Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with:
iodide
Acidified potassium dichromate oxidises iodides to iodine. The ionic equation is:
Cr₂O₇²⁻ + 14H⁺ + 6I⁻ → 2Cr³⁺ + 3I₂ + 7H₂O
iron(II) solution and
Acidified potassium dichromate oxidises iron(II) salts to iron(III). The ionic equation is:
Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O
H_2S
Acidified potassium dichromate oxidises sulphides to sulphur. The ionic equation is:
Cr₂O₇²⁻ + 8H⁺ + 3H₂S → 2Cr³⁺ + 3S + 7H₂O
Explanation
The textbook explains that potassium dichromate acts as a strong oxidising agent in acidic solution. The dichromate ion (Cr₂O₇²⁻) gets reduced to Cr³⁺ by gaining 6 electrons, with a standard reduction potential E° = 1.33V indicating strong oxidising power. The full ionic equations are obtained by adding the dichromate reduction half-reaction to the oxidation half-reactions of the reducing agents. The textbook provides these half-reactions: 6I⁻ → 3I₂ + 6e⁻ (for iodide), 6Fe²⁺ → 6Fe³⁺ + 6e⁻ (for iron(II)), and 3H₂S → 6H⁺ + 3S + 6e⁻ (for H₂S). For the H₂S reaction, the 6H⁺ produced in the oxidation half-reaction cancels with 6 of the 14H⁺ from the reduction half-reaction, leaving 8H⁺ in the final equation.