JEE Chemistry S Block Elements Notes Set 06

Alkali Metals: Group Trends

1. The group 1 elements Li, Na, K, Rb, Cs and Fr have the configuration ns¹. Down the group atomic and ionic radius, density (except K < Na), reactivity and metallic character increase; ionisation energy, melting point, hardness and electronegativity decrease. Cs is the most electropositive and has the lowest ionisation energy of the stable alkali metals.
2. Density: Li 0.53, Na 0.97 and K 0.86 g/cm³ are less dense than water (Li is the lightest metal), while Rb (1.53) and Cs (1.90) are denser. They are silvery soft metals stored under kerosene (Li in paraffin wax or oil because it floats on kerosene).
3. They impart flame colours: Li crimson red, Na golden yellow, K lilac (violet), Rb red-violet and Cs blue. Easy excitation of the outer electron is responsible. Alkali metals dissolve in liquid ammonia to give blue, conducting, reducing solutions.
4. Standard electrode potentials are very negative (Li -3.05 V is the lowest, Na -2.71 V), so they are strong reducing agents. Li has the most negative E° because of its very high hydration energy.
5. Hydration energy and ionic mobility: Li⁺ is the most hydrated ion (smallest), so it has the largest hydrated radius and the lowest ionic mobility in water: Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺.

Chemical Behaviour of Alkali Metals

1. With oxygen: Li gives the normal oxide Li₂O, Na gives the peroxide Na₂O₂ (mainly) and K, Rb, Cs give the superoxides MO₂ (paramagnetic, coloured). Superoxides and peroxides react with water to give H₂O₂ and O₂.
2. With water: 2M + 2H₂O gives 2MOH + H₂ (increasingly violent from Li to Cs). With nitrogen only lithium forms the nitride Li₃N directly. With hydrogen they form ionic hydrides MH. Na in dry NH₃ gives sodamide: 2Na + 2NH₃ gives 2NaNH₂ + H₂.
3. Hydroxides are strong bases and increase in basic strength down the group: LiOH < NaOH < KOH < RbOH < CsOH. Carbonates (except Li₂CO₃) are stable to heat. Li₂CO₃ gives Li₂O + CO₂.
4. Solubility and thermal stability: LiF, LiOH and Li₂CO₃ are sparingly soluble because of high lattice energy, unlike the corresponding salts of the other alkali metals. Nitrates of Na and K give nitrites and O₂ on heating, while LiNO₃ gives Li₂O + NO₂ + O₂.
5. Li differs from the rest and resembles Mg (diagonal relationship): it forms a nitride, its hydroxide and carbonate decompose on heating, its chloride is deliquescent and soluble in ethanol, and it does not form solid bicarbonate.

Important Compounds of Sodium

1. Sodium carbonate (washing soda, Na₂CO₃.10H₂O) is made by the Solvay process: NaCl + NH₃ + CO₂ + H₂O gives NaHCO₃ + NH₄Cl; NaHCO₃ is then heated to Na₂CO₃. CO₂ and NH₃ are recycled and CaCl₂ is the by-product. On heating, washing soda loses water of crystallisation to give anhydrous soda ash. Its solution is alkaline by hydrolysis.
2. Sodium bicarbonate (baking soda, NaHCO₃) is weakly alkaline, gives CO₂ on heating and is used in fire extinguishers and baking powder (mixed with potassium hydrogen tartrate or a mild acid).
3. Caustic soda (NaOH) is made by the electrolysis of brine in the Nelson (diaphragm) cell or the Castner-Kellner (mercury) cell; Cl₂ is obtained at the anode and H₂ at the cathode. NaOH is deliquescent and absorbs CO₂ from the air. With CO at 200 degrees C it gives sodium formate.
4. Glauber's salt is Na₂SO₄.10H₂O, salt cake is anhydrous Na₂SO₄, nitre or saltpetre is KNO₃, and Chile saltpetre is NaNO₃. Potash alum and Mohr's salt are examples of double salts.
5. Biological roles: Na⁺ and K⁺ maintain the ionic balance and osmotic pressure in cells and drive the sodium-potassium pump for nerve impulses.

Alkaline Earth Metals: Group Trends

1. The group 2 elements Be, Mg, Ca, Sr, Ba and Ra have the configuration ns². They are harder, denser and have higher melting points than alkali metals because two electrons take part in metallic bonding. Hydration energies are high (Be²⁺ highest).
2. Flame colours: Be and Mg none, Ca brick red, Sr crimson red, Ba apple green. Sr salts are used in red fireworks, and Ba salts in green ones. Down the group the basic strength and solubility of the hydroxides increase, the thermal stability of the carbonates increases, and the solubility of the sulphates decreases.
3. Solubility of sulphates in water decreases down the group: BeSO₄ > MgSO₄ > CaSO₄ > SrSO₄ > BaSO₄. Hydroxides: Mg(OH)₂ is sparingly soluble, Ba(OH)₂ is quite soluble.
4. Beryllium is anomalous: it has a small size and high electronegativity and resembles aluminium (diagonal relationship). Be(OH)₂ is amphoteric, BeCl₂ is covalent and its oxide is amphoteric. Beryllium does not give a flame colour and is not attacked by water. Be is obtained by electrolysis of fused BeCl₂ (with NaCl).
5. Calcium is obtained by electrolysis of molten CaCl₂. Mg burns in air giving MgO and Mg₃N₂. Group 2 carbides and nitrides are hydrolysed to give C₂H₂ (from CaC₂) and NH₃ (from Mg₃N₂).

Important Compounds of Calcium and Magnesium

1. Quicklime CaO is made by heating limestone: CaCO₃ gives CaO + CO₂. Slaked lime Ca(OH)₂ is formed when CaO is treated with water; its aqueous solution is lime water, which turns milky with CO₂ and clears again with excess CO₂ (formation of Ca(HCO₃)₂).
2. Bleaching powder CaOCl₂ (Ca(OCl)Cl) is made by passing Cl₂ over dry slaked lime. It releases chlorine with dilute acids, acts as an oxidising and bleaching agent, and is used in water disinfection.
3. Gypsum CaSO₄.2H₂O on heating at about 393 K gives plaster of Paris 2CaSO₄.H₂O (half a water molecule per CaSO₄), which sets to a hard mass of gypsum on adding water. The difference in water between gypsum and plaster of Paris is 1.5 molecules per CaSO₄.
4. Cement is a mixture of calcium silicates and aluminates (CaO, SiO₂, Al₂O₃, Fe₂O₃) made by heating limestone and clay, with gypsum added to slow setting. Sorrel cement is a mixture of MgCl₂ and MgO. Lithopone is a white pigment, a mixture of BaSO₄ and ZnS.
5. Anhydrous MgCl₂ cannot be obtained by simply heating the hydrate (hydrolysis occurs); it is made by heating the hydrate in a stream of dry HCl gas. Magnesia is MgO, and MgO is used as a refractory lining in furnaces.
6. Biological role: Mg²⁺ is present in chlorophyll and Ca²⁺ in bones, teeth and in muscle contraction and blood clotting.