(a) How is potassium dichromate prepared from chromite ? How does it react with :
(i) acidified ferrous sulphate solution
(ii) potassium iodide solution
(iii) hydrochloric acid
(iv) conc. `H_(2)SO_(4)` and soluble metal chloride ?
(b) The orange colour of potassium dichromate changes to yellow on the addition of alkali, On acidifying the yellow solution, the colour again changes to Orange red. Explain giving equations. Draw structures of important species formed.

Explanation

(a) Potassium dichromate is prepared from chromite (`FeCr_(2),O_(4)`) by fusing with molten alkali in the presence of air
`FeCr_(2)O_(4)+ 16NaOH+ 7O_(2) to 8Na_(2)CrO_(4)+ 2Fe_(2)O_(3)+ 8H_(2)O`
The solution of sodium chromate is filtered and acidified with dil , `H_(2)SO_(4)` giving sodium dichromate
`2NaCrO_(4)+ 2H^(+) to Na_(2)Cr_(2)O_(7) + 2Na^(+)+ H_(2)O`
Sodium dichromate is more soluble and less stable than potassium dichromate. Potassium dichromate is prepared by mixing a hot concentrated solution of `Na_(2)Cr_(2)O_(7)` and KCl in equimolar quantities.
`Na_(2)Cr_(2)O_(7)+ 2KCI to K_(2)Cr_(2)O_(7) + 2NaCl`
`K_(2)Cr_(2)O_(7)` is obtained as orange crystals,
Reactions (1) `K_(2)Cr_(2)O_(7)` oxidises ferrous sulphate to ferric sulphate.
`6FeSO_(4)+ K_(2)Cr_(2)O_(7)+ 7H_(2)SO_(4) to 3FeSO_(4)+ Cr_(2)(SO_(4))_(3)+ K_(2)SO_(4)+ 7H_(2)O`
OR `Cr_(2)O_(7)^(2-)+ 6Fe^(2+)+ 14H^(+) to 2Cr^(3+)+ 7H_(2)O+ 6Fe^(3+)`
(ii) `K_(2)Cr_(2)O_(7)+ 6KI+ 7H_2SO_(4)+ Cr_(2)(SO_(4))_(3)+ 7H_(2)O+ 3I_(2)`
OR `Cr_(2)O_(7)^(2-)+ 6I^(-)+ 14H^(+) to 2Cr^(3+)+ 3I_(2)+ 7H_(2)O`
(iii) `K_(2)Cr_(2)O_(7) + 14HCI to 2KCI+ 2CrCl_(3)+ 7H_(2)O+ 3Cl_(2)`
(iv) `K_(2)Cr_(2)O_(7)+ 4NaCl+ 6H_(2)SO_(4) to 2KHSO_(4)+ 4NaHSO_(4)+ 2CrO_(2)Cl_(2)+ 3H_(2)O`
(b) Potassium dichromate on heating with alkalies (KOH) changes to potassium chromate, which is yellow :
`K_(2)Cr_(2)O_(7)+ 2KOH to underset("(Yellow)")(underset("Pot. chromate")(2K_(2)CrO_(4)))+ H_(2)O`
On acidifying, the yellow colour change back to orange red due to reversible reaction.
`2K_(2)CrO_(4) + H_(2)SO_(4) to underset("(Orange)")(K_(2)Cr_(2)O_(7)) + K_(2)SO_(4) + H_(2)O`
The two main species involved are `CrO_(4)^(2-)` and `Cr_(2)O_(7)^(2-)` ions. Their structures are given below:

Regular tetrahedral structure