Question

Identify the reactions in which dichromate acts as an oxidising reagent.
I. $Cr _{2} O _{7}^{2-}+6 Fe ^{2+}+14 H ^{+} \longrightarrow 2 Cr ^{3+}+6 Fe ^{3+}+7 H _{2} O$
II. $Cr _{2} O _{7}^{2-}+2 OH ^{-} \longrightarrow 2 CrO _{4}^{2-}+ H _{2} O$
III. $Cr _{2} O _{7}^{2-}+6 I ^{-}+14 H ^{+} \longrightarrow {2 Cr ^{3+}}+3 I _{2}+7 H _{2} O$
IV. $Na _{2} Cr _{2} O _{7}+2 KCl \longrightarrow K _{2} Cr _{2} O _{7}+2 NaCl$

• A. I, IV
• B. I,II
• C. II, III
• D. II, IV

Answer 1

Answer: (B)

In reaction I and III, dichromate acts as an oxidising reagent.

(I) $Cr _{2} O _{7}^{2-}+6 Fe ^{2+}+ 1 4 H ^{+} \longrightarrow 2 Cr ^{3+}+6 Fe ^{3+}+7 H _{2} O$

In the above reaction, $Cr ^{6+}$ reduced to $Cr ^{3+}$ and $Fe ^{2+}$ oxidised to $Fe ^{3+}$

(II) $Cr _{2} O _{7}^{2-}+2 OH ^{-} \longrightarrow 2 CrO _{4}^{2-}+ H _{2} O$

In the above reaction, there is no change occuring in the oxidation state of $Cr$.

(III) $Cr _{2} O _{7}^{2-}+6 I ^{-}+14 H ^{+} \longrightarrow 2 Cr ^{3+} +3 I _{2}+7 H _{2} O$

In the above reaction, $Cr ^{6+}$ has reduced to $Cr ^{3+}$ and $I ^{-}$ has oxidised to $I _{2}$.

(IV) $Na _{2} Cr _{2} O _{7}+2 KCl \longrightarrow K _{2} Cr _{2} O _{7}+2 NaCl$

In the above reaction, no change in the oxidation state of $Cr$ has taken place.