GROUP IV

General Characteristics C non-metal Si Ge Sn Pb All elements can form 4 covalent bonds in compounds. All elements form covalent hydrides (MH4) and covalent chlorides (MCl4). Carbon and silicon do not form ionic compound (except a few metal carbides containing C4- ion). Going down the group, atomic size increase and can lose electron to form cations. Metalloids Metal

Physical properties of the elements. #briggs 270

Going down the group, atomic radius increase  interatomic bonding becomes weaker  attaraction of neighbouring nuclei for intervening electrons gets less. This results in change in bonding from covalent to metallic. Change in structure : C, Si, Ge : Giant molecular Sn, Pb : Giant metallic M.p decreases from C to Ge – due to increase in atomic radius  bond length increase  weaker covalent bond. Hence the melting point and boiling point decreases going down the group.

* Graph of M.P

Electrical conductivity increases going down the group. This is due to increase of metallic character. Pure Si and Ge conduct electricity slightly. Conductivity can be increases by adding impurities  semiconductors. Elements Conductivity Explanation C diamond graphite poor good - no free electrons - all used for bonding. - one electron per carbon is not used for bonding and joins delocalised cloud. Si semiconductor Ge Sn metallic bonding - delocalised electron cloud Pb

* Graph of electrical conductivity

Tetrachlorides All Group IV elements form chlorides with the formula XCl4. Tetrachloride molecules are held together by weak Van der Waals forces. Low boiling point, liquid at room temperature. Going down the group, boiling point of the compound increase (volatility decrease)  number of electrons increase, stronger Van der Waals forces. Shape : Tetrahedral

yellow liquid white solid gas Thermal stability of tetrachlorides decrease down the group due to atomic radius of Group IV atom increase, weaker covalent bonds. CCl4 is very stable to heat. PbCl4 decompose slowly at room temperature to PbCl2 and Cl2 PbCl4  PbCl2 + Cl2 yellow liquid white solid gas CCl4, SiCl4 and GeCl4 are stable to heat, even at high temperature. SnCl4 will decompose on strong heating.

Reaction of tetrachlorides with water. CCl4 does not react with water – no empty orbitals to accept lone pair from oxygen of the water molecule. SiCl4 hydrolysed in cold water to give an acidic solution: SiCl4 + 2H2O  4HCl + SiO2 white fumes GeCl4 hydrolysed by water : GeCl4 + 2H2O  4HCl + GeO2 SnCl4 and PbCl4 are only partially hydrolysed by water.

Oxides of Group IV elements Group IV can exist in two oxidation states to form oxides ( +2 and +4). Ground state : Excited state : Relative stability of higher and lower oxidation states of the elements. Going down the group, the oxidation +4 become less stable. From C to Sn, most stable oxide XO2. But Pb is most stable oxidation state +2. most stable oxide is PbO. This is due to ‘inert pair effect’ where the 2 outer s electrons remains relatively stable and unreactive.

Ge : +4 rather more stable Sn : +4 slightly more stable C : +4 much more stable Si : +4 more stable Ge : +4 rather more stable Sn : +4 slightly more stable Pb : +2 much more stable. Stability of +4 over +2 oxidations state decrease.

E.g of reactions showing relative stabilities of oxides. 1) Oxidation of CO to CO2. CO + ½ O2  CO2 H = very exothermic 2) PbO2 decomposes on heating to form more stable PbO and O2 gas. PbO2  PbO + ½ O2 3) PbCl4 is thermally unstable PbCl4 ——> PbCl2 + Cl2

E [Sn4+/Sn2+] has a small positive value  less tendency for Sn4+ to be reduced to Sn2+. Sn4+ is more stable. E [Pb4+/Pb2+] has a high positive value  high tendency for Pb4+ to be reduced to Pb2+. Pb2+ is more stable. PbO2 is a powerful oxidising agent. * Refer to word document attachment for properties and reaction of Group IV oxides.*

Silicon(IV) oxide – SiO2 Properties of silica bases ceramic: good electrical insulators good thermal insulators – high m.p and b.p. have great rigidity are hard – due to rigid tetrahedral arrangment. Uses : furnace linings glasses for solar panels power line insulators parts of turbines pottery porcelain