Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with:
$(i)$ iodide
$(ii)$ iron $(II)$ solution and
$(iii)$ $H_{2}S$

(N/A) $K_{2}Cr_{2}O_{7}$ acts as a very strong oxidising agent in the acidic medium.
In acidic solution,the dichromate ion is reduced to chromium $(III)$ ion:
$Cr_{2}O_{7}^{2-} + 14H^{+} + 6e^{-} \longrightarrow 2Cr^{3+} + 7H_{2}O$
$(i)$ Reaction with iodide:
$Cr_{2}O_{7}^{2-} + 14H^{+} + 6I^{-} \longrightarrow 2Cr^{3+} + 3I_{2} + 7H_{2}O$
$(ii)$ Reaction with iron $(II)$ solution:
$Cr_{2}O_{7}^{2-} + 14H^{+} + 6Fe^{2+} \longrightarrow 2Cr^{3+} + 6Fe^{3+} + 7H_{2}O$
$(iii)$ Reaction with $H_{2}S$:
$Cr_{2}O_{7}^{2-} + 8H^{+} + 3H_{2}S \longrightarrow 2Cr^{3+} + 3S + 7H_{2}O$