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.

---

(i) K2Cr2O7 oxidizes iodide to iodine

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

---

$Cr_2O^{2-}_7+6I^-+14H^+ \rightarrow 2Cr^{3+}+3I_2+7H_2O$

---

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

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

---

$Cr_2O^{2-}_7+14H^+ +6Fe^{2+}\rightarrow 2Cr^{3+}+6Fe^{3+}+7H_2O$

---

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

---

$Cr_2O^{2-}_7+3H_2S +8H^+ \rightarrow 2Cr^{3+}+3S+7H_2O$