Question

Describe the oxidising action of potassium dichromate and write the ionicequations for its reaction with: (i) iodide (ii) iron(II) solution and (iii) H₂S

Answer 1

K₂Cr₂O₇ acts as a very strong oxidising agent in the acidic medium
K₂Cr₂O₇+4H₂SO₄ \(\rightarrow\) K₂SO₄+Cr₂(SO₄)₃+4H₂O+3[O]
K₂Cr₂O₇ takes up electrons to get reduced and acts as an oxidising agent. The reaction of K₂Cr₂O₇ with other iodide, iron (II) solution, and H₂S are given below.

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(i) K₂Cr₂O₇ oxidizes iodide to iodine

\(Cr_2O^{2-}_7+14H^++6e^-\) \(\rightarrow\) \(2Cr^{3+}{+7H_2O}\)
                      \(2I^-\)\(\rightarrow\)\(I_2+2e^-]\times3\)
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\(Cr_2O^{2-}_7+6I^-+14H^+ \rightarrow 2Cr^{3+}+3I_2+7H_2O\)

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(ii) K₂Cr₂O₇ oxidizes iron (II) solution to iron (III) solution i.e., ferrous ions to ferric ions

\(Cr_2O^{2-}_7+14H^++6e^-\) \(\rightarrow\) \(2Cr^{3+}{+7H_2O}\)
                      \(Fe^{2+}\) \(\rightarrow\) \(Fe^{3+}+e^-]\times6\)
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\(Cr_2O^{2-}_7+14H^+ +6Fe^{2+}\rightarrow 2Cr^{3+}+6Fe^{3+}+7H_2O\)

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(iii) K₂Cr₂O₇ oxidizes H₂S to sulphur
\(Cr_2O^{2-}_7+14H^++6e^-\) \(\rightarrow\) \(2Cr^{3+}{+7H_2O}\)
                      \(H_2S\) \(\rightarrow\) \(S+2H^++2e^-]\times3\)
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\(Cr_2O^{2-}_7+3H_2S +8H^+ \rightarrow 2Cr^{3+}+3S+7H_2O\)