O-LevelChemistryRates of Reaction5070/22 · Oct/Nov 2018 · Q910 Marks

Sulfuric acid reacts with zinc to form zinc sulfate and hydrogen. Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

(a)Describe and explain, using ideas about collisions between particles, how the rate of this reaction changes when the concentration of sulfuric acid is increased. All other conditions stay the same.[2]
(b)Describe and explain, using ideas about collisions between particles, how the rate of this reaction changes when the temperature is decreased. All other conditions stay the same.[2]
(c)The reaction of zinc with sulfuric acid is exothermic. Complete the energy profile diagram for this reaction to show the enthalpy change. [Figure 9.1: An empty energy profile diagram with the y-axis labelled 'energy' and the x-axis labelled 'progress of reaction'.][2]
(d)Calculate the maximum volume of hydrogen, in dm³, formed when 4.55 g of zinc reacts with excess sulfuric acid at room temperature and pressure. volume .......... dm³[2]
(e)The formula of zinc phosphate is Zn₃(PO₄)₂. Calculate the percentage by mass of zinc in zinc phosphate. .......... %[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 2018). The mark or mark type is shown in brackets.

[1] (a) rate of reaction increases AND one or both of: more particles per unit volume / more particles per cm³ / particles closer together/more concentrated particles (1)
[1] (a) collision frequency increases / more particles collide per second (1)
[1] (b) rate of reaction decreases AND one or both of: particles move slower / particles have less kinetic energy (1)
[1] (b) fewer particles have activation energy (or above) / collisions are less successful (1)
[1] (c) reactants on the left and products on the right and reactant level above product level and labels on or just above the energy level lines (1)
[1] (c) vertical arrow downwards between reactants and products (1)
[1] (d) mol Zn = 0.07(0) (1)
[1] (d) (= mol H₂)
[1] (d) volume of H₂ = 1.68 (dm³) (1)
[2] (e) 50.6%/51% (2)
[1] (e) If two marks not obtained, award one mark for: (relative formula mass of zinc phosphate) = 385

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