Question

A number of methods are used for the manufacture of the alkali metal compounds. Accordingly match the processes listed in column-I with the compound(s) listed in column-II.

Answer 1

(A) The mercury cathode cell (Castner-Kellner cell): In this cell mercury flows along the bottom of the cell and is made cathode (as shown in figure (a)). The brine solution flows in the same direction and anode consists of a number of graphite blocks. The brine electrolyzes and since, hydrogen has a high overvoltage at mercury cathode, sodium is preferentially discharged forming amalgam with mercury.
The sodium amalgam flows out and is reacted with water to give NaOH
$2NaHg+2H_{2}O\to 2NaOH+2Hg+H_2$
The mercury is re-circulated to the cell. Hydrogen and chlorine are the two important by-products.
(B) Solvay ammonia soda process :
(a) $2N{H}_3+C{O}_2+H_{2}O\to (N{H}_4{)}_{2}C{O}_3$
$CaC{l}_2+(N{H}_4{)}_{2}C{O}_2\to CaC{O}_3\downarrow +2N{H}_{4}Cl$
$MgC{l}_2+(N{H}_4{)}_{2}C{O}_3\to MgC{O}_3\downarrow +2N{H}_{4}Cl$
(b) $N{H}_3+C{O}_2+H_{2}O\to N{H}_{4}HC{O}_3;N{H}_{4}HC{O}_3+NaCl\stackrel{{30}^{o}C}{\to }NaHC{O}_3+N{H}_{4}Cl$
(c) Calcination to get sodium carbonate :
$2NaHC{O}_3\underset{\left(steam\right)}{\stackrel{{150}^{o}C}{}}N{a}_{2}C{O}_3+C{O}_2+H_{2}O$
(d) $N{H}_{4}HC{O}_3\underset{\left(steam\right)}{\stackrel{\Delta }{}}N{H}_3+C{O}_2+H_{2}O;2N{H}_{4}Cl+Ca(OH{)}_2\underset{\left(steam\right)}{\stackrel{\Delta }{}}2N{H}_3+2H_{2}O+CaC{l}_2$
(e) $CaC{O}_3\stackrel{1375k}{\to }CaO+C{O}_2$
$CaO+H_{2}O\to Ca(OH{)}_2$ (slaked lime)
(C) Hargreaves process :
$4NaCl\left(drylump\right)+2S{O}_2\left(g\right)+2H_{2}O\left(g\right)+O_2\left(g\right)\to 2N{a}_{2}S{O}_4+4HCl$.
(D) Le-Blanc process :
$NaCl\left(orKCl\right)+conc.H_{2}S{O}_4\to NaHS{O}_{4}orKHS{O}_4+HCl$
$NaHS{O}_4+NaCl\stackrel{\Delta }{\to }N{a}_{2}S{O}_4$ (as intermediate product) + HCl
$N{a}_{2}S{O}_4+CaC{O}_3+4C\stackrel{\Delta }{\to }N{a}_{2}C{O}_3$ or $K_{2}C{O}_3+CaS+4CO$