WAEC Chemistry 2013 Theory — Question 3

Model answer

(a)(i) The standard electrode potential of an electrode is the potential difference between the electrode and the hydrogen electrode under standard conditions. The standard conditions are: A temperature of 25°C (298K); A concentration of 1 moldm⁻³; A pressure of 1 atm. (ii) Factors affecting the discharge of ions during electrolysis: Position of ions in the electrochemical series; Concentration of ions; Nature of electrode. (iii) Functions of a salt bridge: It enhances ionic contact between the two half-cell compartments; It maintains electrical neutrality in the compartments. (b) When an aqueous solution of CuSO4 is electrolyzed using copper electrodes, the colour of the CuSO4 (blue) is not discharged. This is because as copper ions leave the electrolyte for the cathode, more copper anode dissolves to replace the one just removed from the electrolyte. The whole electrolysis can therefore be interpreted as dissolution of copper anode and subsequent deposition on the copper cathode: Cu²⁺ + 2e⁻ → Cu(s). (c) Q = It. I=0.2A, t=35mins=(35×60)s=2100s. Q=(0.2×2100)C=420C. 1 mole of Cu (64g) requires 2F (2×96,500C) of electricity. Thus, 420C will only produce (64/(2×96500))×420g = 0.139g.