Question
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
Solution
`K_2Cr_2O_7` acts as a very strong oxidising agent in the acidic medium.
`K_2Cr_2O_7 + 4H_2SO_4 - > K_2SO_4 + Cr_2(SO_4)_3+4H_2O+3[0]`
`K_2Cr_2O_7` takes up electrons to get reduced and acts as an oxidising agent. The reaction of K2Cr2O7with other iodide, iron (II) solution, and H2S are given below.
(i)`K_2Cr_2O_7` oxidizes iodide to iodine
`Cr_2O_7^(2-) + 14H^(+) + 6e^(-) -> 2Cr^(3+) + 7H_2O`
`2I^(-) -> I_2 + 2e^(-)]xx3`
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` Cr_2O_7^(2-) + 6I^(-) + 14H^(+) -> 2Cr^(3+) + 3I_2 + 7H_2O`
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(ii) `K_2Cr_2O_7` oxidizes iron (II) solution to iron (III) solution i.e., ferrous ions to ferric ions.
`Cr_2O_7^(2-) + 14H^(+) + 6e^(-) -> 2Cr^(3+) + 7H_2O`
`Fe^(2+) ->Fe^(3+) + e^(-)] xx6`
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`Cr_2O_7^(2-) + 14H^(+) + 6Fe^(2+) -> 2Cr^(3+) +6Fe^(3+) + 7H_2O`
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(iii) `K_2Cr_2O_7` oxidizes H2S to sulphur.
`Cr_2O_7^(2-) + 14H^(+) + 6e^(-) -> 2Cr^(3+) + 7H_2O`
` H_2S -> S + 2H^(+) + 2e^(-)]xx3`
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`Cr_2O_7^(2-) + 3H_2S + 8H^(+) -> 2Cr^(3+) + 3S + 7H_2O`
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