11th Class Chemistry The s-Block Elements

  • 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 \[2NaHC{{O}_{3}}\xrightarrow{\Delta }\underset{sodium\,carbonate}{\mathop{N{{a}_{2}}C{{O}_{3}}}}\,+{{H}_{2}}O+C{{O}_{2}}\uparrow \]  

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