Question

How is sodium chromate converted into sodium dichromate in the manufacture of potassium dichromate from chromite ore ?
a.) By the action of concentrated sulphuric acid
b.) By roasting with soda ash
c.) By the action of sodium hydroxide
d.) By the action of limestone

Answer 1

Hint: The chromite ore contains the chromite ions in combination with iron. The sodium hydroxide replaces sodium with iron. Initially, chromite ions are present only which are converted to dichromate by sodium.

Complete step by step answer:
Initially, we will see the process of manufacture of potassium dichromate from chromite ore. Let us see that, then we will be able to get our answer.

This is a three step process.
Step 1 :- The first step involves the preparation of sodium chromate. The reaction can be written as -
$4FeC{r_2}{O_4} + 16NaOH + 7{O_2} \to 8NaCr{O_4} + 2F{e_2}{O_3} + 8{H_2}O$

Step 2 :- The step 2 involves the conversion of sodium chromate into sodium dichromate. The reaction is
$2N{a_2}Cr{O_4} + conc.{H_2}S{O_4} \to N{a_2}C{r_2}{O_7} + N{a_2}S{O_4} + {H_2}O$

The sodium chromate produced in the first step reacts with concentrated sulphuric acid to give the sodium dichromate along with sodium sulphate and water.

Step 3 :- The step 3 involves the conversion of sodium dichromate and potassium dichromate.
$N{a_2}C{r_2}{O_{_7}} + 2KCl \to {K_2}C{r_2}{O_7} + 2NaCl$

After this, we get orange precipitates of potassium dichromate.
So, by observing the above, mechanism; we can conclude that the conversion of sodium chromate in sodium dichromate is by action of concentrated sulphuric acid.
So, the correct answer is “Option A”.

Note: The potassium dichromate is less soluble than sodium chloride. Thus, it can be removed by filtration. It must also be noted that the dichromate ions are in equilibrium with the chromate ions at pH 4. On changing the pH value, the ions can be interconverted into each other as per the requirement of the reaction.