Question

Indicate the steps in the preparation of:
(i) $K_{2}C{r}_2{O}_7$ from chromite ore. (ii) $KMn{O}_4$ from pyrolusite ore.

Answer 1

Preparation of potassium dichromate $K_{2}C{r}_2{O}_7$ from chromite ore $FeO\cdot C{r}_2{O}_3\left(FeC{r}_2{O}_4\right)$:
(i) Chromite ore is fused with $NaOH$ or sodium carbonate in air to form sodium chromate.
$4FeC{r}_2{O}_4+16NaOH+7O_2\to 8N{a}_{2}Cr{O}_4+2F{e}_2{O}_3+8H_{2}O$

(ii) Sodium chromate solution is filtered and acidified to form sodium dichromate.
$2N{a}_{2}Cr{O}_4+H_{2}S{O}_4\to N{a}_{2}C{r}_2{O}_7+N{a}_{2}S{O}_4+H_{2}O$
The solution is coled to remove crystallized sodium sulphate. Sodium dichromate remains soluble.

(iii) Equimolar proportions of hot solution of sodium dichromate and KCl are mixed to form potassium dichromate.
$N{a}_{2}C{r}_2{O}_7+2KCl\to K_{2}C{r}_2{O}_7+2NaCl$
Sodium chloride precipitates out in hot solution and is removed by filtration. On cooling, potassium dichromate separates out from mother liquor.

Preparation of potassium permanganate $KMn{O}_4$ from pyrolusite ore $Mn{O}_2$ .

(i) Pyrolusite ore is fused with KOH (or $K_{2}C{O}_3$) in presence of air (or oxidising agents such as $KN{O}_3$ or $KCl{O}_3$) to give potassium manganate $K_{2}Mn{O}_4$ (green mass)
$2MN{O}_2+4KOH+O_2\stackrel{Delta}{\to }2K_{2}Mn{O}_4+2H_{2}O$

(ii) Extraction of green mass with water followed by oxidation with chlorine (or ozone or $C{O}_2$) gives potassium permanganate.
$2K_{2}Mn{O}_4+C{l}_2\to 2KCl+2KMn{O}_4$
On concentrating the solution, dark purple crystals of potassium permanganate separates out.
Note: In neutral or acidic medium, manganate ion will disproportionate to permanganate ion.
$3Mn{O}_4^{2-}+4H^{+}\to 2Mn{O}_4^{-}+Mn{O}_2+2H_{2}O$
Convertion of manganate to permanganate can also be carried by electrolytic oxidation.