Question

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 toluence we use alcoholic potassium permanganate as an oxidant. Why? Write a balanced redox equation for the reaction.
b. When concentrated sulphuric acid 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 og bromine. why?

Answer 1

When conc `H_(2)SO_(4)` is added to an inorgainc mixture containing chloride, a pungent smelling gas `HCl` is produced because a stronger acid displaces a weaker acid from its sail.
`2NaCl+2H_(2)SO_(4) rarr 2NaHSO_(4)+2HCl`

Since HCl is a very weak reducing agent, it can not reduce `H_(2)SO_(4)` to `SO_(2)`, and hence `HCl` is not oxidised to `Cl_(2)`.
However, when the mixture contains bromide ion, the inially produced `HBr` being a stronger reducing agent reduces `H_(2)SO_(4)` to `SO_(2)` and is itself oxidised to produce red vapour of `Br_(2)`.
`2NaBr+2H_(2)SO_(4) rarr 2NaHSO_(4)+2HBr`
`2HBr+H_(2)SO_(4)rarr Br_(2)+SO_(2)+2H_(2)O`