How do you count for the following observations ?
(a) Though alkaline potassium permanganate and acidic potassium permanganate both are used as oxidants, yet in the manufacture of benzoic acid from toluene we use alcoholic potassium permanganate as an oxidant. Why ? Write a balanced redox equation for the reaction.
(b) When concentrated sulphuric acid is added to an inorganic mixture containing chloride, we get colourless pungent smelling gas HCl, but if the mixture contains bromide then we get red vapour of bromine. Why ?

Explanation

When conc. `H_2SO_4` is added to an inorganic mixture containing chloride , a pungent smelling gas HCl is produced because a stronger acid displaces a weaker acid from its salt.
`2NaCl +2H_2SO_4 rarr 2NaHSO_4 +2HCl`

Since HCl is weak reducing agent , it cannot reduce `H_2SO_4` to `SO_2` and therefore , HCl is not oxidised to chlorine.
However , if the mixture contains bromide ion, the initially produced HBr is a stronger reducing agent . Therefore , it reduces `H_2SO_4 " to "SO_2` and itself gets oxidised to produce red vapour of `Br_2`.
`2NaBr + 2H_2SO_4 rarr 2NaHSO_4 rarr 2NaHSO_4 + 2HBr`
`2 HBr+H_2SO_4 rarr Br_2+SO_2+2H_2O`