Important Compounds of Alkali Metals

Sodium carbonate, Na₂CO₃·10H₂O (washing soda). One of the most important inorganic industrial chemicals, made by the Solvay process. Ammonia is first converted to ammonium carbonate, which is then converted to ammonium bicarbonate by passing excess carbon dioxide into a sodium chloride solution that is saturated with ammonia. This ammonium bicarbonate reacts with the sodium chloride present to give sodium bicarbonate (which, being only poorly soluble, precipitates out) and ammonium chloride. The precipitated sodium bicarbonate is isolated and heated to give sodium carbonate:

2NH₃ + H₂O + CO₂ → (NH₄)₂CO₃

(NH₄)₂CO₃ + H₂O + CO₂ → 2NH₄HCO₃

NH₄HCO₃ + NaCl → NH₄Cl + NaHCO₃

2NaHCO₃ →(Δ)→ Na₂CO₃ + CO₂ + H₂O

The ammonia consumed in the process is recovered by treating the resulting ammonium chloride solution with calcium hydroxide (calcium chloride is formed as a by-product).

Properties: sodium carbonate, commonly called washing soda, crystallises as the white decahydrate, Na₂CO₃·10H₂O. It is water-soluble and gives an alkaline solution. On heating it first loses water of crystallisation to become the monohydrate, and above 373 K the monohydrate becomes completely anhydrous, turning into a white powder called soda ash: Na₂CO₃·10H₂O → Na₂CO₃·H₂O + 9H₂O, then Na₂CO₃·H₂O → Na₂CO₃ + H₂O.

Uses: mainly for laundering (hence "washing soda"); in water treatment, to convert hard water to soft water; and in manufacturing glass, paper, paint and similar products.

Sodium chloride, NaCl (cooking/table salt). Isolated by evaporating sea water, which contains 2.7-2.9% NaCl by mass -- around 50 lakh tonnes of salt are produced annually in India this way, by solar evaporation. The crude salt obtained by crystallising brine also contains sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities. To purify it: insoluble impurities are first removed by filtering the crude salt solution (using the minimum amount of water), and then pure sodium chloride is crystallised out by passing HCl gas through the filtrate -- the calcium and magnesium chloride impurities, being more soluble than NaCl, stay behind in solution. Sodium chloride melts at 1081 K and has a solubility of 36.0 g per 100 g of water at 273 K, a solubility that does not rise appreciably with temperature.

Uses: as common/table salt for domestic use, and as the raw material for preparing important inorganic compounds such as NaOH and Na₂CO₃.

Sodium hydroxide (caustic soda). Manufactured commercially by electrolysing brine (concentrated aqueous NaCl) in a Castner-Kellner cell, which uses a mercury cathode and a carbon anode. At the cathode, sodium metal is discharged and immediately combines with the mercury to form a sodium amalgam, while chlorine gas is liberated at the anode:

At cathode: Na⁺ + e⁻ → Na(amalgam)

At anode: Cl⁻ → ½Cl₂↑ + e⁻

The sodium amalgam thus obtained is then treated with water to give sodium hydroxide: 2Na(amalgam) + 2H₂O → 2NaOH + 2Hg + H₂↑.

Properties: a white, translucent, deliquescent solid that dissolves in water to give a strongly alkaline solution; it melts at 591 K. A sodium hydroxide solution's exposed surface reacts with atmospheric CO₂ to form a surface layer of Na₂CO₃. …