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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?
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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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