• # question_answer 21) Starting with sodium chloride, how would you proceed to prepare : (i) sodium metal (ii) sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate?

(i) Sodium is obtained by carrying electrolysis of a fused mixture of $NaCl$ (40%) and $CaC{{l}_{2}}$(60%) in Down's cell at ${{600}^{{}^\circ }}C$using iron cathode and graphite anode. Na is discharged at cathode and$C{{l}_{2}}$is evolved at the anode. (ii) Sodium hydroxide is obtained by electrolysis of aqueous solution of $NaCl$ in Castner-Kellner process. In this cell, mercury cathode and iron anode is used. The electrode processes are given below: $NaCl\xrightarrow{Water}N{{a}^{+}}(aq)+C{{l}^{-}}(aq)$ Anode: $2C{{l}^{-}}\to C{{l}_{2}}+2{{e}^{-}}$ Cathode: $N{{a}^{+}}+{{e}^{-}}\to Na$ $Na+Hg\to Na/Hg$(amalgam) $Na/Hg+{{H}_{2}}O\to \underset{Caustic\,soda}{\mathop{NaOH}}\,+Hg+\frac{1}{2}{{H}_{2}}\uparrow$ (iii) Sodium peroxide is obtained by heating sodium in excess of air. $4Na+{{O}_{2}}\to 2N{{a}_{2}}O$ $2N{{a}_{2}}O+{{O}_{2}}\to 2N{{a}_{2}}{{O}_{2}}$ (iv) Sodium carbonate is obtained from aqueous $NaCl$by solvay's process Step 1: Brine $(Aq\text{ }NaCI)$ is saturated with ammonia to form ammonical brime. $\underset{Brine}{\mathop{(NaCl+{{H}_{2}}O)}}\,+N{{H}_{3}}(g)\to \underbrace{NaCl+N{{H}_{4}}OH}_{Ammonical\,Brine}$ Step 2: Carbon dioxide when passed through ammonical brime, we get residue of sodium bicarbonate. $N{{H}_{4}}OH+C{{O}_{2}}\to N{{H}_{4}}HC{{O}_{3}}$ $N{{H}_{4}}HC{{O}_{3}}+NaCl\to \underset{(Residue)}{\mathop{NaHC{{O}_{3}}}}\,+N{{H}_{4}}Cl$ Thermal decomposition of $NaHC{{O}_{3}}$ gives the residue of sodium carbonate $2NaHC{{O}_{3}}\xrightarrow{\Delta }\underset{sodium\,carbonate}{\mathop{N{{a}_{2}}C{{O}_{3}}}}\,+{{H}_{2}}O+C{{O}_{2}}\uparrow$