Which of the following reactions is/are involved in the preparation of potassium dichromate?
$(I) 4 F e C r_{2} O_{4} + 8 N a_{2} C O_{3} + 7 O_{2} \to$
$(I I) N a_{2} C r O_{4} + H_{2} S O_{4} \to$
$(I I I) N a_{2} C r_{2} O_{7} + 2 K C l \to$

(I) and (II)

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(II) and (III)

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(I) and (III)

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(I), (II) and (III)

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Solution

The correct option is D (I), (II) and (III)
Dichromates are generally prepared from chromate, which in turn are obtained by the fusion of chromite ore $(F e C r_{2} O_{4})$ with sodium or potassium carbonate in free access of air. The reaction with sodium carbonate occurs as follows:
$(I) 4 F e C r_{2} O_{4} + 8 N a_{2} C O_{3} + 7 O_{2} \to 8 N a_{2} C r O_{4} + 2 F e_{2} O_{3} + 8 C O_{2}$
The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate can be crystallised.
$(I I) 2 N a_{2} C r O_{4} + H_{2} S O_{4} \to N a_{2} C r_{2} O_{7} + N a_{2} S O_{4} + H_{2} O$
Sodium dichromate is more soluble than potassium dichromate. The latter is therefore, prepared by treating the solution of sodium dichromate with potassium chloride.
$(I I I) N a_{2} C r_{2} O_{7} + 2 K C l \to K_{2} C r_{2} O_{7} + 2 N a C l$

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