JEE Chemistry The P Block Elements Notes Set 18

Group 13: Boron Family

1. The group 13 elements B, Al, Ga, In and Tl have the configuration ns²np¹. Boron is a non-metal (hard, high melting) and the rest are metals. Boron's small size and high ionisation energy make its compounds covalent. Al is the most abundant metal in the crust.
2. Oxidation states: +3 is common; +1 becomes more stable down the group because of the inert pair effect, so Tl⁺ is more stable than Tl³⁺. Ga melts at 303 K (30 degrees C) and expands on solidification.
3. Boron halides BX₃ are electron deficient Lewis acids (acidity BBr₃ > BCl₃ > BF₃). AlCl₃ exists as the dimer Al₂Cl₆ with bridging chlorine atoms in the solid and in non-polar solvents, and in water it gives [Al(H₂O)₆]³⁺ and Cl^-. Preparation: B₂O₃ + 3C + 3Cl₂ gives 2BCl₃ + 3CO and Al₂O₃ + 3C + 3Cl₂ gives 2AlCl₃ + 3CO.
4. Oxides and hydroxides: B₂O₃ is acidic, Al₂O₃ and Al(OH)₃ are amphoteric (Al(OH)₃ dissolves in excess NaOH to give [Al(OH)₄]^- or AlO₂^-), and Ga₂O₃ is amphoteric. Orthoboric acid H₃BO₃ is a weak monobasic Lewis acid: B(OH)₃ + H₂O gives [B(OH)₄]^- + H⁺. On heating it gives metaboric acid HBO₂ and then boric oxide B₂O₃.
5. Diborane B₂H₆: boron is sp³ hybridised; two terminal BH₂ groups lie in a plane with two bridging hydrogens above and below (three-centre two-electron or banana bonds). It has four terminal B-H bonds (normal) and two bridges. Angles: terminal H-B-H about 120 degrees, bridging about 97 degrees. B₃N₃H₆ (borazine) is called inorganic benzene.
6. Borax Na₂B₄O₇.10H₂O gives a glassy bead (borax bead test) on heating, which is used to detect coloured metal ions. Boron carbide B₄C is extremely hard.

Group 14: Carbon Family

1. Group 14 elements C, Si, Ge, Sn and Pb have ns²np². Metallic character, atomic size and stability of the +2 state increase down the group (inert pair effect), so Pb²⁺ is more stable than Pb⁴⁺ and PbO₂ is a strong oxidiser.
2. Carbon shows catenation and p-pi bonding. Allotropes: diamond (sp³, hardest), graphite (sp², conducting layers) and fullerenes. Carbon suboxide C₃O₂ is linear (O=C=C=C=O).
3. CO is a neutral oxide and a reducing agent, formed by passing CO over solid NaOH at about 473 K to give sodium formate. Dry ice is solid CO₂, which sublimes at -78 degrees C. Oxalic acid with conc. H₂SO₄ gives CO and CO₂.
4. Silicon is the second most abundant element in the crust (in silica and silicates). SiO₂ is a covalent network solid. SiF₄ hydrolyses to give silicic acid and H₂SiF₆. Carborundum is SiC, very hard. Silicones are polymers with Si-O-Si chains.
5. Glass is a super-cooled liquid (amorphous solid). Crooke's glass absorbs UV light and pyrex (borosilicate) resists heat. Zeolites are aluminosilicates with a three-dimensional cage-like structure used as cation exchangers.
6. Tin and lead: white tin changes to grey tin below 286 K (tin pest), which crumbled the tin buttons of old uniforms. Solder is an alloy of Pb and Sn. Lead storage cells use Pb and PbO₂. Red lead Pb₃O₄ is a mixed oxide, 2PbO.PbO₂.
7. Tin with concentrated HNO₃ gives metastannic acid (hydrated SnO₂). Pb is not attacked by dilute H₂SO₄ or HCl because an insoluble coating forms.

Group 15: Nitrogen Family

1. Group 15 elements N, P, As, Sb and Bi have ns²np³. Metallic character increases down the group. Stability of the +3 state increases down the group (inert pair), while the basic character of the oxides increases: N₂O₃, P₂O₃ acidic, As₂O₃ amphoteric, Bi₂O₃ basic.
2. Nitrogen is inert because of the strong N-N triple bond. Pure N₂ is obtained by thermal decomposition of barium azide or ammonium nitrite, and ammonium dichromate gives Cr₂O₃ + N₂ + 4H₂O. Nitrogen forms NCl₃ but not NCl₅ because it has no d-orbitals; P forms PCl₃ and PCl₅ using 3d orbitals.
3. Oxides of nitrogen: N₂O (laughing gas, neutral, supports combustion), NO (neutral), N₂O₃, NO₂ (mixed anhydride giving nitrate and nitrite with alkali) and N₂O₅. The ONO bond angle is largest in NO₂⁺ (180 degrees, linear), then NO₂ (134 degrees), and smallest in NO₂^- (115 degrees).
4. Ammonia is made by heating NH₄Cl with Ca(OH)₂ and made by the Haber process (N₂ + 3H₂ gives 2NH₃). Catalytic oxidation of NH₃ over Pt gives NO in the Ostwald process, used for nitric acid. NH₃ over hot CuO gives N₂. In water it exists as NH₃, NH₄⁺ and OH^-. Hydrazoic acid HN₃ is the acidic hydride of nitrogen, and the azide ion N₃^- is a pseudohalide.
5. Nitric acid: concentrated HNO₃ decomposes on heating to NO₂, H₂O and O₂. Dilute nitric acid with Zn gives N₂O, with Cu gives NO, and with Mg or Mn in cold dilute acid, hydrogen. It oxidises P to H₃PO₄, S to H₂SO₄ and I₂ to HIO₃.
6. Phosphorus: white phosphorus is P₄ (tetrahedral, P-P-P angle 60 degrees), waxy, poisonous, burns spontaneously at about 303 K and is stored under water. Red phosphorus is polymeric and used in safety matches. White P with NaOH gives phosphine: P₄ + 3NaOH + 3H₂O gives PH₃ + 3NaH₂PO₂.
7. Oxoacids of phosphorus: H₃PO₂ (hypophosphorous, monobasic), H₃PO₃ (phosphorous, dibasic), H₃PO₄ (orthophosphoric, tribasic), HPO₃ (metaphosphoric, polymeric) and H₄P₂O₇ (pyrophosphoric). PCl₃ with water gives H₃PO₃, and P₂O₃ + 3H₂O gives 2H₃PO₃. Superphosphate of lime is a mixture of calcium dihydrogen phosphate and gypsum.