Redox Reactions

Concept:In acidic medium, dichromate ion ($\mathrm{Cr_2O_7^{2-}}$) is a strong oxidising agent.It oxidises iodide ($\mathrm{I^-}$) to iodine ($\mathrm{I_2}$) and itself gets reduced to chromium(III) ion ($\mathrm{Cr^{3+}}$).Explanation:In $\mathrm{K_2Cr_2O_7}$, each chromium atom has an oxidation state of $+6$ (since oxygen is $-2$, total charge $-2$ gives $2x + 7(-2) = -2 \rightarrow x = +6$).When $\mathrm{KI}$ is added to acidified $\mathrm{K_2Cr_2O_7}$, the reduction half‑reaction is:$$\mathrm{Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O}$$Thus, each chromium atom gains three electrons and its oxidation state decreases from $+6$ to $+3$.The final product contains $\mathrm{Cr^{3+}}$ ions, so the oxidation state is $+3$.Answer:$+3$ (Option B)