O-LevelChemistryRates of Reaction5070/21 · May/June 2018 · Q710 Marks

A scientist investigates the reaction of iron with steam in a closed system. A dynamic equilibrium mixture is established. 3Fe(s) + 4H2O(g) ⇌ Fe3O4(s) + 4H2(g)

(a)Explain why the concentrations of steam and of hydrogen do not change once the dynamic equilibrium mixture has been established.[1]
(b)The pressure of the equilibrium mixture is increased. The temperature of the closed system is kept constant. Predict and explain what will happen, if anything, to the composition of the equilibrium mixture.[2]
(c)(i)The temperature of the equilibrium mixture is increased. The pressure within the closed system is kept constant. The position of equilibrium shifts to the left hand side. What conclusion can be made about the enthalpy change of the reaction?[1]
(c)(ii)Describe and explain what happens to the rate of reaction.[1]
(d)(i)Dilute sulfuric acid reacts with Fe3O4 to form three compounds, A, B and C. • A is iron(II) sulfate. • B is iron(III) sulfate. • C is a colourless liquid. Name compound C.[1]
(d)(ii)Construct the equation for this reaction.[2]
(d)(iii)Describe a chemical test for iron(III) ions. test observation[2]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[1] 7(a) rate of forward reaction = rate of backward reaction
[2] 7(b) (composition) does not change (1) same number of moles of gas on left hand side and right hand side (1)
[1] 7(c)(i) exothermic / enthalpy change is negative
[1] 7(c)(ii) rate increases because particles have more energy / particles moving faster / more successful collisions / more energetic collisions (1)
[1] 7(d)(i) water
[2] 7(d)(ii) Fe3O4 + 4H2SO4 → Fe2(SO4)3 + FeSO4 + 4H2O correct formulae (1) balanced equation – dependent on correct formulae (1)
[2] 7(d)(iii) test - add (aqueous) sodium hydroxide / (aqueous) ammonia (1) observation - brown ppt / brown solid (1)

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