How do you count for the following observations?
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?
Solution 1
When conc. H2SO4 is added to an inorganic mixture containing bromide, initially HBr is produced. HBr, being a strong reducing agent reduces H2SO4 to SO2 with the evolution of red vapour of bromine.
`2NaBr + 2H_2SO_4 -> 2NaHSO_4 + 2HBr`
`2HBr + H_2SO_4 -> Br_2 + SO_2 + 2H_2O`
(red vapour)
But, when conc. H2SO4 is added to an inorganic mixture containing chloride, a pungent smelling gas (HCl) is evolved. HCl, being a weak reducing agent, cannot reduce H2SO4 to SO2.
`2NaCl + 2H_2SO_4 -> 2NaHSO_4 + 2HCl`
Solution 2
When cone. H2S04 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 -> 2NaHSO_4 + 2HCl`
Stronger acid Weaker acid
`HCl + H_2SO_4 -> Cl_2 + SO_2 + 2H_2O`
Since HCl is a very weak reducing agent, it can not reduce H2S04 to S02and hence HCl is not oxidised to Cl2.
However, when the mixture contains bromide ion, the initially produced HBr being a strong reducing agent than HCl reduces H2S04to S02 and is itself oxidised to produce red vapour of Br2.
`2NaBr + 2H_2SO_4 -> 2NaHSO_4 + 2Hbr`
`2HBr + H_2SO_4 -> Br_2 + SO_2 + 2H_2O`
