(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.
(a)(i) Observable changes (electrolysis of dilute CuCl2 with platinum electrodes)
- The blue colour of the solution gradually fades as copper (II) ions are discharged.
- A reddish-brown deposit of copper forms on the cathode.
- A gas (chlorine, with a choking smell) is evolved at the anode as bubbles.
(a)(ii) Quantity of electricity
Q = I × t = 0.21 × (2 × 3600) = 0.21 × 7200 = 1512 C (coulombs).
(a)(iii) Preferential discharge: when more than one type of ion is attracted to an electrode, only one type is discharged in preference to the others (based on its position in the electrochemical series, its concentration and the nature of the electrode).
(a)(iv) The position of the ion in the electrochemical series (concentration of the ion or nature of the electrode are also acceptable).
(a)(v) A (metallic) conductor conducts electricity by the movement of electrons and is not chemically decomposed, whereas an electrolyte conducts electricity by the movement of ions (in molten or aqueous state) and is chemically decomposed.
(b)(i) Concentration of the reactants and temperature.
(b)(ii) The sulphur (S). As it is formed the mixture becomes cloudy (turbid); the time taken for a mark placed under the flask to disappear can be measured, giving the rate.
(b)(iii) A thermometer (to measure temperature) and a stop watch/clock (to measure time).
(c)(i) In both rusting and burning a substance combines with oxygen (iron combines with oxygen to form rust; a fuel combines with oxygen when it burns). Both therefore involve a gain of oxygen (loss of electrons), which is oxidation; rusting is slow oxidation while burning is rapid oxidation.
(c)(ii)
- I. Oxidation: Mg(s) → Mg2+(aq) + 2e-. Reduction: Fe2+(aq) + 2e- → Fe(s).
- II. The oxidizing agent is Fe2+ (the iron (II) ion).
- III. The oxidation number of iron changes from +2 to 0, a decrease of 2.
(d)(i) Tin: cassiterite (tin (IV) oxide, SnO2). Iron: haematite (Fe2O3).
(d)(ii) Copper is used for making electrical wires and cables, and for making coins (or alloys such as brass and bronze).
(d)(iii) Solder (an alloy of tin and lead); bronze is also acceptable.
(a)(i) Observable changes (electrolysis of dilute CuCl2 with platinum electrodes)
- The blue colour of the solution gradually fades as copper (II) ions are discharged.
- A reddish-brown deposit of copper forms on the cathode.
- A gas (chlorine, with a choking smell) is evolved at the anode as bubbles.
(a)(ii) Quantity of electricity
Q = I × t = 0.21 × (2 × 3600) = 0.21 × 7200 = 1512 C (coulombs).
(a)(iii) Preferential discharge: when more than one type of ion is attracted to an electrode, only one type is discharged in preference to the others (based on its position in the electrochemical series, its concentration and the nature of the electrode).
(a)(iv) The position of the ion in the electrochemical series (concentration of the ion or nature of the electrode are also acceptable).
(a)(v) A (metallic) conductor conducts electricity by the movement of electrons and is not chemically decomposed, whereas an electrolyte conducts electricity by the movement of ions (in molten or aqueous state) and is chemically decomposed.
(b)(i) Concentration of the reactants and temperature.
(b)(ii) The sulphur (S). As it is formed the mixture becomes cloudy (turbid); the time taken for a mark placed under the flask to disappear can be measured, giving the rate.
(b)(iii) A thermometer (to measure temperature) and a stop watch/clock (to measure time).
(c)(i) In both rusting and burning a substance combines with oxygen (iron combines with oxygen to form rust; a fuel combines with oxygen when it burns). Both therefore involve a gain of oxygen (loss of electrons), which is oxidation; rusting is slow oxidation while burning is rapid oxidation.
(c)(ii)
- I. Oxidation: Mg(s) → Mg2+(aq) + 2e-. Reduction: Fe2+(aq) + 2e- → Fe(s).
- II. The oxidizing agent is Fe2+ (the iron (II) ion).
- III. The oxidation number of iron changes from +2 to 0, a decrease of 2.
(d)(i) Tin: cassiterite (tin (IV) oxide, SnO2). Iron: haematite (Fe2O3).
(d)(ii) Copper is used for making electrical wires and cables, and for making coins (or alloys such as brass and bronze).
(d)(iii) Solder (an alloy of tin and lead); bronze is also acceptable.
