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Concept 04 / 05The s-Block Elements

Important Compounds of Sodium: Na2CO3, NaCl, NaOH and NaHCO3

Manufacture and properties of sodium carbonate by the Solvay process, washing soda and its hydrates, purification of common salt, sodium hydroxide by the Castner-Kellner cell, sodium hydrogencarbonate, and their principal uses.

Figure 1
Sodium carbonate (washing soda, Na2CO3⋅10H2O\mathrm{Na_{2}CO_{3} \cdot 10H_{2}O}). Made by the Solvay (ammonia-soda) process (see figure). Carbon dioxide is passed into brine saturated with ammonia: 2NH3+H2O+CO2→(NH4)2CO3\mathrm{2NH_{3} + H_{2}O + CO_{2} \rightarrow (NH_{4})_{2}CO_{3}} (NH4)2CO3+H2O+CO2→2NH4HCO3\mathrm{(NH_{4})_{2}CO_{3} + H_{2}O + CO_{2} \rightarrow 2NH_{4}HCO_{3}} NH4HCO3+NaCl→NH4Cl+NaHCO3\mathrm{NH_{4}HCO_{3} + NaCl \rightarrow NH_{4}Cl + NaHCO_{3}} (precipitates, being sparingly soluble) 2NaHCO3→Na2CO3+CO2+H2O\mathrm{2NaHCO_{3} \rightarrow Na_{2}CO_{3} + CO_{2} + H_{2}O} (on heating). Ammonia is recovered: 2NH4Cl+Ca(OH)2→2NH3+CaCl2+2H2O\mathrm{2NH_{4}Cl + Ca(OH)_{2} \rightarrow 2NH_{3} + CaCl_{2} + 2H_{2}O}, with Ca(OH)2\mathrm{Ca(OH)_{2}} and CO2\mathrm{CO_{2}} from limestone. The only by-product is CaCl2\mathrm{CaCl_{2}}. The process cannot make K2CO3\mathrm{K_{2}CO_{3}}, because KHCO3\mathrm{KHCO_{3}} is too soluble to precipitate. Properties: the decahydrate effloresces; on heating it loses water to the monohydrate, which becomes anhydrous (soda ash) above 373 K373\,\mathrm{K}. Its solution is alkaline by hydrolysis of CO32−\mathrm{CO_{3}^{2-}}. Uses: water softening, laundering, glass, soap, borax and caustic soda manufacture, laboratory reagent. Sodium chloride. Obtained from sea water (about 2.72.7 to 2.9%2.9\% by mass) by evaporation. Crude salt contains CaSO4\mathrm{CaSO_{4}}, Na2SO4\mathrm{Na_{2}SO_{4}}, CaCl2\mathrm{CaCl_{2}} and MgCl2\mathrm{MgCl_{2}} (the last two are deliquescent, making crude salt damp). Purification: dissolve in minimum water, filter, and pass HCl gas; pure NaCl crystallises by the common ion effect, while the more soluble Ca and Mg chlorides stay in solution. Sodium hydroxide (caustic soda). Made by electrolysis of brine in the Castner-Kellner cell, with a mercury cathode and carbon anode: cathode: Na++e−+Hg→Na-amalgam\mathrm{Na^{+} + e^{-} + Hg \rightarrow Na\text{-}amalgam}; anode: Cl−→12Cl2+e−\mathrm{Cl^{-} \rightarrow \frac{1}{2}Cl_{2} + e^{-}}. The amalgam is treated with water: 2Na-amalgam+2H2O→2NaOH+2Hg+H2\mathrm{2Na\text{-}amalgam + 2H_{2}O \rightarrow 2NaOH + 2Hg + H_{2}}. The mercury cathode prevents sodium from reacting with water at the electrode and keeps chlorine and alkali apart. (Modern plants prefer membrane cells to avoid mercury pollution.) Properties: white, deliquescent, very soluble with evolution of heat; absorbs CO2\mathrm{CO_{2}} from air to form Na2CO3\mathrm{Na_{2}CO_{3}}. Uses: soap, paper, artificial silk, petroleum refining, purification of bauxite, mercerising cotton. Sodium hydrogencarbonate (baking soda, NaHCO3\mathrm{NaHCO_{3}}). Made by saturating sodium carbonate solution with CO2\mathrm{CO_{2}}: Na2CO3+H2O+CO2→2NaHCO3\mathrm{Na_{2}CO_{3} + H_{2}O + CO_{2} \rightarrow 2NaHCO_{3}}. On heating it releases CO2\mathrm{CO_{2}}, which makes dough rise. Uses: baking powder, mild antiseptic, antacid, fire extinguishers. Key Takeaways: - Solvay: NaHCO3\mathrm{NaHCO_{3}} precipitates because it is sparingly soluble; heating gives Na2CO3\mathrm{Na_{2}CO_{3}}; NH3\mathrm{NH_{3}} is recycled. - K2CO3\mathrm{K_{2}CO_{3}} cannot be made by the Solvay process. - Castner-Kellner: mercury cathode forms sodium amalgam; NaOH, H2\mathrm{H_{2}} and Cl2\mathrm{Cl_{2}} are co-products. - Pure NaCl: pass HCl into saturated solution (common ion effect). - Pitfall: washing soda is Na2CO3⋅10H2O\mathrm{Na_{2}CO_{3} \cdot 10H_{2}O}; baking soda is NaHCO3\mathrm{NaHCO_{3}}. Brain-Teaser 1: What maximum mass of Na2CO3\mathrm{Na_{2}CO_{3}} can be produced from 117 g117\,\mathrm{g} of NaCl by the Solvay process? Solution: 117 g117\,\mathrm{g} NaCl =2 mol= 2\,\mathrm{mol}, giving 2 mol2\,\mathrm{mol} NaHCO3\mathrm{NaHCO_{3}} and then 1 mol1\,\mathrm{mol} Na2CO3=106 g\mathrm{Na_{2}CO_{3}} = 106\,\mathrm{g}. Brain-Teaser 2: 2.86 g2.86\,\mathrm{g} of washing soda is heated strongly to constant mass. What is the residue? Solution: Na2CO3⋅10H2O\mathrm{Na_{2}CO_{3} \cdot 10H_{2}O} (M=286M = 286): 0.01 mol0.01\,\mathrm{mol}. Strong heating removes all water, leaving 0.01 mol0.01\,\mathrm{mol} Na2CO3=1.06 g\mathrm{Na_{2}CO_{3}} = 1.06\,\mathrm{g}; the carbonate itself does not decompose. Brain-Teaser 3: Why is a mercury cathode used in the Castner-Kellner cell instead of iron? Solution: At an ordinary cathode, water (not Na+\mathrm{Na^{+}}) would be reduced, giving H2\mathrm{H_{2}}, and the NaOH formed would mix with chlorine. Hydrogen has a high overvoltage on mercury, so Na+\mathrm{Na^{+}} is discharged instead and dissolves in mercury as an amalgam, which is decomposed with water in a separate compartment to give pure NaOH.

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Important Compounds of Sodium: Na2CO3, NaCl, NaOH and NaHCO3 — DPP

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