) sulfate can be prepared by reacting excess iron powder with sulfuric acid. Describe the essential practical details to prepare pure dry crystals of iron(II) sulfate.[2]

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 2014). The mark or mark type is shown in brackets.

[1] B8 (a) (i) concentration of ethanoate = 0.45 mol/dm³ (1)
[1] B8 (a) (i) mass = 0.45 × 59 × 200/1000 = 5.31/5.3g (1)
[1] B8 (a) (ii) 0.17/300 = 5.67 × 10⁻⁴ / 5.7 × 10⁻⁴ (mol/dm³/s) (1)
[1] B8 (a) (iii) rate of reaction decreases with time/reaction slows down (1)
[1] B8 (a) (iii) concentration (of H⁺ ions) decreases/concentration (of reactants) decreases/concentration (of ethyl ethanoate) decreases (1)
[1] B8 (a) (iii) collision frequency reduced (1)
[1] B8 (b) Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)2(s) correct formulae (1)
[1] B8 (b) correct state symbols (dependent on correct formulae) (1)
[1] B8 (c) filter (off iron) (1)
[1] B8 (c) heat filtrate to crystallisation point then leave to crystallise/evaporate off some of the water from filtrate then leave to crystallise/partially evaporate filtrate and leave to crystallise AND dry crystals with filter paper (1)

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