Question

How is potassium dichromate prepared from a sample of chromite ore? Give balanced equations for the chemical reactions involved.

Answer 1

Preparation pf potassium dichromate from chromite ore involves following steps:

(i) Conversion of chromite ore to sodium chromate by roasting finely powdered chromite ore with sodium carbonate and quick lime in excess of air.

$\underset{Chromiteore}{4FeC{r}_2{O}_4}+\underset{carbonate}{\underset{Sodium}{8N{a}_{2}C{O}_3}}+7O_2\stackrel{CaO+air}{\to }\underset{Ferricoxide}{2F{e}_2{O}_3}+\underset{chromate}{\underset{Sodium}{8N{a}_{2}Cr{O}_4}}+8C{O}_2$

(ii) Conversion of sodium chromate to sodium dichromate by the action of $conc.H_{2}S{O}_4$.

$2N{a}_{2}Cr{O}_4+\underset{conc.}{H_{2}S{O}_4}\to \underset{dichromate}{\underset{Sodium}{N{a}_{2}C{r}_2{O}_7}}+N{a}_{2}S{O}_4+H_{2}O$

(iii) Conversion of sodium dichromate to potassium dichromate by treating it with KCl solution.

$N{a}_{2}C{r}_2{O}_7+2KCl\to \underset{dichromate}{\underset{Potassium}{K_{2}C{r}_2{O}_7}}+2NaCl$

From resulting solution potassium dichromate is crystallised.