Question

Explain the manufacture of potassium dichromate from chromite ore $\left( {{\text{FeC}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{4}}}} \right)$.

Answer 1

Hint:The chemical formula for potassium dichromate is ${{\text{K}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}$. The manufacturing of potassium dichromate from chromite ore involves three steps. In this process, sodium chloride is formed as a by-product. The chromite ore is also known as iron chromite ore.

Complete step by step solution:
We have to explain the manufacture of potassium dichromate from chromite ore $\left( {{\text{FeC}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{4}}}} \right)$. The manufacturing of potassium dichromate from chromite ore involves three steps.
The first step is the reaction of chromite ore with sodium carbonate $\left( {{\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}} \right)$ in excess of oxygen. Instead of sodium carbonate potassium carbonate can also be used. The reaction is as follows:
${\text{4FeC}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{4}}} + 8{\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}} + 7{{\text{O}}_2} \to {\text{8N}}{{\text{a}}_{\text{2}}}{\text{Cr}}{{\text{O}}_{\text{4}}} + {\text{2F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}} + {\text{8C}}{{\text{O}}_{\text{2}}}$
The products formed when chromite ore reacts with sodium carbonate in excess of oxygen are sodium chromate $\left( {{\text{N}}{{\text{a}}_{\text{2}}}{\text{Cr}}{{\text{O}}_{\text{4}}}} \right)$, ferric oxide $\left( {{\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}} \right)$ and carbon dioxide $\left( {{\text{C}}{{\text{O}}_{\text{2}}}} \right)$. Sodium chromate is yellow in colour.
The second step is the reaction of sodium chromate with an acidified solution of sulphuric acid \[\left( {{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}} \right)\]. The reaction is as follows:
${\text{2N}}{{\text{a}}_{\text{2}}}{\text{Cr}}{{\text{O}}_{\text{4}}} + {{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}} \to {\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}} + {\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}} + {{\text{H}}_{\text{2}}}{\text{O}}$
The products formed when sodium chromate reacts with an acidified solution of sulphuric acid are sodium dichromate $\left( {{\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}} \right)$, sodium sulphate $\left( {{\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}} \right)$ and water $\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)$.
The third step is the reaction of sodium dichromate with potassium chloride \[\left( {{\text{KCl}}} \right)\]. The reaction is as follows:
${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}} + 2{\text{KCl}} \to {{\text{K}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}} + {\text{2NaCl}}$
The products formed when sodium dichromate reacts with potassium chloride are potassium dichromate $\left( {{{\text{K}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}} \right)$ and sodium chloride $\left( {{\text{NaCl}}} \right)$.

Note:We can see that sodium chloride is formed as the by-product. Now, to separate potassium dichromate and sodium chloride, sodium chloride is precipitated and then is removed from the hot solution using filtration. The mother liquor contains potassium dichromate.