10.16 Starting with sodium chloride how would you proceed to prepare (i) sodium metal (ii) sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate ?

Answers (1)
R Rakesh

(i) Sodium metal can be extracted from sodium chloride.In this process electrolysis of fused NaCl(40%) and CaCl_{2}(60%) at a temp. of 1123K in Downs cell.

steel cathode and graphite block is act as anode. sodium and calcium deposit at cathode .

NaCl\rightarrow Na^{+}+Cl^{-}  (Molten state)

At Cathode-Na^{+}+e^{-}\rightarrow Na

At Anode-        \\Cl^{-}\rightarrow Cl+e^{-}\\ Cl+Cl\rightarrow Cl_{2}

(ii) NaOH can be prepared by the electrolysis of sodium chloride. By Castner- kellner process.  A brine solution is electrolysed using a mercury cathode and a carbon anode. Sodium metal discharged at the cathode combines with mercury to form sodium amalgam. Chlorine gas is evolved at the anode.

At Cathode   Na^{+}+e^{-}\rightarrow Na-amalgam

At anode-  Cl^{-}\rightarrow 1/2Cl_{2}+e^{-}


(iii) NaCl is electrolysed for the formation of Na metal (in downs process). this sodium metal is then heated on aluminium trays  in air to form sodium peroxide.

2Na+O_{2}\rightarrow Na_{2}O_{2}

(iv) Sodium carbonate is preapred by solvay process.NaHCO_{3} is precipitated in a reaction of sodium chloride and ammonium hydrogen carbonate.

\\2NH_{3} +H_{2}O+CO_{2}\rightarrow (NH_{4})_{2}CO_{3}\\ (NH_{4})_{2}CO_{3}+H_{2}O+CO_{2}\rightarrow NaHCO_{3}\\ (NH_{4})_{2}CO_{3} +NaCl\rightarrow NH_{4}Cl+NaHCO_{3}

this NaHCO_{3} crystals are heatedto give sodium carbonate.

2NaHCO_{3}\rightarrow Na_{2}CO_{3}+CO_{2}+H_{2}O