Question

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

Answer 1

Hint:The heating of ores in presence of oxygen is termed as roasting, while the heating of ores in absence of oxygen is calcination.
Preparation of reagents from ores of metal requires a number of steps involving roasting, followed by the treatment of the products, depending on the desired final reagent.


Complete solution:
Potassium dichromate has the chemical formula ${{K}_{2}}C{{r}_{2}}{{O}_{7}}$ and the chemical formula for chromite ore is $FeC{{r}_{2}}{{O}_{4}}$.
The preparation of potassium dichromate by the use of chromite ore follows a series of steps involving conversions of chromite ore and the other products which would be formed in the reactions.
In the first step the chromite ore reacts with sodium carbonate in presence of oxygen gas along with quick lime having the chemical formula $CaO$ and air in excess. This process of heating an ore in presence of oxygen is called roasting. This process can be chemically represented as
$4FeC{{r}_{2}}{{O}_{4}}+8N{{a}_{2}}C{{O}_{3}}+7{{O}_{2}}\xrightarrow{CaO+air}2F{{e}_{2}}{{O}_{3}}+8N{{a}_{2}}Cr{{O}_{4}}+8C{{O}_{2}}$
In this reaction we can see that four molecules of the chromite ore reacts with eight molecules of sodium carbonate and seven molecules of molecular oxygen in presence of quick lime and excess of air in order to give two molecules of ferric oxide having chemical formula $F{{e}_{2}}{{O}_{3}}$ and eight molecules of sodium chromate which is represented by the formula $N{{a}_{2}}Cr{{O}_{4}}$ along with eight molecules of carbon dioxide with formula $C{{O}_{2}}$. Now in the next step we would use this sodium chromate, and convert it to sodium dichromate by the use of concentrated sulphuric acid. This reaction can be chemically represented as,
\[~2N{{a}_{2}}Cr{{O}_{4}}+conc.{{H}_{2}}S{{O}_{4}}\to N{{a}_{2}}C{{r}_{2}}{{O}_{7}}+N{{a}_{2}}S{{O}_{4}}+{{H}_{2}}O\]
Here we can see that two molecules of sodium chromate reacts with one molecule of concentrated sulphuric acid which has the chemical formula \[{{H}_{2}}S{{O}_{4}}\], in order to form one molecule of sodium dichromate which has the chemical formula $N{{a}_{2}}C{{r}_{2}}{{O}_{7}}$ along with sodium sulphate which has the formula \[N{{a}_{2}}S{{O}_{4}}\]and water which is represented as \[{{H}_{2}}O\].
Now the last step involves the conversion of this sodium dichromate which was obtained from the previous reaction, to potassium dichromate by using potassium chloride which has the chemical formula $KCl$. The chemical equation can be represented as,
$N{{a}_{2}}C{{r}_{2}}{{O}_{7}}+KCl\to {{K}_{2}}C{{r}_{2}}{{O}_{7}}+2NaCl$
Upon reaction of sodium dichromate with two molecules of potassium chloride, the formation of one molecule of potassium chloride having the chemical formula ${{K}_{2}}C{{r}_{2}}{{O}_{7}}$ and sodium chloride represented by $NaCl$ takes place.

From this solution the crystalline potassium dichromate is formed.


Note:Potassium dichromate can be prepared by chromite ores, by first roasting the ores with sodium carbonate in presence of quick lime. Then treating the sodium chromate, which was obtained as a product of the first reaction, with sulphuric acid, in order to get sodium dichromate. Then finally treating it with potassium chloride to get potassium dichromate.