Question

Write the steps in the preparation of following compounds.
(i) ${K_2}C{r_2}{O_7}$ from chromite ore
(ii) $KMn{O_4}$ from pyrolusite ore

Answer 1

Hint: We get metals in the form of ores from the earth and these ores need to be processed for converting them to useful compounds and pure metals. This process is called metallurgy and there can be different reactions in this process.

Complete step by step answer:
We will write all the steps in the preparation of both the compounds with accompanying reactions involved.
(i) Preparation of ${K_2}C{r_2}{O_7}$ from chromite ore -
For the preparation of potassium dichromate (${K_2}C{r_2}{O_7}$ ) from the chromite ore $(FeO.C{r_2}{O_3}{\text{ or }}FeC{r_2}{O_4})$, the ore is crushed and the following reactions are done on the powder.
$1.$ The chromite ore powder is reacted with the aqueous alkali solution like sodium hydroxide or sodium carbonate in the presence of air to give sodium chromate.
$4FeC{r_2}{O_4} + 16NaOH + 7{O_2} \to 8N{a_2}Cr{O_4} + 2F{e_2}{O_3} + 8{H_2}O$
$2.$ The sodium chromate solution is then acidified with sulphuric acid to give sodium dichromate.
$2N{a_2}Cr{O_4} + {H_2}S{O_4} \to N{a_2}C{r_2}{O_7} + N{a_2}S{O_4} + {H_2}O$
$3.$The solution consisting of sodium dichromate is made to react with potassium chloride to give potassium dichromate. The hot solution has sodium chloride precipitates in it, which are filtered and the solution is cooled to give pure crystalline potassium dichromate.
$N{a_2}C{r_2}{O_7} + 2KCl \to {K_2}C{r_2}{O_7} + 2NaCl$
(ii) Preparation of $KMn{O_4}$ from pyrolusite ore –
The preparation of potassium permanganate $(KMn{O_4})$ from the pyrolusite ore $(Mn{O_2})$ involves two steps, oxidative fusion and electrolytic oxidation.
$1.$ In oxidative fusion the ore is reacted with a strong base (or alkali) solution in the presence of oxygen to give manganate ion $(MnO_4^{2 - })$ .
$
  2Mn{O_2} + 3KOH + {O_2} \to 2{K_2}Mn{O_4} + 2{H_2}O \\
  {K_2}Mn{O_4} \to MnO_4^{2 - } + 2{K^ + } \\
 $
$2.$ The manganate ion thus obtained is oxidized electrochemically to permanganate ion which combines with potassium ion in the solution to give potassium permanganate.
$MnO_4^{2 - } \to$ (electrocatalytic oxidation) $\to MnO_4^ - + {e^ - }$

Note:
It is very important to keep in mind that all the reactions written should be balanced. The important steps need to be followed and correlated.A balanced equation is an equation for a chemical reaction in which the number of atoms for each element in the reaction and the total charge is the same for both the reactants and the products. In other words, the mass and the charge are balanced on both sides of the reaction.