Which of the following reactions is an example of redox reaction?

Which of the following reactions is an example of redox reaction?
  1. $\mathrm{Cr}_{2} \mathrm{O}_{3}+6 \mathrm{HCl} ightarrow 2 \mathrm{CrCl}_{3}+3 \mathrm{H}_{2} \mathrm{O}$
  2. $2 \mathrm{CrO}_{4}^{2-}+\mathrm{H}^{+} ightleftharpoons \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+\mathrm{OH}^{-}$
  3. $\mathrm{Cr} \mathrm{O}_{4}^{2-}+6 \mathrm{I}^{-}+14 \mathrm{H}^{+} ightarrow 2 \mathrm{Cr}^{3+}+3 \mathrm{I}_{2}+7 \mathrm{H}_{2} \mathrm{O}$
  4. $\mathrm{CrO}_{3}+2 \mathrm{NaOH} ightarrow \mathrm{Na}_{2} \mathrm{CrO}_{4}+\mathrm{H}_{2} \mathrm{O}$

Solution

In reactions (a), (b) and (d), no change in oxidation numbers of elements occur. Only in the choice (c), the oxidation number of $\mathrm{Cr}$ changes from $+6$ to $+3$ while that of $\mathrm{I}$ from $-1$ to $0$ .

Asked in: JEE-TOPICTESTS-CHEMISTRY

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