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General Characteristics of the Compounds of Alkali Metals
All the common compounds of the alkali metals are generally ionic in nature. General characteristics of some of their compounds are discussed here.
Oxides and Hydroxides
On combustion in excess of air, alkali metals forms normal oxides with formula M2O. They react with water to form corresponding hydroxides which are basic in nature.
M2O + H2O → 2 MOH
Alkali metals apart from lithium form peroxides in addition to normal oxides upon combustion with excess air. These peroxides produce hydroxides and H2O2 upon reacting with water.
M2O2 + 2 H2O → 2MOH + H2O2
(M = Na, K, Rb, Cs)
Except lithium and sodium, all the other alkali metals form superoxides also. These superoxides also gives basic hydroxides upon treatment with water.
2 MO2 + 2OH2 → 2 MOH + H2O2 + O2
(M = K, Rb, Cs)
Under appropriate conditions pure compounds M2O, M2O2 or MO2 may be prepared.
Properties of Oxides and Hydroxides:
The oxides and the peroxides are colourless when pure, but the superoxides are yellow or orange in colour. The peroxides are diamagnetic while the superoxides are paramagnetic. Sodium peroxide is widely used as an oxidising agent.
The hydroxides which are obtained by the reaction of the oxides with water are all white crystalline solids. The alkali metal hydroxides are strong bases. They dissolve in water with evolution of heat on account of intense hydration.
The alkali metal halides, MX, (X=F, Cl, Br, I) are colourless crystalline solids with high melting points. They can be prepared by the reaction of the appropriate oxide, hydroxide or carbonate with aqueous hydrohalic acid (HX).
As the electropositive character of alkali metal increases from Li to Cs, the ease with which the metals form halides increases from Li to Cs. All halides are ionic in nature except LiBr and LiI. Except LiF, all other halides are soluble in water. The low solubility of LiF in water is due to its high lattice enthalpy (small size of Li+ and F–). Due to the presence of covalent nature both LiBr and LiI are soluble in organic solvents.
Salts of Oxo-Acids
Alkali metals form salts with all the oxo-acids. Most of these salts are soluble in water and are thermally stable. As the electropositive character increases down the group, the stability of the carbonates and bicarbonates increases.
This is due to the decrease in polarising power of alkali metal cations. The carbonates (M2CO3) of alkali metals are remarkably stable up to 1273 K, above which they first melt and then eventually decompose to form oxides. However, Li2CO3 is considerably less stable and decomposes readily.
This is presumably due to large size difference between Li+ and CO32- which makes the crystal lattice unstable. Being strongly basic, alkali metals except lithium form solid bicarbonates. No other metal forms solid bicarbonates.
M2CO3 + CO2 + H2O → 2 MHCO3
(M = Na, K, Rb, Cs)
Important Compounds of Alkali Metals:
Sodium Carbonate Na2CO3.10H2O (Washing Soda):
Sodium carbonate is one of the important inorganic compounds used in industries. It is prepared by Solvay process. In this process, ammonia is converted into ammonium carbonate which then converted to ammonium bicarbonate by passing excess carbon dioxide in a sodium chloride solution saturated with ammonia.
The ammonium bicarbonate thus formed reacts with the sodium chloride to give sodium bicarbonate and ammonium chloride. As sodium bicarbonate has poor solubility, it gets precipitated. The sodium bicarbonate is isolated and is heated to give sodium carbonate. The equations involved in this process are,
2NH3 + H2O + CO2 → (NH4)2CO3
(NH4)2CO3 + H2O + CO2 → 2 NH4HCO3
NH4HCO3 + NaCl → NH4Cl + NaHCO3
2NaHCO3 → Na2CO3 + CO2 + H2O
The ammonia used in this process can be recovered by treating the resultant ammonium chloride solution with calcium hydroxide. Calcium chloride is formed as a by-product.
Sodium carbonate, commonly known as washing soda, crystallises as decahydrate which is white in colour. It is soluble in water and forms an alkaline solution. Upon heating, it looses the water of crystallisation to form monohydrate. Above 373 K, the monohydrate becomes completely anhydrous and changes to a white powder called soda ash.
Na2CO3.10H2O → Na2CO3.H2O + 9H2O
Na2CO3.H2O → Na2CO3 + H2O
- Sodium carbonate known as washing soda and is used mainly for laundering.
- It is also used in water treatment to convert the hard water to soft water.
- It is used in the manufacturing of glass, paper, paint etc…
Sodium Chloride NaCl (Cooking Salt or Table salt):
Sodium chloride is isolated by evaporation from sea water which contains 2.7 to 2.9% by mass. Approximately 50 lakh tons of salt are produced annually in India by solar evaporation. Crude sodium chloride can be obtained by crystallisation of brine solution which contains sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities. Pure sodium chloride can be obtained from crude salt as follows.
Firstly removal of insoluble impurities by filtration from the crude salt solution with minimum amount of water. Sodium chloride can be crystallised by passing HCl gas into this solution. Calcium and magnesium chloride, being more soluble than sodium chloride, remain in solution.
Sodium chloride melts at 1081K. It has a solubility of 36.0 g in 100 g of water at 273 K. The solubility does not increase appreciably with increase in temperature.
- It is used as a common salt or table salt for domestic purpose.
- It is used for the preparation of many inorganic compounds such as NaOH and Na2CO3
Sodium hydroxide is prepared commercially by the electrolysis of brine solution in Castner-Kellner cell using a mercury cathode and a carbon anode. Sodium metal is discharged at the cathode and combines with mercury to form sodium amalgam. Chlorine gas is evolved at the anode. The sodium amalgam thus obtained is treated with water to give sodium hydroxide.
Na+ + e– → Na(amalgam)
Cl– → ½ Cl2↑+ e–
2Na(amalgam) + 2H2O → 2NaOH + 2Hg + H2↑
Sodium hydroxide is a white, translucent and deliquescent solid, that dissolves in water to give a strong alkaline solution. It melts at 591 K. The sodium hydroxide solution at the surface reacts with the CO2 in the atmosphere to form Na2CO3
- Sodium hydroxide is used in the purification of bauxite (ore of Aluminium) and petroleum refining
- It is used in the textile industries for mercerising cotton fabrics
- It is used in the manufacture of soap, paper and artificial silks.
Sodium Bicarbonate NaHCO3 (Baking Soda):
Sodium hydrogen carbonate or sodium bicarbonate is used in baking cakes pastries etc. It is called so because it decomposes on heating to generate bubbles of carbon dioxide, leaving holes in cakes or pastries and making them light and fluffy.
This compound is prepared by saturating a solution of sodium carbonate with carbon dioxide. The white crystalline powder of sodium bicarbonate, being less soluble, precipitated out.
- Primarily used as an ingredient in baking.
- Sodium hydrogen carbonate is a mild antiseptic for skin infections.
- It is also used in fire extinguishers.