CHMB31H3 Chapter Notes - Chapter 13: Boron, Thermal Shock, Lithium Aluminium Hydride

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9 Nov 2011
Chapter 13: The Group 13 Elements
The Essentials
Group 13 Elements: boron, aluminum, gallium, indium and thallium.
Boron is the only nonmetal in the group.
Aluminum is the most abundant Group 13 element.
There is an increase in metallic character from B to Tl; B is a nonmetal, Al is a
metalloid and Ga, In and Tl are all metals.
Boron’s diagonal relationship with silicon.
oBoron and silicon form acidic oxides, B2O3 and SiO2; aluminum forms an
amphoteric oxide.
oBoron and silicon form many polymeric oxide structures and glasses.
oBoron and silicon form flammable, gaseous hydrides; aluminum
hydride is a solid.
Aluminum is an electropositive metal but it is very inert because of the
presence of a passivating surface oxide film, not oxidized by air.
Gallium is brittle at low temperatures but liquefies at 30°C and has the widest
liquid range of 30-2420°C.
All of the elements form hydrides oxides and halides in the +3 oxidation state.
oMany of the compounds made from lighter Group 13 elements have
incomplete octets and act as Lewis acids (electron pair acceptor).
oBoranes: binary hydrogen compounds of B.
oBoron trihalides are monomeric in the gas, liquid and solid state unlike
the rest of the halides in Group 13.
o4 B(OH)3 (s) + heat 2 B2O3 (s) + 6 H2O (l)
oB2O3 (l) + 2 NH3 + heat 2 BN (s) + 3 H2O (g)
BN structure consists of planar sheets of atoms like graphite.
BN and graphite have similar physical properties, both used as
lubricants but BN is a white, nonconducting solid.
The +1 oxidation state becomes more stable down the group and is the most
stable oxidation state for compounds of thallium (due to inert pair effect).
oBecause the F- ion is so small, the trifluorides are mechanically hard
ionic solids that have much higher melting points and sublimation
enthalpies than other halides; they are not very soluble and do not act
as a Lewis acid.
oThe heavier trihalides of Al, Ga and In are soluble in a wide variety of
polar solvents and are excellent Lewis acids.
The most important oxosalts of Group 13 are called alums, MAl(SO4)2 12 H2O,
where M is a univalent cation like Na+, K+, Rb+, Cs+, Tl+ or NH4+.
oGallium and indium can also form analogous series of salts but B and Tl
do not because B is too small and a Tl atom is too large.
oAlum is used widely to describe other compounds with the general
formula MIM’III(SO4)2 12 H2O, where M’ is often a d metal, such as Fe.
Boron forms an extensive range of polymeric, cage-like compounds which
include the borohydrides, metallaboranes and the carboranes.
oCloso Structure: borohydrides with the formula [BnHn]2-.
oNido Structure: when boron clusters have the formula [BnHn+4] formula.
oArachno Structure: when clusters of boron have the formula [BnHn+6].
oMetallaboranes: borons that form many metal-containing clusters.
oCarboranes: a large family of clusters that contain both B and C atoms.
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