O-LevelChemistryStoichiometry5070/22 · Oct/Nov 2022 · Q910 Marks

Magnesium carbonate reacts with dilute hydrochloric acid. MgCO3 + 2HCl → MgCl2 + CO2 + H2O When 25.0cm³ of dilute hydrochloric acid is added to excess magnesium carbonate, the volume of carbon dioxide gas produced at room temperature and pressure is 120cm³.

(a)(i)Calculate the concentration, in mol/dm³, of the dilute hydrochloric acid. concentration[3]
(a)(ii)The reaction is repeated at a higher temperature. All other conditions stay the same. Describe how the rate of reaction changes. Explain your answer using ideas about collisions between particles.[2]
(a)(iii)The reaction is repeated using a higher concentration of hydrochloric acid. All other conditions stay the same. Describe how the rate of reaction changes. Explain your answer using ideas about collisions between particles.[2]
(b)Duralumin is an alloy of aluminium, magnesium, copper and manganese. State the meaning of the term alloy.[1]
(c)Copper is purified by electrolysis using an impure copper anode and a pure copper cathode. Construct the ionic equation for the reaction taking place at the cathode.[1]
(d)A metal object can be electroplated with another metal. State one use of electroplating.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[3] 9(a)(i) mol CO2 = 120 / 24000 OR 5 × 10-3 (1); mol HCl = M1 × 2 OR 1 × 10-2 (1); concentration of HCl = 0.4 (mol/dm³) (1)
[2] 9(a)(ii) rate increases (no mark alone); particles move faster / particles have more kinetic energy (1); more successful collisions / more collisions above that of activation energy / more particles with energy greater than activation energy(1)
[2] 9(a)(iii) rate increases (no mark alone); particles more crowded / more particles per unit volume / particles are closer together (1); greater collision frequency (1)
[1] 9(b) mixture of a metal with another element (1)
[1] 9(c) Cu2+ + 2e- → Cu
[1] 9(d) (makes object) more resistant to corrosion / harder (surface)

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