Group 14 Flashcards
Elements?
Carbon behaves as non metal, silicon germanium metalloids, tin and lead metallic, tin shows some amphoteric behaviour dissolving in both acid and alkali to generate acid, all have stable octets and therefore electron precise
Inert pair effect?
+4 oxidation state becomes less important with respect to the +2 oxidation state as descended down the group
Catenation?
Ability of an element to form covalent bonds with itself to give chains or rings - particularly important for carbon, catenation becomes less important as the group is descended partly due to the decreasing bond enthalpies of the X-X bonds and partly because the heavier atoms are larger and have orbitals that are available for reaction
Multiple bonds?
Double and triple bonds are also more stable for carbon that for other group 14 elements, for heavier members of group 14 multiply bonded compounds are much rarer because of the distance sensitivity of pi overlap
Allotropes for carbon?
Graphite, diamonds and the fullerenes, graphite most stable of these at room temperature and pressure
Silicon?
Elemental silicon obtained by reduction of SiO2 with coke in an electric furnace. Silicon is a semiconductor and it conductivity is strongly affected by impurities. Silicon is purified by converting it into SiHCl3, SiH4, SiCl4 which is distilled and heated to decompose it back to silicon
Si + 3HCl –> SiHCl3 + H2
2SiHCl3 –> Si + SiCl4 + 2HCl
recrystallised by dipping a crystal seed into molten silicon then pulling the growing crystal upwards
Carbon oxides?
CO, CO2 and C3O2 carbon suboxide lower stability
CO and CO2 are colourless, odourless gases under standard conditions and they are formed by the combustion of carbon or organic material
C + O2 –> CO2 in excess oxygen
2C + O2 –> 2CO in limited oxygen
Carbon dioxide thermodynamically favoured product of oxidation reactions, formed when combustion occurs. Carbon monoxide toxoid binds to Fe in haemoglobin preventing transportation of oxygen round the body
Carbon dioxide in water?
Carbon dioxide dissolves in water and is present in solution in equilibrium with carbonic acid H2CO3 and the H3O+, HCO3- and CO32- ions
CO2 + H2O H2CO3 H3O+ + H2O + HCO3- CO32- + H3O +
Silicon oxides?
Silicon dioxide SiO2 forms a range of covalent network structure all of which contain Si-O bonds, quartz most common form
Si-O bonds and C-O bonds?
Covalent network structures of SiO2 are very different from the discrete molecules formed by CO2. This difference is related to the relative strengths of the single and double bond to oxygen. When calculated to form C-O network structure is highly exothermic so silica network like structure for carbon dioxide is highly unflavoured with respect to gaseous molecules
Si-O the silicon network structure is strongly endothermic so is more favoured
Key factors in difference in structure are high strength of C=O bond compare to Si=O bond which arises from the distance sensitivity of the pi bonding and the high strength of the
Si-O bond compared to the C-O bond which arises largely as a consequence of the greater ionic character of the Si-O bond, calculation use enthalpy rather than gibbs energy but this can be used to show a general trend rather than include entropy
SiO2 and acids?
With exception of hydrofluoric acid it does not react with acids.
On heating with sodium carbonate SiO2 is converted to sodium silicate Na4SiO4
SiO2 + 2Na2CO3 –> Na4SiO4 + 2CO2
Silicates?
SiO2 + Na2CO3 —> Na4SiO4 + 2CO2 produces sodium silicate
Silicates react with acids to form a precipitate of hydrated silicon dioxide known as silica gel
SiO4 4- + 4H3O+ + (x-6)H2O –> SiO2.XH2O
can be used as a drying agent due to large surface area
Silicon forms many extended silicate anions that are based on SiO4 tetrahedra with oxygen atoms shared between adjacent tetrahedra the tetrahedra can link together to form cyclic anions, extended chains or extended sheets. In aluminosilicates some of the silicon atoms have been exchanged for aluminium atoms, good conductors used in microchips, good thermal properties used in advanced ceramics
Silicones?
Silicones are prepared by the hydrolysis of silicon chlorides such as Me2SiCL2. They hydrolysis initially forms Me2Si(OH)2 which then undergoes condensation polymerisation to form the silicone. Silicone chains can contain four different structural units which different in the number of the Si-O bonds they contain. All have a strong Si-O bond polymer backbone and this give good thermal stability. Properties depends on nature of substituents and chain length eg longer chain length the more viscous the silicone. Can be used as lubricants waterproof coatings and insulators. Cross linking of chains gives silicon rubbers maintain strength and flexibility over a wide temperature range and are used in adhesives and sealants, extensive cross linking gives rise to a resin
Carbon halides?
Carbon tetrahalides all covalent molecules with tetrahedral geometries, C-X bonds are polar but the dipoles cancel out in the molecules so carbon tetrahalides are non polar solvents, under standard conditions CF4 is a gas, CCl4 liquid, both are inert and CBr4 and CI4 solids
Stability of carbon tetrahalides?
Decrease as halogen atom becomes larger since C-X bond enthalpy decreases in the order
C-F > C-Cl > C-Br > C-I
consistent with reception in orbital overlap as the halogen atomic orbitals become larger and more diffuse
