$\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$ paper acidified with dilute $\mathrm{H}_{2} \mathrm{SO}_{4}$ turns green when exposed to

JEE Main · 2023 · 24 Jan 2023 · Shift 2 · d and f Block Elements

K_{2} Cr_{2} O_{7}

paper acidified with dilute

H_{2} SO_{4}

turns green when exposed to

Answer: (D)

Acidified potassium dichromate is orange from chromium in the

+ 6

state and turns green when that chromium is reduced to

+ 3

. Sulphur dioxide is a reducing gas: it hands electrons to the dichromate, itself being oxidised to sulphate, while

C r_{2} O_{7}^{2 -}

is reduced to green

C r^{3 +}

, as in

3 S O_{2} + C r_{2} O_{7}^{2 -} + 2 H^{+} \to 3 S O_{4}^{2 -} + 2 C r^{3 +} + H_{2} O

. The gas that greens the paper is sulphur dioxide.

JEE Main · 2023 · 24 Jan 2023 · Shift 2 · Single correct option