O-LevelChemistryRates of Reaction5070/21 · Oct/Nov 2015 · Q810 Marks

A sample of 0.030g of small pieces of magnesium is added to 20cm³ of 0.10mol/dm³ hydrochloric acid. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

(a)(i)Show by calculation which reactant is in excess.[3]
(a)(ii)What would you observe in this reaction?[1]
(b)The graph shows how the volume of hydrogen gas produced, at room temperature and pressure, changes with time. [Figure B8.1: A graph showing Volume of hydrogen /cm³ on the y-axis (0-30) against Time /s on the x-axis (0-100). The curve starts at (0,0), increases, and then plateaus at 24 cm³ around 60s.] Calculate the total mass of hydrogen formed. mass of hydrogen =[2]
(c)The experiment is repeated at the same temperature and pressure. The same mass of magnesium is added but magnesium powder is used instead of small pieces. The rate of reaction is faster. Explain why.[2]
(d)(i)Magnesium reacts with nitrogen when heated. The ionic compound magnesium nitride, Mg3N2, is formed. Construct the equation, including state symbols, for this reaction.[1]
(d)(ii)Deduce the charge on the nitride ion.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2015). The mark or mark type is shown in brackets.

[3] B8 (a) (i) mol Mg (= 0.030/24) = 1.25 × 10⁻³ (1) mol HCl (= 0.10 × 20/1000) = 2 × 10⁻³ (1) mol HCl required to react with 1.25 × 10⁻³ mol Mg is 2.5 x 10⁻³ so Mg in excess (1)
[1] (ii) bubbles/effervescence/fizzing/tube gets hot/magnesium reduces on size (1)
[2] (b) mol of gas(= 24/24000) = 1.0 × 10⁻³ (1) mass of hydrogen (= 2 × 1.0 × 10⁻³) = 2.0 × 10⁻³ (g)
[2] (c) greater surface area (1) more frequent collisions (of H⁺ ions with Mg) (1)
[1] (d) (i) 3Mg(s) + N₂(g) → Mg₃N₂(s) (1)
[1] (ii) 3⁻ / –3 (1)

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