Describe the oxidising action of potassium dichromate and write the ionic equations of reaction with:
(i). Iodide
(ii). Iron (II) solution and
(III). `H_2S`

Explanation

`K_(2)Cr_(2)O_(7)` acts as a very strong oxidising agent in the acidic medium.
`K_(2)Cr_(2)O_(7)+4H_(2)SO_(4) to K_(2)SO_(4)+CR_(2)(SO_(4))_(3)+4H_(2)O+3[O]`
`K_(2)Cr_(2)O_(7)` takes up electrons to get reduced and acts as an oxidising agent. The reaction of `K_(2)Cr_(2)O_(7)` with other iodide, iron (II) solution, and `H_(2)S` are given below.
(i) `K_(2)Cr_(2)O_(7)` oxidizes iodide to iodine.
`{:(Cr_(2)O_(7)^(2-)+14H^(+)+6e^(-) to 2Cr^(3+)+7H_(2)O),(" "2I^(-) to I_(2)+2e^(-)xx3),(underline(bar(Cr_(2)O_(7)^(2-)+6I^(-)+14H^(+) to 2Cr^(3+)+3I_(2)+7H_(2)O))):}`
(ii) `K_(2)Cr_(2)O_(7)` oxidizes iron (II) solution to iron (III) solution i.e., ferrous ions to ferric ions.
`{:(Cr_(2)O_(7)^(2-)+14H^(+)+6e^(-) to 2Cr^(3+)+7H_(2)O),(" "Fe^(2+) to Fe^(3+)+e^(-)xx6),(underline(bar(Cr_(2)O_(7)^(2-)+14H^(+)+6Fe^(2+) to 2Cr^(3+)+6Fe^(3+)+7H_(2)O))):}`
(iii) `K_(2)Cr_(2)O_(7)` oxidizes H2S to sulphur.
`{:(Cr_(2)O_(7)^(2-)+14H^(+)+6e^(-) to 2Cr^(3+)+7H_(2)O),(" "H_(2)S to S+2H^(+)+2e^(-)xx3),(underline(bar(Cr_(2)O_(7)^(2-)+3H_(+)S+8H^(+) to 2Cr^(3+)+3S+7H_(2)O))):}`