Chemistry - Salt Analysis Question with Solution | TestHub

When ${\mathrm{Cl}}^{-}$ salt is heated with ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ + conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$ red vapours of ${\mathrm{CrO}}_2{\mathrm{Cl}}_2$ are evolved which on passing into NaOH solution gives yellow solution of ${\mathrm{CrO}}_4^{2-}$ ions. Latter further gives yellow ppt. of ${\mathrm{PbCrO}}_4$ with $Pb(C{H}_{3}COO{)}_2$ and acetic acid.

$4{\mathrm{Cl}}^{-}+{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+6{\mathrm{H}}^{+}⟶2{\mathrm{CrO}}_2{\mathrm{Cl}}_2\uparrow +3{\mathrm{H}}_2\mathrm{O}$

${\mathrm{CrO}}_2{\mathrm{Cl}}_2+4{\mathrm{OH}}^{-}⟶{\mathrm{CrO}}_4^{2-}+2{\mathrm{Cl}}^{-}+2{\mathrm{H}}_2\mathrm{O}$

while on heating a bromide salt with conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$ & ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$, instead of ${\mathrm{CrO}}_2{\mathrm{Br}}_2$, reddish brown fumes of ${\mathrm{Br}}_2$ are formed due to reducing character of HBr formed during the reaction. ${\mathrm{Br}}_2$ dissolves in NaOH & No colour is produced.

$6{\mathrm{Br}}^{-}+{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+{\mathrm{H}}^{+}⟶3{\mathrm{Br}}_2\uparrow +{\mathrm{Cr}}^{3+}+7{\mathrm{H}}_2\mathrm{O}$

$2\mathrm{NaOH}+{\mathrm{Br}}_2⟶\mathrm{NaBr}+\mathrm{NaOBr}+{\mathrm{H}}_2\mathrm{O}$