This question was attempted by many candidates and the performance was good.
In(a)(i), candidates correctly stated Faraday’s first law of electrolysis that the mass of an element discharged during electrolysis is directly proportional to the quantity of electricity passed.
In (a)(ii), candidates could mention two factors that influence the preferential discharge of ions during electrolysis from among the following:
- position of the ions in the electrochemical series
- concentration of the ions in the electrolyte
- nature of the electrodes.
In (a)(iii), candidates correctly calculated the time as follows:
Ag+(aq) + e- Ag (s)
1 mole of electrons Ξ IF = 96500C
96500C deposit 1 mole of Ag
Q = It
t = 96500C
In (a)(iv), candidates correctly gave two applications of electrolysis from the following applications:
- extraction of metals
- preparation of some named compounds/elements.
In (b)(i), candidates knew that X is more electropositive.
In (b)(ii), candidates correctly gave the change in oxidation of the less electropositive species as + 2 to O and in (b)(iii) they correctly wrote the half cell equation thus:
I. X(s) → + X2+((aq) + 2ҽ-
II. X2+ (aq) + 2ҽ- → Y(s).
In (c)(i), only very few candidates could explain why copper plated iron pillar would corrode when the surface is scratched. The expected answer was that iron is higher than copper in the electrochemical series hence, more reactive than copper. When the surface is scratched, an electrolytic cell develops and the iron is oxidized by forming a brown deposit of iron (III) compound. In (c)(ii), most candidates correctly gave one advantage of lead acid battery cell over Leclanche dry cell.
In (d)(i), most candidates correctly wrote an equation for the dissociation of each of the acids thus:
In (d)(ii), candidates correctly gave the basicity of H3PO4 and HCOOH as 3 and 1 respectively.
In (d)(iii), most candidates gave a correct arrangement of the solution in order of increasing pH values as NH4Cl, KNO3, Na2CO3.