CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 17(a)

(a) Metals in the middle of the reactivity series (zinc, iron, lead, copper) are moderately reactive and occur mainly as sulphides or carbonates. Their ores are first converted into oxides, by roasting (sulphide ore heated strongly in excess air) or calcination (carbonate ore heated strongly in limited air), and the oxides are then reduced to the metals by heating with carbon, e.g. $\text{ZnO}+\text{C}\to \text{Zn}+\text{CO}$. Metals high up in the series (potassium, sodium, calcium, magnesium, aluminium) are very reactive and have a greater affinity for oxygen than carbon has. So carbon cannot reduce their oxides, and these metals are obtained by electrolytic reduction of their molten compounds (sodium, calcium and magnesium from their molten chlorides, aluminium from its molten oxide). The same process cannot be applied because carbon holds oxygen less strongly than these metals do: heating their oxides with carbon does not set the metal free, so they are obtained by electrolytic reduction. (b) Sodium is extracted by the electrolysis of molten (fused) sodium chloride. When an electric current is passed, ${\text{Na}}^{+}$ ions move to the cathode (negative electrode) and gain electrons to form sodium metal, which collects there; ${\text{Cl}}^{-}$ ions move to the anode (positive electrode) and lose electrons, and chlorine gas is given off. At the cathode: ${\text{Na}}^{+}+e^{-}\to \text{Na}$ At the anode: $2{\text{Cl}}^{-}\to {\text{Cl}}_2+2e^{-}$ Overall: $2\text{NaCl(l)}\to 2\text{Na(l)}+{\text{Cl}}_2\text{(g)}$