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Question
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
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.
\[\ce{2NaBr + 2H_2SO_4 -> 2NaHSO_4 + 2HBr}\]
\[\ce{2HBr + H_2SO_4 -> \underset{red vapour}{Br_2 + SO_2} + 2H_2O}\]
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.
\[\ce{2NaCl + 2H_2SO_4 -> 2NaHSO_4 + 2HCl}\]
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