Group 18 Flashcards
Elements?
All have filled octet, all present to some extent in atmosphere, neon argon, krypton, xenon and radon all obtained form fractional distillation of air, helium occurs in natural gas deposits formed as a result of radioactive decay, helium has lowest boiling point and at atmospheric pressure does not form a solid even when cooled to temperature of abolsute zero, liquid helium used as coolant has no viscosity can move rapidly through any pore
Group 18 compounds?
High enthalpy changes of ionisation but thee decrease with increasing size of the atom, this means that the reactivity of the elements increases dow the group and this is witnessed by the range of compounds that the elements form. Xenon has the most extensive chemistry and the majority of compounds are those with the most electronegative elements fluorine and oxygen. No stable compounds of helium or neon are known and the only compound of argon to have been prepared is HArD which is only stable below -246 degrees. Krypton forms the stable KrF2 and compounds have been isolated that contains KrF+ and Kr2F3+
Xenon fluorides?
XeF2, XeF4, XeF6 Xe + F2 --> XeF2 Xe + 2F2 --> XeF4 Xe + 3F2 --> XeF6 XeF2 molecules are linear, XeF4 are square planar, both consistent with VSEPR predictions, XeF6 does not form a structure based on a pentagonal bipyramid instead in the gas phase it has a distorted octahedral shape caused by the lone pair of electrons being directed through one of the triangular faces of the octahedra. The lone pair moves around the octahedra so the molecule is fluxional. In the solid state xenon hexafluoride is ionic with the formula XeF5+F-
Other xenon fluorides?
All strong fluorination agents and act as a fluoride ion donors and acceptors. XeF2 reacts with AsF5 to form an ionic compound containing the XeF+ cation
XeF2 + AsF5 —> XeF+AsF6-
Other xenon fluoride ions such as Xe2F3+, XeF5+ and XeF82+ and XeF5- are also known
Xenon oxides?
Formed by hydrolysis of xenon fluorides
6XeF4 + 12H2O —> 2XeO3 + 4Xe + 3O2 + 24HF
this is a disproportionation reactions where xenon(IV) reacts to given xenon (VI) and xenon (0), xenon trioxide is an explosive white solid, it is stable in aqueous solution but reacts with hydroxide ions to form HXeO4-
XeO3 + OH- —> HXeO4-
This ion is unstable with respect to disproportionation and decomposes to form XeO6- which contains xenon (VIII) and xenon gas
HXeO4- + 2OH- —> XeO64- + 4Xe + O2 + 2H2O
xenon tetraoxide is also known but it decomposes explosively to xenon and O3, HXeOXeH has been prepared
Other xenon compounds?
xenon compounds with bonds to carbon, nitrogen, chlorine and metal atoms have all been characterised. Typically electron withdrawing groups are needed on the carbon or nitrogen substitutes in order to stabilise the compounds, for exmaple the phenyl cation C6H5Xe+ is unknown but the C6F5Xe+ exists
p block group 13 organometallic chemistry?
metals and metalloids of the p block form organometallic compounds many of which are useful in synthesis and in industry. In group 13 the aluminium organometallic compounds are particularly useful and triethylaluminium is used with TiCl4 to form catalysts for the polymerisation of ethene and propene
Trimethylaluminium is a volatile reactive liquid that is flammable inner and explodes on contact with water, it has a structure consisting of Al2Me6 dimers, although the structure is superficially similar to that of Al2Cl6, Al2Me6 is electron deficient whereas Al2Cl6 is not this is because the methyl group does not have a lone pair or other filled orbital to donate a metal centre, the bridging methyl groups in Al2Me6 are involved in 3 centre 2 electron bonds similar to those involving the bridging hydrogen atoms in diborane B2H6 the dimeric structure of Al2Me6 is in contrast to the structure of BMe3 which is monomeric the greater the size of the aluminium atom the greater the lewis acidity of AlMe3 are both factors in this difference
p block group 14 organometallic chemistry?
Carbon forms covalent bonds to the other group 14 atoms and silyl groups such SiMe3 are useful in organic chemistry the C-Si bond is relatively strong and tetralkylsilicon compounds have high thermal stabilities. The bonds decrease in strength with increasing size of the group 14 element due to poorer orbital overlap and the thermal stability of the compounds decreases in line with this. However tetralkyltin and tetraalkyllead compound are more stable to reductio than their halide analogues. The organolead compound PbEt4 has been used as anti knock agent
Organometallic trends?
As the electronegativity of the metal or metalloid increases the bonding in its organometallic compounds become more covalent. As a consequence the compounds become less reactive. So whereas s block organometallic are highly reactive to air and water group 14 organometallics are more stable
Laser chemistry?
Ar —> Ar* (excited state)
Ar* + F2 –> ArF* + F
ArF* —> Ar + F + energy
Useful also Ar, Kr and Xe with F and Cl
Important reactions?
Kr + F2 --> KrF2 Xe + F2 --> XeF2 Xe + 2F2 ---> XeF4 Xe + 3F2 ---> XeF6 2XeF2 + 2H2O ---> O2 + Xe + 4HF XeF6 + 3H2O ---> 6HF + XeO3 XeF6 + H2O --> XeOF4 + 2HF
Electron affinity?
Values close to zero as adding electrons to higher shell which experience strong shielding of those below
