Ncert Exercises & Solutions

8.15 Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with:

(i) iodide (ii) iron(II) solution and (iii) H2S

Step 1:

K₂Cr₂O₇ is a strong oxidising agent. It will oxidize iodides to iodine, sulfides to sulphur, and iron(II) salts to iron(III).

Step 2:

The ionic equation is as follows:

K₂Cr₂O₇ + 7H₂SO₄ + 6KI → Cr₂(SO₄)₃ + 3I₂ + 4K₂SO₄ + 7H₂O

\(\begin{aligned} \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}+4 \mathrm{H}_{2} \mathrm{SO}_{4} &+3 \mathrm{H}_{2} \mathrm{~S} \rightarrow \mathrm{K}_{2} \mathrm{SO}_{4}+\mathrm{Cr}_{2}\left(\mathrm{SO}_{4}\right)_{3}+7 \mathrm{H}_{2} \mathrm{O}+3 \mathrm{~S} \end{aligned}\)

\(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+6 \mathrm{Fe}^{2+}+14 \mathrm{H}^{+} \rightarrow 2 \mathrm{Cr}^{3+}+6 \mathrm{Fe}^{3+}+7 \mathrm{H}_{2} \mathrm{O}\)

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