JEE Chemistry The P Block Elements Notes Set 11

Group 14 at a Glance

1. Group 14 contains carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb), with the outer configuration ns²np².
2. Carbon is a non-metal, silicon and germanium are metalloids, and tin and lead are metals.
3. Group 14 elements are smaller than group 13 elements of the same period, so their ionisation enthalpies and electronegativities are higher.
4. The covalent radius increases sharply from C to Si and only slightly after Si because of poor shielding by the inner d and f electrons.

Oxidation States and Inert Pair Effect

1. Carbon and silicon show mainly +4. Germanium is mostly +4 with few +2 compounds, tin shows both +2 and +4, and lead is more stable as +2.
2. Inert pair effect: the stability of the +2 state increases down the group, because the ns² electrons take part in bonding less readily. Sn(II) is a reducing agent (Sn²⁺ goes to Sn⁴⁺), and Pb(IV) compounds such as PbO₂ are strong oxidising agents.
3. PbO₂ is a stronger oxidising agent than SnO₂ because Pb²⁺ is more stable than Pb⁴⁺, while Sn⁴⁺ is more stable than Sn²⁺.
4. Most tetrahalides are covalent. PbF₄ and SnF₄ are exceptions and are ionic.
5. CCl₄ is not hydrolysed because carbon has no d orbitals and cannot expand its covalency beyond four, whereas SiCl₄ is readily hydrolysed: SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl.
6. Carbon shows the greatest catenation. The tendency falls in the order C–C > Si–Si > Ge–Ge > Sn–Sn > Pb–Pb because the bond enthalpy decreases as size increases.
7. Carbon forms pπ-pπ multiple bonds, but silicon is too large for effective sideways overlap, which is why no form of silicon resembles graphite.

Allotropes of Carbon

1. Diamond: every carbon is sp³ hybridised and bonded tetrahedrally to four others in a giant covalent network. It is extremely hard and an electrical insulator, so it is used as an abrasive and for cutting tools.
2. Graphite: every carbon is sp² hybridised, bonded to three others in flat hexagonal layers held together by weak van der Waals forces. The fourth electron is delocalised, so graphite conducts electricity along the layers and is soft and slippery.
3. Fullerenes (for example C₆₀) are cage molecules made of five- and six-membered carbon rings (12 pentagons and 20 hexagons in C₆₀), made by heating graphite in an electric arc in an atmosphere of helium or argon. They were discovered by Smalley, Curl and Kroto.
4. Silicon can be crystallised in a diamond-like structure; the crystalline form is a semiconductor.

Oxides of Carbon and Silicon

1. Carbon monoxide is formed when carbon burns in a limited supply of oxygen: 2C + O₂ → 2CO. On a large scale it is made from steam and hot coke: C + H₂O → CO + H₂ (water gas).
2. Producer gas is a mixture of CO and N₂ in the molar ratio 1 : 2, obtained by passing air over hot coke: 2C + O₂ + 4N₂ → 2CO + 4N₂. Both water gas and producer gas are used as industrial fuels.
3. CO is poisonous because it combines with haemoglobin to form carboxyhaemoglobin, which cannot carry oxygen. CO is a neutral oxide and a reducing agent: ZnO + CO → Zn + CO₂.
4. Carbon dioxide forms by complete combustion of carbon and fuels, and in the laboratory by CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O. It is the anhydride of carbonic acid (H₂CO₃ → H₂O + CO₂).
5. CO₂ turns lime water milky (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O), and the milkiness disappears with excess CO₂ because soluble calcium hydrogencarbonate forms: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂.
6. Excess CO₂ contributes to global warming because it absorbs heat radiated by the earth and radiates part of it back.
7. Silicon dioxide (silica) is a covalent three-dimensional network in which every silicon is bonded tetrahedrally to four oxygen atoms. Hydrofluoric acid dissolves it: SiO₂ + 4HF → SiF₄ + 2H₂O, and SiF₄ + 2HF → H₂SiF₆.

Silicates, Zeolites and Silicones

1. The basic unit of all silicates is the SiO₄⁴⁻ tetrahedron. Feldspar, zeolites, mica and asbestos are natural silicates.
2. Zeolites are aluminosilicates containing Na⁺, K⁺ or Ca²⁺. They are used for removing the permanent hardness of water and as catalysts.
3. Silicones are synthetic organosilicon polymers with the repeating unit (R₂SiO)n. They are made by hydrolysing alkyl chlorosilanes and then condensing: Si heated with CH₃Cl in the presence of copper gives CH₃SiCl₃, (CH₃)₂SiCl₂, (CH₃)₃SiCl and a little (CH₃)₄Si.
4. (CH₃)₂SiCl₂ gives linear chains, CH₃SiCl₃ gives cross-linked polymers, and (CH₃)₃SiCl is a chain terminator that limits the chain length.
5. Silicones are water-repellent, heat-resistant and chemically inert, and are used as sealants, greases, and in electrical insulation.