(a)(i) List three observable changes that take place when a dilute solution of copper (II) chloride is electrolysed using platinum electrodes. Stage III (ii) Calculate the quantity of electricity used during electrolysis when a current of 0.21 ampere flows for 2 hours. (iii) State what is meant by the term preferential discharge of ions in electrolysis (iv) Give one factor which influences the preferentia! discharge of ions during electrolysis. (v) State one Q difference between-a conductor and an electrolyte.
(b) Consider the reaction represented by the equation below: Na\(_2\)S\(_2\)O\(_{3(aq)}\) + 2HCI\(_{(aq)}\) \(\to\) 2NaCI\(_{(aq)}\) + H\(_2\)O\(_{(l)}\) + SO\(_{2(g)}\) + S\(_{(s)}\) (i) List two factors that can affect the rate of this reaction. (i) Which of the products can be readily used to measure the rate of the reaction. Give a reason for your answer. (iii) Name two instruments that can be used to measure factors in (b)(i) above.
(c)(i) State the reasons for regarding rusting and burning as oxidation processes. (ii) I. Write the balanced half equations for the following redox reaction: Mg\(_{(s)}\) + Fe\(^{2+}_{(aq)}\) ----> Mg\(^{2+}_{(aq)}\) + Fe\(_{(s)}\) II. Which of the reactants is the oxidizing agent? Ill. State the change in the oxidation number of the oxidizing agent.
(d)(i) State one ore from which each of the following metals can be extracted. I. Tin II. Iron (ii) List two uses of copper (iii) Name one alloy of tin.
Explanation
(a)(i) The green colour of copper (II) chloride is gradually discharged, the solution gradually turns from green colourless. A brown or reddish brown is deposited on the cathode while oxygen gas is form on the lode. (ii) Q = It = 0.21 x 60 x 60 x 2 = 1512 coulombs (iii) Preferential discharge of ions in ectrolysis is the selective discharge of ions in preference to others at the electrodes due to certain factors. iv) —Nature of electrodes —Concentration of ions in the electrolyte —Position of the ions in the ectrochemical series. (v) A conductor conducts through electrons while an electrolyte conducts through ions. OR A conductor does not decompose when current passes through it while an electrolyte decomposes.
(b) Na\(_2\)S\(_2\)SO\(_{3(aq)}\) + 2HCI\(_{(aq)}\) —> 2NaCI\(_{(aq)}\) + H\(_2\)O\(_{(l)}\) + SO\(_{2(aq)}\) + S\(_{(s)}\) (i) The two factors are temperature and pncentration. (ii) SO\(_2\), because the volume can be measured OR Sulphur because it is a coloured solid. iii) Stop watch, thermometer, burette and graduated cylinder.
(c)(i) Both rusting and burning require xygen. (ii) (i) Mg\(_{(s)}\) —› Mg\(^{2+}\) + 2e\(^-\) ; Fe\(^{2+}\) + 2e\(^-\) —> Fe\(_{(S)}\) (ii) Fe\(^{2+}\) iii) from +2 to zero
(d)(i) I. Cassiterite II. Iron pyrite, haematite or magnetite. (ii) Copper is used in making the following: —Electrical conducting rires and cables — Pipes and radiators in central heating systems —Alloys, Coinage metals, jewellery, household utensils. (iii) Bronze, soft solder, type metal.
