JEE Chemistry S Block Elements Notes Set 02

Alkaline Earth Metals (Group 2)

Group 2 (Be, Mg, Ca, Sr, Ba, Ra) has ns2 configuration and shows +2 state. They are harder and denser than alkali metals, with higher melting points (irregular variation: Be 1560 K, Mg 923 K, Ca 1115 K, Ba 1000 K). Their compounds are generally less soluble than those of group 1, because the higher charge gives greater lattice energy. Reducing power increases down the group, with Ba the strongest.

Anomalous Behaviour of Beryllium

Be, with a very small size and high polarising power, forms covalent compounds, forms complexes such as [BeF4]2-, and does not impart colour to the flame. BeH2 is a polymeric, electron-deficient covalent hydride, unlike the ionic hydrides of the other members. Be and Mg do not form peroxides in normal conditions. Be is amphoteric, dissolving in NaOH to give Na2[Be(OH)4] and H2, and it is passivated by concentrated nitric acid. Be resembles aluminium (diagonal relationship): both are amphoteric, both are passive in HNO3, both have chlorides that are Lewis acids, and both give carbides that react with water to give methane (Be2C and Al4C3). Mg burns in air to give MgO and Mg3N2, and Mg3N2 on hydrolysis gives NH3.

Important Compounds of Calcium

Quicklime (CaO): obtained by heating limestone; CaCO3 gives CaO + CO2. Slaked lime Ca(OH)2 is made by adding water to quicklime; a suspension is milk of lime and the clear solution is lime water, which turns milky with CO2 and clear again in excess CO2 due to soluble Ca(HCO3)2.
Plaster of Paris is CaSO4.1/2H2O, made by heating gypsum at about 393 K; when mixed with water, it takes up water to form gypsum and sets hard. Cement setting is also due to hydration and is exothermic.

Important Compounds of Sodium

Sodium carbonate by the Solvay (ammonia-soda) process: NaCl + NH3 + CO2 + H2O gives NaHCO3 (precipitated in cold brine) + NH4Cl; on heating, 2NaHCO3 gives Na2CO3 + H2O + CO2. CO2 and ammonia are recycled (NH3 is recovered from NH4Cl with Ca(OH)2), and CaCl2 is the by-product. Potassium carbonate cannot be made this way because KHCO3 is too soluble.
NaOH is made by electrolysis of brine in a membrane, diaphragm or mercury cell: Cl2 at the anode, H2 at the cathode. NaOH absorbs CO2 from air to give Na2CO3, so its solution loses strength on standing. NaHCO3 and NaOH cannot coexist in solution since they react to give Na2CO3. NaHCO3 and Mg(OH)2 are used in antacids.

Biological Importance

Na+ and K+ maintain osmotic balance and nerve impulse transmission (the sodium-potassium pump). Mg2+ activates ATP-using enzymes and is present in chlorophyll; Ca2+ is needed for bones, teeth, muscle contraction and blood clotting.