Write the balanced equations in the manufacture of potassium dichromate from chromite ore.

Explanation

(i) Preparation of `K_(2)Cr_(2)O_(7)` from chromite ore.
(a) Potassium dichromate is obtained by the fusion of chromite ore `(FeCr_(2)O_(7))` with sodium (or potassium) carbonate in the presence of excess of air.
`4FeCr_(2)O_(4) + 8Na_(2)CO_(3) + 7O_(2) rarr 8Na_(2)CrO_(4) underset("Yellow solution")+ 2Fe_(2)O_(3) + 8CO_(2)`
(b) The solution is filtered and treated with sulphuric acid.
`Na_(2)CrO_(4) + H_(2)SO_(4) rarr Na(2)Cr_(2)O_(7) + Na_(2)SO_(4) + H_(2)O `
(c) Now sodium dichromate is treated with potassium chloride. As a result, potassium dichromate is produced.
`Na_(2)Cr_(2)O_(7) + 2KCI rarr K_(2)Cr_(2)O_(7)underset("Potassium dichramate crystals") + 2NaCl`
(iii) Effect of increasing pH on `K_(2)Cr_(2)O_(7)` solution.
`Cr_(2)O_(7)^(2-)underset("Orange") + 2OH^(-) rarr CrO_(4)^(2-) +H_(2)O`
On increasing pH `K_(2)CrO_(7)` changes into `K_(2)CrO_(4)` (orange to yellow).