Choose the reactions correctly given by the dichromate ion:
(A) \(Cr_2O_7^{2-} + 14H^+ + 6Fe^{2+} \longrightarrow 2Cr^{3+} + 6Fe^{3+} + 7H_2O\)
(B) \(Cr_2O_7^{2-} + 14H^+ + 3H_2S \longrightarrow 2Cr^{3+} + SO_4^{2-} + 7H_2O\)
(C) \(Cr_2O_7^{2-} + 14H^+ + 3Sn^{2+} \longrightarrow 2Cr^{3+} + 3Sn^{4+} + 7H_2O\)
(D) \(Cr_2O_7^{2-} + 14H^+ \longrightarrow 3Cr^{3+} + 2H_2O\)
(E) \(Cr_2O_7^{2-} + 2OH^- \longrightarrow 2CrO_4^{2-} + H_2O\)
Choose the correct answers from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A, C, E only
In acidic solution the dichromate ion is a strong oxidising agent:
\( \ce{Cr2O7^{2-}} \) + \( \ce{14H^{+}} \) + 6e− → \( \ce{2Cr^{3+}} \) + \( \ce{7H2O} \) (E° = 1.33 V)
It oxidises iron(II) to iron(III), tin(II) to tin(IV), iodides to iodine and sulphides to sulphur. Each full equation is this half-reaction plus the oxidation half-reaction taken enough times to give 6 electrons. In alkaline solution dichromate changes to chromate without any change in the oxidation state of Cr (+6).
(A) Cr2O72− + 14H+ + 6Fe2+ → 2Cr3+ + 6Fe3+ + 7H2O — Correct. \( \ce{Fe^{2+}} \) → \( \ce{Fe^{3+}} \) + e−, taken 6 times; charges balance (+24 on each side).
(B) Cr2O72− + 14H+ + 3H2S → 2Cr3+ + SO42− + 7H2O — Incorrect. \( \ce{H2S} \) is oxidised to sulphur (\( \ce{H2S} \) → S + \( \ce{2H^{+}} \) + 2e−, taken 3 times), and the printed equation is not even balanced (3 S on the left, 1 on the right). The correct equation is
\( \ce{Cr2O7^{2-}} \) + \( \ce{8H^{+}} \) + \( \ce{3H2S} \) → \( \ce{2Cr^{3+}} \) + 3S + \( \ce{7H2O} \)
(C) Cr2O72− + 14H+ + 3Sn2+ → 2Cr3+ + 3Sn4+ + 7H2O — Correct. \( \ce{Sn^{2+}} \) → \( \ce{Sn^{4+}} \) + 2e−, taken 3 times; charges balance (+18 on each side).
(D) Cr2O72− + 14H+ → 3Cr3+ + 2H2O — Incorrect. There is no reducing agent to supply the 6 electrons, and the equation is unbalanced (2 Cr and 7 O on the left; 3 Cr and 2 O on the right).
(E) Cr2O72− + 2OH− → 2CrO42− + H2O — Correct. Chromates and dichromates are interconvertible with pH; in alkali the orange dichromate becomes yellow chromate.
NCERT (2026-27 edition):
Potassium Dichromate – “… Sodium and potassium dichromates are strong oxidising agents; the sodium salt has a greater solubility in water and is extensively used as an oxidising agent in organic chemistry. Potassium dichromate is used as a primary standard in volumetric analysis. In acidic solution, its oxidising action can be represented as follows:”
\( \ce{Cr2O7^2- + 14H+ + 6e- -> 2Cr^3+ + 7H2O} \) (E° = 1.33 V)
Potassium Dichromate – “… The chromates and dichromates are interconvertible in aqueous solution depending upon pH of the solution.”
\( \ce{Cr2O7^2- + 2OH- -> 2CrO4^2- + H2O} \)