Question

Answer the following questions briefly.
(i) What is the actual reducing agent of haematite in blast furnace ?
(ii) Give the equations for the recovery of lead from galena by air reduction.
(iii) Why is sodium chloride added during electrolysis of fused anhydrous magnesium chloride.
(iv) Zinc, not copper, is used for the recovery of metallic silver from the complex `[Ag(CN)_(2)]`. Explain.
(v) Why is chalcocite roasted and not calcinated during recovery of copper ?

Answer 1

(i) For iron, first by calcination and roasting, the ferrous oxide is oxidised to ferric oxide. Then, in blast furnace, smelting is done, where it is reduced to get iron.
`4FeO + O_(2) rarr 2Fe_(2)O_(3)`
`Fe_(2)O_(3) + CO rarr 2Fe_(3)O_(4) + CO_(2)`
`Fe_(2) O_(3) + Co rarr FeO + CO_(2)`
`FeO + CO rarr Fe + CO_(2)`.
Carbon monoxide is the actual reducing agent of haematite in the blast furnace.
(ii) Lead is mainly extracted from sulphide ore called galena. Roasting is done followed by reduction with carbon. Self-reduction finally takes place.
`2PbS + 3O_(2) rarr 2PbO + SO_(2)`
`PbS + 2O_(2) rarr PbSO_(4)`
`PbSO_(4) + PbO rarr 2Pb + 2SO_(2)`
`PbS + 2PbO rarr 3Pb + SO_(2)`
(iii) The anhydrous magnesium chloride is fused with `NaCl` and anhydrous calcium chloride. The mixture is electrolysed at `700^@C` in the presence of an inert gas in an electrolytic cell. Magnesium is discharged at the cathode. The purpose of the addition of `NaCl` and `CaCl_(2)` to anhydrous `MgCl_(2)` is to lower the fusion temperature and make the fused mass a good conductor of electricity. Sodium prevents the hydrolysis of magnesium chloride.
(iv) Zinc is more reducing and cheaper than copper.
(v) Chalcocite is a sulphide ore of copper , therefore, it must be roasted in the presence of air to get oxide.