JEE Chemistry The P Block Elements Notes Set 13

Group 14: Carbon Family

Carbon is a non-metal, Si and Ge are metalloids, Sn and Pb are metals. Configuration is ns2 np2.

Catenation and Carbides

1. Catenation is the self-linking tendency of an element. It is greatest for carbon and falls down the group, following the bond energies C-C 348, Si-Si 297, Ge-Ge 260 and Sn-Sn 240 kJ per mole.
2. Ionic (salt-like) carbides are grouped by the hydrocarbon released on hydrolysis. C1 type: \( \ce{Al4C3 + 12H2O -> 4Al(OH)3 + 3CH4} \) and Be2C. C2 type: \( \ce{CaC2 + 2H2O -> Ca(OH)2 + C2H2} \). C3 type: \( \ce{Mg2C3 + 4H2O -> 2Mg(OH)2 + CH3C#CH} \) (propyne).
3. Covalent carbides are SiC and B4C. Interstitial carbides are formed by transition metals; carbon sits in the voids, the metal keeps its lustre and conductivity.
4. SiC (carborundum) is made from sand and coke at 2000 to 2500 C. It is diamond-like, very hard, inert and attacked by no acid except phosphoric acid. It is used as an abrasive.

Carbon Monoxide and Related Gases

1. CO burns with a blue flame. It reduces PdCl2 solution to black palladium: \( \ce{CO + PdCl2 + H2O -> CO2 + Pd + 2HCl} \).
2. CO is estimated with iodine pentoxide: \( \ce{I2O5 + 5CO -> I2 + 5CO2} \), and the iodine is titrated with thiosulphate. It is absorbed by ammoniacal cuprous chloride.
3. In the Mond process impure Ni is converted to volatile \( \ce{Ni(CO)4} \) at about 50 C, which decomposes at about 150 C to pure Ni.
4. Water gas (CO + H2) has a higher calorific value than producer gas (CO + N2 + H2), because N2 does not burn.
5. Teflon is poly(tetrafluoroethylene), made by polymerising \( \ce{CF2=CF2} \) (from \( \ce{CHCl3} \) with HF, then pyrolysis of \( \ce{CHClF2} \)). It withstands about 500 to 550 C.

Silicon, Silicones and Silica

1. Si is the second most abundant element in the crust (about 27 percent) after oxygen. It occurs as silica and silicates such as feldspar, kaolinite and asbestos.
2. Crude silicon: \( \ce{SiO2 + 2C -> Si + 2CO} \) in an electric furnace. Very pure silicon for semiconductors: reduce SiCl4 or SiHCl3 with H2, then zone refine.
3. Silicon is unreactive at room temperature except towards fluorine. It forms SiO2 with O2 at 1173 K and Si3N4 with N2 at 1673 K.
4. Silanes \( \ce{Si_nH_{2n+2}} \) exist only for SiH4 and Si2H6, obtained by treating magnesium silicide with acid. SiH4 is more reactive than CH4 because Si is less electronegative than H, Si has vacant d orbitals and is large, so a nucleophile can attack it easily.
5. CCl4 is not hydrolysed, but SiCl4 is, giving Si(OH)4 and HCl because Si can expand its octet using d orbitals.
6. Silicones are organosilicon polymers with Si-O-Si chains. Hydrolysis of \( \ce{R2SiCl2} \) gives linear (or cyclic) silicones; \( \ce{R3SiCl} \) is a chain stopper that fixes the chain length; \( \ce{RSiCl3} \) gives cross links and a harder 3-D polymer.
7. Silicones are electrical insulators and water repellents, are used in polishes, as antifoaming agents and lubricants.
8. Silica is a 3-D network of SiO4 tetrahedra with sp3 silicon, giving a high melting point. It is attacked only by HF: \( \ce{SiO2 + 4HF -> SiF4 + 2H2O} \), and fused with bases or carbonates gives silicates, as in glass making. Silica gel is a drying agent.

Tin and Lead

1. SnO and SnO2 are amphoteric. Tin dissolves in hot alkali giving stannate.
2. SnCl2 is a good reducing agent: \( \ce{Sn^2+ + 2Fe^3+ -> Sn^4+ + 2Fe^2+} \); it reduces Hg(II) to Hg, Cu(II) to Cu(I) and nitrobenzene to aniline. It hydrolyses, so anhydrous SnCl2 cannot be made from the hydrate.
3. A piece of tin is kept in SnCl2 solution because air oxidises Sn(II) to Sn(IV) and the metal reduces it back: \( \ce{SnCl4 + Sn -> 2SnCl2} \).
4. SnCl4.5H2O is called butter of tin (a mordant). \( \ce{(NH4)2SnCl6} \) is pink salt used in calico printing. SnS2 is mosaic gold.
5. Lead oxides: PbO, red lead \( \ce{Pb3O4} \) which is 2PbO.PbO2, sesquioxide \( \ce{Pb2O3} \) which is PbO.PbO2, and brown PbO2. Dilute nitric acid dissolves the PbO part of red lead and leaves PbO2 behind.
6. PbO2 is a strong oxidising agent: it oxidises SO2 to PbSO4 and Mn(II) to permanganate in nitric acid. Tetraethyl lead was the antiknock additive.