Question

Write the equations involved in the preparation of potassium dichromate from chromite ore.

Answer 1

The chromite ore is an oxide mineral composed of chromium, iron, and oxygen having the formula $FeC{r_2}{O_4}$. It is dark gray to black in colour with a metallic to submetallic lustre and having high specific gravity. Chromite is important because it is the only economic ore of chromium,

Complete answer:
The balanced equations in the manufacture of ${K_2}C{r_2}{O_7}$ ​ from chromite ore are as given below:
The first step involves the roasting- conversion of chromite ore into sodium chromate. In this step, Potassium dichromate (${K_2}C{r_2}{O_7}$​​) is prepared from chromite ore $FeC{r_2}{O_4}$ ​. The chromite ore is fused with sodium or potassium carbonate in free access of air.
$4FeC{r_2}{O_4} + 8N{a_2}C{O_3} + 7{O_2} \to 8N{a_2}Cr{O_4} + 2Fe{O_3} + 8C{O_2}$
The second step involves the conversion of sodium chromate into sodium dichromate. In this step, the yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, $N{a_2}C{r_2}{O_7}.2{H_2}O\;$ can be crystallized.
$2N{a_2}Cr{O_4}\; + {H_2}S{O_4} \to N{a_2}C{r_2}{O_7} + N{a_2}S{O_4} + {H_2}O$
The last step involves the conversion of sodium dichromate into potassium dichromate. In this step, Sodium dichromate is more soluble than potassium dichromate. Hence, sodium dichromate when fused with KCl forms orange crystals of potassium dichromate (${K_2}C{r_2}{O_7}$).
$N{a_2}C{r_2}{O_7} + 2KCl \to {K_2}C{r_2}{O_7}\; + 2NaCl$

Note:
The chemical formula for potassium dichromate is ${K_2}C{r_2}{O_7}$ and its molecular mass is $294.185g/mol$. It is an ionic compound with two potassium ions i.e., ${K^ + }$ and the negatively charged dichromate ion i.e., $C{r_2}{O_7}^ -$, in which two hexavalent chromium atoms with oxidation state $+ 6$ are each attached to three oxygen atoms as well as a bridging oxygen atom.