JEE Chemistry The P Block Elements Notes Set 10

Group 13 at a Glance

1. Group 13 contains boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl). Their outer configuration is ns²np¹.
2. Boron is a non-metal (a metalloid) and the others are metals. The metallic character increases down the group because ionisation energy falls.
3. Boron shows a diagonal relationship with silicon: both are hard semiconductors, form only covalent compounds, exist in crystalline and amorphous forms, and form acidic oxides (B₂O₃, SiO₂) that dissolve in alkali.
4. The chlorides of both are hydrolysed by water to the oxoacid: BCl₃ + 3H₂O → H₃BO₃ + 3HCl and SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl.

Atomic Properties and Trends

1. Group 13 atoms are smaller than the group 2 atoms in the same period because the nuclear charge is higher and the added electron enters the same shell.
2. Atomic radius does not rise smoothly: Al (143 pm) is larger than Ga (135 pm) because the ten d electrons of Ga screen the nucleus poorly.
3. First ionisation energy falls sharply from B to Al, then rises slightly at Ga because of poor d-electron shielding. It is lower than that of the corresponding group 2 element.
4. The third ionisation energy of thallium is very high, so Tl prefers +1.
5. The common oxidation states are +3 and +1. The stability of +1 increases in the order Al < Ga < In < Tl because of the inert pair effect (the ns² pair is reluctant to take part in bonding).
6. Boron and aluminium form covalent compounds because the sum of their three ionisation energies is very high, so they do not form 3+ ions easily.

Boranes and Diborane

1. None of the group 13 elements reacts directly with hydrogen. Boron forms many hydrides called boranes, with the general formulas B_nH_n+4 (nido) and B_nH_n+6 (arachno).
2. Diborane is made industrially by 2BF₃ + 6LiH → B₂H₆ + 6LiF. In the laboratory, 2NaBH₄ + I₂ → B₂H₆ + 2NaI + H₂, or 4BCl₃ + 3LiAlH₄ → 2B₂H₆ + 3AlCl₃ + 3LiCl.
3. Structure: each boron is sp³ hybridised. There are four terminal B–H bonds (2-centre 2-electron bonds) and two bridging hydrogens that form 3-centre 2-electron bonds. The bridge B–H bonds are longer than the terminal bonds.
4. Lower boranes are colourless gases and higher boranes are volatile liquids or solids. They catch fire in air because boron has a strong affinity for oxygen: B₂H₆ + 3O₂ → B₂O₃ + 3H₂O (large heat release).
5. Diborane is hydrolysed by water: B₂H₆ + 6H₂O → 2B(OH)₃ + 6H₂.
6. With alkali metal hydrides in ether it gives borohydrides: B₂H₆ + 2MH → 2M⁺[BH₄]⁻.
7. With ammonia, 3B₂H₆ + 6NH₃ → 3[BH₂(NH₃)₂]⁺[BH₄]⁻ at low temperature, and on heating with excess NH₃ gives borazine: 3B₂H₆ + 6NH₃ → 2B₃N₃H₆ + 12H₂. Borazine is a ring like benzene and is called inorganic benzene.
8. Aluminium hydride (AlH₃)n is a polymer with Al–H–Al bridges. LiAlH₄ is made by 4LiH + AlCl₃ → LiAlH₄ + 3LiCl in ether and is a strong reducing agent.

Halides, Oxides and Hydroxides

1. Boron trihalides are monomeric, electron-deficient and act as Lewis acids by accepting an electron pair (for example BF₃ + :NH₃ → F₃B←NH₃).
2. Boron is too small to hold four large halide ions, so BCl₃ does not dimerise, but aluminium chloride exists as the dimer Al₂Cl₆ with two bridging chlorine atoms, and anhydrous AlCl₃ is an important Lewis acid catalyst.
3. In AlCl₃ every aluminium reaches an octet in the dimer. Molten AlBr₃ is a poor conductor of electricity because it is covalent.
4. B₂O₃ and B(OH)₃ are acidic. Al₂O₃ and Al(OH)₃ are amphoteric, reacting with both acids and bases. For example 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂.
5. B₂O₃ dissolves in alkali to give borates: B₂O₃ + 6NaOH → 2Na₃BO₃ + 3H₂O.

Borax and Boric Acid

1. Borax is Na₂[B₄O₅(OH)₄]·8H₂O (written Na₂B₄O₇·10H₂O). In water it hydrolyses: Na₂B₄O₇ + 7H₂O → 2NaOH + 4H₃BO₃, so the solution is alkaline and borax acts as a cleansing agent.
2. On heating, borax loses water, swells and fuses to a clear glassy bead: Na₂B₄O₇ → 2NaBO₂ + B₂O₃. The bead dissolves coloured metal oxides to form metaborates, which is the borax bead test (cobalt gives blue Co(BO₂)₂).
3. Orthoboric acid, B(OH)₃, is made by acidifying borax solution: Na₂B₄O₇ + H₂SO₄ + 5H₂O → 4H₃BO₃ + Na₂SO₄.
4. In solid boric acid, planar B(OH)₃ units are joined by hydrogen bonds to form layers, giving a flaky, soapy solid.
5. Boric acid is a weak monobasic Lewis acid. It accepts OH⁻ from water: B(OH)₃ + 2H₂O → [B(OH)₄]⁻ + H₃O⁺.
6. On heating, boric acid loses water in steps: H₃BO₃ → HBO₂ (metaboric acid) → H₂B₄O₇ (tetraboric acid) → B₂O₃.
7. Boron is used as a neutron absorber in nuclear reactors (control rods) because it captures neutrons.

Anomalous Behaviour of Boron and Aluminium Compounds

1. Boron is a hard non-metallic poor conductor with a very high melting point; aluminium is a soft metal and a good conductor.
2. Boron forms only covalent compounds, aluminium forms some ionic compounds as well.
3. Boron halides are monomers; aluminium halides are dimers. Boron hydrides are fairly stable and aluminium hydride is polymeric and less stable.
4. In water the +3 ions of Al, Ga and In exist as octahedral aqua ions [M(H₂O)₆]³⁺. Aluminium sulphate forms double salts called alums, M₂SO₄·Al₂(SO₄)₃·24H₂O, where M is Na⁺, K⁺ or NH₄⁺.
5. Aluminium alloys (with Cu, Mn, Mg, Si) are used in aircraft, packaging and utensils, and aluminium is used as a cable conductor and in the thermite process for Cr and Mn.
6. Aluminium utensils should not be kept in water for long periods because the protective oxide layer is slowly attacked in the presence of air and moisture.