2007 PJC Prelims Paper 3 FAQs

A Note for Q3c
"Do I have to memorise the colours of all transition element complexes?"
The answer is NO!

Only for Cr, Mn, Fe and Cu. Examples: Cr3+, CrO42-, Cr2O72-, Mn2+, MnO2, MnO4-, Fe2+, Fe3+, Cu2+, [Cu(NH3)4]2+.

Explanation for Q5b
NaX + H2SO4 --> NaHSO4 + HX, X = Cl, Br or I
This is NOT a redox reaction!!!

SO42- does not oxidise Cl- to Cl2.
SO42- + 4H+ + 2e- <--> SO2 + 2H2O, E = +0.17V
Cl2 + 2e- <--> 2Cl-, E = +1.36V
Ecell = +0.17 - (+1.36) = -1.19V (< 0)

SO42- oxidises HBr to Br2,
eqn: 2HBr + H2SO4 --> Br2 + SO2 + 2H2O

SO42- oxidises HI to I2,
eqn: 2HI + H2SO4 --> I2 + SO2 + 2H2O or 8HI + H2SO4 --> 4I2 + H2S +4H2O
SO2 has an irritating (or pungent) smell while H2S smells like rotten eggs.