Question

Reddish brown gas is obtained when the following are treated with conc. ${{H}_{2}}S{{O}_{4}}$:
(A) $B{{r}^{-}}$
(B) $N{{O}_{3}}^{-}$
(C) $N{{O}_{2}}^{-}$
(D) ${{I}^{-}}$

Answer 1

when sulphuric acid is added to solution containing nitrate brown fumes appear. When sulphuric acid is added to a solution containing bromide ion, reddish brown gas is obtained with pungent odor.

Complete solution step by step:
-In qualitative analysis, for detection of anions preliminary tests are performed.
-In the first step, a preliminary test is performed using dilute sulphuric acid.
-if no positive result is obtained with dilute sulphuric acid, then conc. Sulphuric acid is used.
- when a solution containing nitrite ion is treated with conc. sulphuric acid, reddish brown fumes of nitrogen dioxide are evolved. sulphuric acid acts as an oxidizing agent as it has the ability to release nascent oxygen.
-When solution containing bromide ion is treated with conc. sulphuric acid, reddish brown fumes of bromine gas are evolved.
when a solution containing nitrate ion is treated with conc. Sulphuric acid, brown fumes of nitrate gas are evolved.
-when solution containing iodide ion is treated with conc. sulphuric acid, violet vapors of iodine gas with pungent odor are evolved. Iodide ions are strong reducing agents, iodide ions are oxidized to iodine gas and sulphuric acid is reduced to Sulphur dioxide.

Reddish brown gas is obtained when the following are treated with conc. ${{H}_{2}}S{{O}_{4}}$ (A) $B{{r}^{-}}$ and (C) $N{{O}_{2}}^{-}$.

Note: Bromide ions are strong reducing agents; they reduce sulphuric acid to Sulphur dioxide gas and bromide ions are oxidized to bromine gas. Sulphuric acid reduces nitrite ion to nitrite ion to nitrogen dioxide responsible for reddish brown fumes. Nitrate ions also give positive brown ring tests.