QUESTION 23 (6 marks)
a) Identify the products of the electrolysis of
i) molten sodium chloride:
[1 mark]
ii) dilute aqueous sodium chloride solution:
[1 mark]
b) Explain how the nature of the electrolyte affects the products generated when a dilute aqueous solution of sodium chloride undergoes electrolysis.
[4 marks]
QCAA guide · typeset solution
QCAA sample response and mark allocation
23 | a) | i) Na(l) and Cl2(g) | • provides Na(l) and Cl2(g) [1 mark] | |
ii) H2(g) and O2(g) | • provides H2(g) and O2(g) [1 mark] | |||
b) | In a dilute solution of aqueous sodium chloride, sodium ions, chloride ions, hydrogen ions, hydroxide ions and water molecules are present. The concentration and the Eo value of the species create competition at the electrodes and affect the products formed. Na+ and H+ compete to be reduced at the cathode. The E0 value for reducing H+ is more positive; therefore, H+ is preferentially reduced and H2 gas is formed rather than Na metal. Cl− and OH− compete to be oxidised at the anode. As the concentration of Cl− is low in a dilute NaCl solution, OH− is preferentially oxidised and O2 gas is produced rather than Cl2 gas. | • identifes that Na+, Cl−, OH−, H+, and H2O are present [1 mark] • identifies that concentration and Eo values of the species affects products [1 mark] • identifies that H+ is preferentially reduced, producing H2 gas due to a more positive Eo value [1 mark] • identifies that OH− is preferentially oxidised, producing O2 gas due to a higher concentration of ions [1 mark] | For H2O, accept OH− and H+. |
QCAA sample response and marking criteria reproduced from the official guide.
Compare your working with the guide above.