In which of the following there is a change in oxidation number ?
(a) An aqueous solution of `CrO_(4)^(2-)` is acidified.
(b) `SO_(2)` gas is passed through acidified `Cr_(2)O_(7)^(2-)` solution.
(c) `Cr_(2)O_(7)^(2-)` solution is made alkaline.
(d)` CrO_(2)Cl_(2)` is dissolved in NaOH.

Explanation

(A) `[ overset(+6) (Cr_(2))O_(7) ]^(2-) + 2OH^(-) rarr 2[overset(_6)(Cr)O_(4)]^(2-) +H_(2)O`. No change in ox. no.
(v)` [2overset(+6)(Cr)O_(4)]^(2-)+ 2H^(+) rarr [ overset(+6)(Cr_(2)O_(7))]^(2-)+ H_(2)O`, No change in ox. no.
(c ) `[overset(+6)(Cr_(2))O_(7)]^(2-) + 3SO_(2)+ 2H^(+) rarr 2 [ overset(3+)(Cr)]^(3+)+ 3 [ overset( +6)(SO_(4))]^(2-)+H_(2)O` , Ox. no. changes
(d) `overset( + 6) (Cr)O_(2)Cl_(2) + 2NaOH rarr Na_(2)overset( 6) (Cr)O_(2)`. No change in ox. no.