2007 ACJC Prelims Paper 3 FAQs

A Note for Q1d(ii)
Hot alkaline aqueous iodine (i.e. I2(aq), Na(OH)(aq), heat) will not distinguish the compounds because there is neither methyl ketone nor RCH(OH)CH3 after alkaline hydrolysis.

Explanation for Q2a(i)
M3+ + 3e- <--> M, E= x V
A more positive x means that the reduction of M3+ to M is more feasible. It also implies that M3+ oxidises other substances more readily (while M3+ itself is reduced).

Explanation for Q3b(ii)
Relative oxidising power of halogens can be compared using:
(1) E values of X2/X-, e.g. E of F2/F- is more +ve than Cl2/Cl-, so F2 is more oxidising than Cl2
(2) delta H of "X2 + 2e- --> 2X-", e.g. delta H is more negative for X = Cl than X = Br, so the reduction of X2 to X- is more energetically feasible for X = Cl, i.e. Cl2 oxidises other substances more readily because Cl2 is reduced more readily
(3) displacement, e.g. Cl2 displaces Br-(aq) from solution
(4) oxidation state of product formed, e.g. Cl2 oxidises S2O32- to SO42- but I2 oxidises S2O32- to S4O62-