Question

What happens when—
(i) Chromite ore reacts with potassium carbonate in presence of air.
(ii) Potassium dichromate reacts with ferrous sulphate in presence of sulphuric acid.
(iii) Potassium permanganate is heated up to 513 K.

Hint:

Remember: (i) Roasting chromite with alkali in air yields yellow chromate; (ii) \(\mathrm{Cr_2O_7^{2-}}\) is a strong oxidant in acid, converting \(\mathrm{Fe^{2+}}\to \mathrm{Fe^{3+}}\); (iii) On heating, \(\mathrm{KMnO_4}\) releases oxygen—useful for lab O$_2$ prep.

Answer 1

(i) Oxidative roasting of chromite with K$_2$CO$_3$ (air):
Chromite (\(\mathrm{FeCr_2O_4}\)) is oxidised to potassium chromate; iron becomes \(\mathrm{Fe_2O_3}\). \[ \boxed{\,4\,\mathrm{FeCr_2O_4} + 8\,\mathrm{K_2CO_3} + 7\,\mathrm{O_2} \;\longrightarrow\; 8\,\mathrm{K_2CrO_4} + 2\,\mathrm{Fe_2O_3} + 8\,\mathrm{CO_2}\,} \] (ii) \(\mathrm{K_2Cr_2O_7}\) + \(\mathrm{FeSO_4}\) in acidic medium:
Dichromate oxidises \(\mathrm{Fe^{2+}}\) to \(\mathrm{Fe^{3+}}\) and is reduced to \(\mathrm{Cr^{3+}}\). \[ \boxed{\,\mathrm{Cr_2O_7^{2-}} + 6\,\mathrm{Fe^{2+}} + 14\,\mathrm{H^+} \;\longrightarrow\; 2\,\mathrm{Cr^{3+}} + 6\,\mathrm{Fe^{3+}} + 7\,\mathrm{H_2O}\,} \] In terms of salts: \[ \mathrm{K_2Cr_2O_7} + 6\,\mathrm{FeSO_4} + 7\,\mathrm{H_2SO_4} \rightarrow \mathrm{K_2SO_4} + \mathrm{Cr_2(SO_4)_3} + 3\,\mathrm{Fe_2(SO_4)_3} + 7\,\mathrm{H_2O} \] (iii) Heating \(\mathrm{KMnO_4}\) to \(\sim 513\,\mathrm{K}\):
Permanganate disproportionates giving manganate, manganese dioxide and oxygen. \[ \boxed{\,2\,\mathrm{KMnO_4} \;\xrightarrow{\,\Delta\,}\; \mathrm{K_2MnO_4} + \mathrm{MnO_2} + \mathrm{O_2}\,} \]