(a) A solution of CuSO\(_4\) was electrolyzed between pure copper electrodes and the following results were obtained:
Mass of copper anode before experiment = 7.20 g
Mass of copper anode after experiment = 4.00 g
Mass of copper cathode before experiment = 5.75 g
From the information provided,
(i) calculate the mass of the cathode, after the experiment.
(ii) write an equation for the reaction at the I. anode, II. cathode.
(iii) state whether the colour of the solution would change during the electrolysis. Give a reason for your answer.
(iv) if the electrolysis was carried out for 1 hour 20 minutes with a current of 2.0 amperes, determine the value of the Faraday.
(b) Consider the reaction represented by the following equation:
MnO\(^-_4\) + I\(^-\) + H\(^+\) \(\to\) I\(_2\) + H\(_2\)O + Mn\(^{2+}\)
Write balanced half equation for the (i) oxidation reaction, (ii) reduction reaction.
(c)(i) Describe briefly how tin can be extracted from its ore.
(ii) State one use of tin.
(iii) Mention one property that makes tin suitable for the use stated in (c)(ii)
(d)(i) What is meant by the term pollution?
(ii) Explain why it is dangerous to run a generator in a closed room.
(a) Electrolysis of CuSO4 between pure copper electrodes
(i) Mass of cathode after the experiment
With copper electrodes the copper dissolved at the anode is deposited on the cathode, so mass lost by the anode equals mass gained by the cathode.
Mass dissolved at anode \( = 7.20 - 4.00 = 3.20\text{ g} \)
Mass of cathode after \( = 5.75 + 3.20 = \mathbf{8.95\text{ g}} \)
(ii) Electrode equations
I. Anode (oxidation): \[ Cu_{(s)} \to Cu^{2+}_{(aq)} + 2e^- \]
II. Cathode (reduction): \[ Cu^{2+}_{(aq)} + 2e^- \to Cu_{(s)} \]
(iii) The colour of the solution does not change. The blue Cu2+ ions removed at the cathode are exactly replaced by Cu2+ ions formed as the copper anode dissolves, so the concentration of the blue Cu2+ ions stays constant.
(iv) Value of the Faraday
Time \( = 1\text{ h }20\text{ min} = 80 \times 60 = 4800\text{ s} \)
Quantity of electricity \( Q = It = 2.0 \times 4800 = 9600\text{ C} \)
Moles of Cu deposited \( = \dfrac{3.20}{63.5} = 0.0504\text{ mol} \)
Each Cu2+ needs 2 electrons, so moles of electrons \( = 2 \times 0.0504 = 0.1008\text{ mol} \)
Faraday \( = \dfrac{9600}{0.1008} = \mathbf{9.52 \times 10^{4}\ C\,mol^{-1}} \) (about 95 200 C, close to the accepted 96 500 C).
(b) MnO4- + I- + H+ reaction
(i) Oxidation: \[ 2I^- \to I_2 + 2e^- \]
(ii) Reduction: \[ MnO_4^- + 8H^+ + 5e^- \to Mn^{2+} + 4H_2O \]
(c) Extraction of tin
(i) Tin occurs chiefly as cassiterite, SnO2. The ore is concentrated by froth flotation or gravity separation, roasted in air to burn off sulphur and arsenic impurities, then the oxide is reduced with carbon (coke) in a furnace:
\[ SnO_2 + 2C \to Sn + 2CO \]
The molten tin is run off and refined.
(ii) Use of tin: coating steel to make tin-plate for food cans (or making solder and alloys such as bronze).
(iii) Property: tin is resistant to corrosion and non-toxic, so it protects the steel and does not contaminate food.
(d) Pollution
(i) Pollution is the introduction of harmful or unwanted substances (pollutants) into the environment, causing damage to living things or the surroundings.
(ii) A generator undergoes incomplete combustion in a closed room and produces the poisonous gas carbon monoxide, CO. Carbon monoxide combines irreversibly with the haemoglobin of the blood (forming carboxyhaemoglobin), preventing the blood from carrying oxygen. This leads to suffocation and death.
(a) Electrolysis of CuSO4 between pure copper electrodes
(i) Mass of cathode after the experiment
With copper electrodes the copper dissolved at the anode is deposited on the cathode, so mass lost by the anode equals mass gained by the cathode.
Mass dissolved at anode \( = 7.20 - 4.00 = 3.20\text{ g} \)
Mass of cathode after \( = 5.75 + 3.20 = \mathbf{8.95\text{ g}} \)
(ii) Electrode equations
I. Anode (oxidation): \[ Cu_{(s)} \to Cu^{2+}_{(aq)} + 2e^- \]
II. Cathode (reduction): \[ Cu^{2+}_{(aq)} + 2e^- \to Cu_{(s)} \]
(iii) The colour of the solution does not change. The blue Cu2+ ions removed at the cathode are exactly replaced by Cu2+ ions formed as the copper anode dissolves, so the concentration of the blue Cu2+ ions stays constant.
(iv) Value of the Faraday
Time \( = 1\text{ h }20\text{ min} = 80 \times 60 = 4800\text{ s} \)
Quantity of electricity \( Q = It = 2.0 \times 4800 = 9600\text{ C} \)
Moles of Cu deposited \( = \dfrac{3.20}{63.5} = 0.0504\text{ mol} \)
Each Cu2+ needs 2 electrons, so moles of electrons \( = 2 \times 0.0504 = 0.1008\text{ mol} \)
Faraday \( = \dfrac{9600}{0.1008} = \mathbf{9.52 \times 10^{4}\ C\,mol^{-1}} \) (about 95 200 C, close to the accepted 96 500 C).
(b) MnO4- + I- + H+ reaction
(i) Oxidation: \[ 2I^- \to I_2 + 2e^- \]
(ii) Reduction: \[ MnO_4^- + 8H^+ + 5e^- \to Mn^{2+} + 4H_2O \]
(c) Extraction of tin
(i) Tin occurs chiefly as cassiterite, SnO2. The ore is concentrated by froth flotation or gravity separation, roasted in air to burn off sulphur and arsenic impurities, then the oxide is reduced with carbon (coke) in a furnace:
\[ SnO_2 + 2C \to Sn + 2CO \]
The molten tin is run off and refined.
(ii) Use of tin: coating steel to make tin-plate for food cans (or making solder and alloys such as bronze).
(iii) Property: tin is resistant to corrosion and non-toxic, so it protects the steel and does not contaminate food.
(d) Pollution
(i) Pollution is the introduction of harmful or unwanted substances (pollutants) into the environment, causing damage to living things or the surroundings.
(ii) A generator undergoes incomplete combustion in a closed room and produces the poisonous gas carbon monoxide, CO. Carbon monoxide combines irreversibly with the haemoglobin of the blood (forming carboxyhaemoglobin), preventing the blood from carrying oxygen. This leads to suffocation and death.
