G14 Flashcards
how does oxidation state trend down G14
ranges from +4 to -4 in hydrides.
+4 OS becomes less important with respect to the 2+ OS going down group
what are the two common allotropes of carbon
Diamond and graphite
describe properties of diamond
sp3 C, 4 sigma bonds per C, strong in all directions, dense solid
high thermal conductivity
electrical insulator
C-C length 1.54 angstrum
describe properties of graphite
sp2 trig planar
3sigma 1pi bond per C
2D delocalised pi orbital
alternate layers lie above one another,
stronger intralayer and weaker interlayer bonding,
lubricant
electrical conductor parallel to layer
forms intercalation compounds
what is the issue with diamond
diamond is metastable as graphite is thermodynamically most stable form and so diamond conversion in graphite is favoured. this is very slow
what are examples and properties of fullerenes
C60,70,76,78,80,84
sp2 carob
hexagonal/pentagonal rings like a football
all C same env 1 CNMR peak
bond lengths: 1.39 and 1.46 due to localised bonds
localised c=c undergo addition
what structures do Si and Ge form
diamond like structure a multiple bonds are weaker between 2nd/3rd row elements - graphite like structure unfavourable
what polymorphs do Tin form
metallic white-tin exists at 298 K where Sn is 6-coordinate
grey-tin when white tin cooled to 286 K contrains 4-coordinate Sn in a diamond like structure
what are the uses of Carbon
diamond very hard used in cutting tools and abrasives
graphite inert, thermal stability, electrical conductivities
carbon black = vulcanized rubber
what are uses of Si, Sn and Pb
Si used in steal and electronics as semiconductor
Silica (SiO2) glass and construction
Sn coat metals corrosion prevention and used in superconducting magnets
Pb in plumbing and paints
what are general comments on the reactivity of G14
carbon doesn’t exhibit valency > 4
Si more reactive than C
going down group electropositive character and reactivity of elements increase
+4 OS dominant for C, Si and Ge compounds stability of +2 increases down (inert pair effect)
what can Si combine with at high temperature
combines with O2, halides, S8, N2, P4, C and B
what is catenation
how does this trend down G14
ability to form bonds with its self
decreases down group C»Si>Ge>Sn>Pb
how are the hydrides of Si, Sn and Ge formed
SiCl4 + LiAlH4 -> SiH4 + LiCl + AlCl3
SnCl4 + LiAlH4 -> SnH4 + LiCl + AlCl3
GeO2 + Na[BH4] -> GeH4 + Na[BO2]
all analogues of CH4 and are very unstable compared to CH4
how does SiH4 decompose
SiH4 + 2O2 -> SiO2 + 2H2O
SiH4 -> Si + H2 (add heat)
which g14 tetrahalides are formed
known for all elements except for PbI4 as Pb-I bond is not strong enough to compensate for promotional energy required for hybridisation
All the halides are volatile covalent except SnF4 and PbF4 which are ionic
how are G14 tetrahalides formed
CH4 + 4Cl2 -> CCl4 + 4HCl (add heat)
E + 2Cl2 -> ECl4
(E= Si , Ge, Sn)
GeO2 + 4HCl -> GeCl4 + 2H2O
how does PbCl4 decompose
in contrast to others decomposes to PbCl2 and Cl2
due to inert pair effect which favours the +2 OS for Pb
what makes EX4 lewis acids when E=Si Ge Sn Pb
use empty d orbitals to accept electrons
(G13 EX3 compounds used empty p orbitals to accept e)
eg:
SiF4 + F- -> SiF5- 10e-, trigonal bipyramidal, sp3d hybridised
how does hydrolysis of tetra-halides occur
hydrolysis is thermodynamically favourable for all G14 elements
CX4 species are resistant to hydrolysis (kinetic reasons)
EX4+2H2O -> EO2 + 4HX
Si Ge Sn Pb can all extend coordination number beyond 4 which allows easy attack of EX4 by water.
C has no low-lying vacant orbitals and can’t do this
describe G14 oxides
carbons forms CO and CO2 CO very strong due to efficient 2p-2p pi bonding
SiO2 forms polymeric 3D solids
dioxides of Ge Sn and Pb are all non-volatile solids
GeO2 display structure that closely resembles SiO2
