What is the oxidation state of chromium in the final product formed when KI reacts with acidified potassium dichromate solution?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → +3
Acidified potassium dichromate is a strong oxidising agent: it oxidises iodide ions to iodine and is itself reduced to chromium(III) ions.
Chromium in \( \ce{Cr2O7^2-} \): 2x + 7(−2) = −2, so x = +6.
Reduction half-reaction:
\( \displaystyle \ce{Cr2O7^2- + 14H^+ + 6e^- -> 2Cr^3+ + 7H2O} \)
Oxidation half-reaction:
\( \displaystyle \ce{6I^- -> 3I2 + 6e^-} \)
Full ionic equation (sum of the two):
\( \displaystyle \ce{Cr2O7^2- + 14H^+ + 6I^- -> 2Cr^3+ + 3I2 + 7H2O} \)
Chromium in the product: \( \ce{Cr^3+} \), oxidation state +3; each Cr gains 3 electrons.
NCERT (2026-27 edition):
4.4.1 Oxides and Oxoanions of Metals – “… In acidic solution, its oxidising action can be represented as follows:
\( \displaystyle \ce{Cr2O7^2- + 14H^+ + 6e^- -> 2Cr^3+ + 7H2O} \quad (E^{\circ}=1.33\,\mathrm{V}) \)
Thus, acidified potassium dichromate will oxidise iodides to iodine, sulphides to sulphur, tin(II) to tin(IV) and iron(II) salts to iron(III).”