140.

Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with:
(a) iodide    (b) iron (II) solution and     (c) H₂S.

Potassium dichromate, K₂Cr₂O₇ is a strong oxidising agent and is used as a primary standard in volumetric analysis involving oxidation of iodides, ferrous ion and S^2– ions etc. In acidic solution, its oxidising action can be represented as follows:
Cr₂O₇^2– + 14H⁺ + 6e^– → 2Cr³⁺ + 7H₂O;
(E⁺ = 1.33 V)

(a)    It oxidises potassium iodide to iodine.
Cr₂O₇^2– + 14H⁺ 61^– → 2Cr³ + 7H₂O + 3I₂

(b)    It oxidises iron(II) salt to iron (III) salt
Cr₂O₇^2– + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O

(c)    It oxidises H₂S to S
Cr₂O₇^2– + 8H⁺ + 3H₂S → 2Cr³⁺ + 7H₂O + 3S