Transcript THE GROUP 13 ELEMENTS - University of the Witwatersrand

THE GROUP 13 ELEMENTS
Include boron, aluminum, gallium, indium and thallium.
Boron is the only nonmetal in the group many of their compounds of the
elements are electron deficient and act as Lewis acids. Aluminum is a metalloid
And gallium, indium and thallium are metals.
1
2
13
14
15
16
17
0
He
Li
Be
B
C
Mg
Al
Si
Ca
Ga
Ge
Sr
In
Sn
Ba
Tl
Pb
Ra
N
O
F
Ne
Properties of Elements
Covalent radius/pm
B
Al
Ga
In
Tl
80
125
125
150
155
143
141
166
171
Metallic radius/pm
Ionic radius(M+3)/pm
27
53
62
94
98
Melting point/°C
2300
660
30
157
304
Boiling point/°C
3930
2470
2400
2000
1460
1st I.E.I1/kj.mol-1
799
577
579
558
590
2nd I.E.I2/kj.mol-1
2427
1817
1979
1821
1971
3rd I.E.I3/kj.mol-1
3660
2745
2963
2704
2878
Electron affinity,
Ea/kj.mol-1
26.7
42.5
28.9
28.9
Pauling electronegativity 2.0
1.5
1.6
1.7
1.8
Eө (M3+;M)/V
-1.68
-0.53
-0.34
+1.26
-0.89
Page 288 Sh&At
Trends from the table
Electronic configuration – ns2 np1 Generally exhibit +3 oxidation state
and changes to +1 as the group is descended.
Covalent, metallic and ionic radii increases from B to Tl.
Electronegativity: Ga is more electronegative than Al due to the alternation
effect. (consequence of increased nuclear charges of the 4p elements due to
the presence of their poorly shielding 3d electrons.
Distinct chemical properties from those of other elements in a group:
- Boron forms acidic oxides, B2O3, and aluminium forms amphoteric oxide, Al2O3
- Boron forms polymeric oxide structure.
- Boron form flammable, gaseous hydrides, aluminium hydride is a solid
- Boron is electron deficient and making all its neutral compounds Lewis acids.
Element
Symbol IE (kJ.mol-1)

Electronic
Configuration
Boron
B
801
2.0
[He] 2s2 2p1
Aluminium
Al
577
1.5
[Ne] 3s2 3p1
Gallium
Ga
579
1.6
[Ar]
Indium
In
558
1.7
[Kr]
Thallium
Tl
590
1.8
[Xe] 4f14 5d10 6s2
6p1
Oxidation state
III*
(1)
III*
3d10 4s2 4p1
I
III*
4d10 5s2 5p1
I
III*
I*
III
MEAN BOND ENTHALPIES (kJ/mol-1)
H
F
Cl
Br
I
Boron
334
757
536
423
220
Aluminium 284
664
511 444
370
Gallium
274
577
481
444
339
Indium
243
506
439
414
331
Thallium
188
445
372
334
272
Occurrence and recovery
Al – most abundant and Tl and In are least abundant.
B:- Borax; Na2B4O5(OH)4.8H2O and kernite; Na2B4O5(OH)4.2H2O;
Borax - Na2B4O5(OH)4.8H2O → boric acid, B(OH)3 → boron oxide, B2O3
→reduced with Mg, hydrofluoric acid, HF
Pure boron is produced by reduction of BBr3 vapour with H2:
2 BBr3(g) + 3 H2(g) → 2 B(g) + 6 HBr(g)
Al:- clays and aluminosilicate minerals and commercially as bauxite.
Gallium oxide occurs naturally as an impurity in bauxite and indium is obtained
in the Pb and Zn ores. Thallium compounds are found in the flue dust.
Aluminium alloys
Al is the widely used nonferrous metal, it’s light, has high electrical and thermal
Conductivity and high reflectance. Small percentages (less than 2 %) of other
metal gives desirable weight to strength ratio.
Al/Mn – most widely used and contains Fe and Si in traces to give added strength
and hardness
Al/Mg – have good ductility and corrosion resistant but less strong.
Al/Si – have good fluidity and used in welding wires.
Al/Li – have very low density and high elasticity.
Al/Cu/Mg – used in making aircrafts.
Al/Cu – used in satellites and space vehicles.
Other uses of Al alloys include building and construction – walls, gutters, window
frames, roofs and doors, aerosol cans, drink cans, sheets and foils and cooking
utensils – toys, tools, refrigerators, cosmetic tubes and airconditioning units and
in cars – body panels, engine blocks, wheels, bumpers and radiators.
Production of Aluminium (as in Alusaf)
First step: Purification of the ore – Bayer’s process.
Bauxite is crushed and digested with caustic soda under pressure.
Al2O3 dissolves to form sodium aluminate.
. 3H2O
Al2O3
Extraction
+
2NaAlO2 + 4H2O
2NaOH
decomposition
or
Al2O3
. 3H2O
+
2NaOH
2Na[Al(OH)4
 The solution is diluted thereby impurities like TiO2, sodium aluminium silicate
and iron(III) oxide precipitate out. They are filtered out.
 CO2 is bubbled through the liquor to reduce the pH to enable Al(OH)3
precipitate and then filtered and washed.
 Al(OH)3 is calcined at ~1300°C to give pure Al2O3 + 3H2O.
2Al(OH)3
Al2O3 + 3H2O
Second step: Smelting process
This is done by electrolysis using the Hall-Héroult process.
Al2O3 is dissolved in molten cryolite, Na3[AlF6] and electrolysed in a graphitelined steel tank which serves as the cathode and carbon as anode.
Calcium fluoride (fluospar) is added to lower the m.p.
Molten aluminium (m.p. 600) is drained from the bottom of the cell at intervals.
Cryolite is made synthetically to augment the mined cryolite.
2Al(OH)3 + 3NaOH + 6HF  Na3[AlF6] + 6H2O
(Cryolite improves the electrical conductivity of the cell as Al2O3 is a poor
conductor. It also serves as an added impurity and lowers the m.p. of the
mixture to about 900°C).
Typical electrolyte composition ranges are Na3[AlF6] (80-85%), CaF2 (5-7%),
AlF3 (5-7%), Al2O3 (2-8% - intermittently recharged).
The following dissociations occur;
2AlF3
Al3+ + AlF63-
Al2O3
AlO2- + AlO+; AlO+
Al3+ + O2-
 Al is preferentially discharged at the cathode and oxygen at the anode.
Al3+ + 3e
Al
2AlO2
Al2O3 + ½O2 + 3e
Carbon anode burns to CO2, so the anode is replaced periodically.
Some aluminium fog dispersed in the electrolyte, reduces CO2 to CO. Exit
gases contain about 30% CO. The overall anode reaction seems to be
Al2O2F42- + 2AlF63- + C  4AlF4- + CO2 + 4e
and at the cathode
AlF63- + 3e  Al + 6FOverall cell reaction:
3Al2O2F42- + 10AlF63- + C  12AlF4- + 3CO2 + 4Al + 24F-
Series of carbon anodes
Steel
casing
cathode
Cryolite and Fluospar
Aluminium tapped and transferred to
reverbaratory furnace before being
cut into inguts.
BORON COMPOUNDS
HYDRIDES
Figure 12.12 page 304
Boron hydride compounds – Borane, the simplest form is diborane – B2H6 its
structure is described as 2c,2e and 3c,2e bonds; can be prepared by metathesis
of boron halides with either LiAlH4 or LiBH4 in ether.
3 LiBH4(et) + 4 BF3 (et) = 2 B2H6 (g) + 3 LiBF4(et)
All the boranes are electron deficient, colourless and diamagnetic.
Higher boranes are classified according to their electron count:
Type
Formula
skeletal electron pairs Examples
Closo
BnHn2n+1
B5H52- to B12H122Nido
BnHn+4
n+2
B2H6, B5H9, B6H10
Arachno
BnHn + 6
n+3
B4H10, B5H11
Hypho
BnHn + 8
n+4
none
Wade’s Rules: established by Kenneth Wade in the 1970s based on correlation
between the number of electrons, the formula and the shape of the molecules.
This apply to a class of polyhedra called deltahedra because they are made up
of triangular faces resembling Δ. For molecular and anionic boranes - predict
shapes of molecule or anion from its formula.
H
B
H
H
H
B-H bonds – 2c-2e
B-H-B bonds – 3c-2e
B
H
H
Structure of diborane
Diborane, like all boranes, is electron-deficient. There are 12 electrons (6 from
H and 3 each from B). The four B-H bonds use 8 electrons, leaving 2 electrons
each for the B-H-B bonds. The B-H-B bonds are therefore electron –deficient
(short of 4 electrons)
Characteristics reactions of
boranes and borohydrides
• Cleavage of BH2 unit from diborane or tetraborane by
NH3.
• Deprotonation of large boron hydrides by bases.
• Reaction of boron hydrides with borohydride ions to
produce larger borohydride anions.
• Fridel-Crafts type substitution for hydrogen in
pentaborane and some larger boron hydrides
H
H
-
B
H
H
-
-
B
H
-
B
H
B4H10
B
H
H
H
H
+ 2 :NH3
H
+
H N H
- H
H
+
B
N H
H
H
H
H
+
H
-
B
H
-
H
B
-
B
H
H
H
BORON TRIHALIDES
Are useful Lewis acids as a result of incomplete octet, with BCl3 stronger than
BF3. They consist of trigonal planar BX3 molecules. BCl3, BF3 are gases; BBr3
is a volatile liquid and BI3 is a solid.
B(OH)3
B(NH2)3
B(OR)3
H2O
Protolysis
RNH2
ROH
BX3
PR3
NR3
Complex formation
SR2
X3BNR3
X3BPR3
X3BSR2
REACTIONS OF BX3 COMPOUNDS
BORON OXYGEN COMPOUNDS
Important oxides – B2O3, polyborates and borosilicate glasses.
2 B(OH)3(s) Δ→
B2O3(s) +
3 H2O(l)
An example of cyclic polyborate anion, [B3O6]3- - three coordinate B atom
and [B(OH)4]- four coordinate B atom. Polyborates form by sharing O atom with
the neighboring B atom. The rapid cooling of molten B2O3 lead to the formation
of borate glasses.
Sodium perborate, commonly written as NaBO3.4H2O contains peroxide ion –
O22-, and hence the accurate formula is Na2[B2(O2)2(OH)4].6H2O. It is used as a
bleach in laundry powders, automatic dishwashing powders and whitening
toothpaste.
O
O
O
-
B
B
-
HO
-
O
B
O
OH
O
O
-
B
O
LEWIS ACIDITY
Diborane and other hydrides act as Lewis acids and cleaved by reaction with
Lewis bases;
B2H6 + 2 N(CH3)3 → 2 H3B-N(CH3)3 Symmetric cleavage
B2H6 + 2NH3 → [BH4]- + [BH2(NH3)2]+ Unsymmetric cleavage
HYDROBORATION, METAL BORIDES – SUPERCONDUCTING ABILITY OF MgB2
METALLABORANES, CARBORANES
SELF STUDY
COMPOUNDS WITH OTHER
ELEMENTS – Al, Ga, In AND Tl
HYDRIDES
LiAlH4 and LiGaH4 are good precursors for metal hydride complexes, MH3L2 and
sources for H- ion for the metalloid hydrides – SiH4. Aluminium hydride, AlH3 is a
solid similar to the s-block metal hydrides although not readily available. Pure
Ga2H6 has been prepared recently and the hydrides of Tl and In are very
unstable.
Some of the reactions involving the hydrides:
- 4 LiH + ECl3 → (Δ, ether) LiEH4 + LiCl (E = Al, Ga)
- LiAlH4 + SiCl4 → (THF) LiAlCl4 + SiH4
- LiEH4 + [(CH3)3NH]Cl → (CH3)3N-EH3 + LiCl + H2 (E = Al, Ga)
- (CH3)3N-EH3 + N(CH3)3 → ((CH3)3N)2EH3 + LiCl + H2 (E = Al, Ga)
HALIDES AND TRIHALIDES
All elements form trihalides in their +3 oxidation state, however +1 becomes
Common with Tl forming a stable monohalide. Trihalides of Al, Ga and In are
Lewis acids. Generally prepared by reaction of the electropositive metal with
hydrogen halides such as HCl, HBr gases.
2 Al (s) + 6 HCl (g) → (100 °C) AlCl3 (s) + 3 H2 (g)
Trifluorides are mechanically harder than others and they have high melting
points and sublimation enthalpies and have low solubility. Their reactivity
towards most donors is low (not Lewis acids) and despite that AlF3 and GaF3
form slats such as Na3AlF6 (Cryolite)
– used as a solvent for bauxite in the production of aluminium) and Na3GaF6.
Thallium trihalides are less stable than those of light congeners. The triiodide,
TlI3 is a compound of +1 and +3 as it contains the I3- not the I- ion. Confirmed
by the standard electrode potentials which indicates that Tl(III) is rapidly
reduced to Tl(I) by iodide:
Tl3+ (aq) + 2 e- → Tl+ (aq) EӨ = +1.26 V
I3- (aq) + 2 e- → 3 I- (aq) EӨ = +0.55 V
However, in excess iodide Tl(III) is stabilized by the formation of the complex;
TlI3 (s) + I- → TlI4- (aq)
Generally the +1 oxidation state becomes more stable progressively from Al to
Tl.
(Read more in page 310)
OXYGEN COMPOUNDS
Al and Ga form α and β forms of oxide in which the elements are in their +3
oxidation state; Tl forms an oxide in which it’s in +1 oxidation state and a
peroxide. α-alumina, Al2O3 is the most stable form and is very hard and a
refractory material. In mineral form called corundum and as a gemstone is
called sapphire, ruby, emerald or amethyst depending o the amount of metal ion
impurities. Dehydration of Al(OH)3 at temperatures below 900 °C result in the
formation of γ-alumina, which is metastable polycrystalline form with a defect
spinel structure. The α and γ forms of Ga2O3 have the same structureas their Al
analoques. The metastable form is β-Ga2O3, which has a ccp structure with
Ga(III) in distorted octahedral and tetrahedral sites. In forms In2O3 and Tl forms
Tl(I) oxide and peroxide, Ti2O and Tl2O2. The elements all form a series of salts
called alums – MAl(SO4)2.12H2O where M is univalent cation such as Na+; K+;
Rb+; Cs+; Tl+ or NH4+. Alums are thought of double salts containing the hydrated
trivalent cation [Al(H2O)6]3+. The remaining water molecules form hydrogen
bonds with the sulfate and the cations.
+3 Ox. State
B2O3
Al2O3
Ga2O3
In2O3
Tl2O3
acidic
amphoteric amphoteric basic
basic
+1 Ox. State
Tl2O
basic
Al2O3 and Al(OH)3 are amphoteric


Al(OH)3  3H(aq)
 Al3(aq)
 3H2O (Al(OH)3 a base)

Al(OH)3  OH(aq)
 [Al(OH)4 ]-(aq) (Al(OH)3 an acid)

3
Al3( aq

[Al(H
O
)
]
)
2
6
 Ga2O3 and Ga(OH)3 are amphoteric and dissolve in alkali to form gallates.
 In2O3 and Tl2O3 are completely basic and the metals form neither hydrates
nor hydroxides.
 TlOH is a strong base and is soluble in water, like Group 1A hydroxide.
 Tl+ compounds are extremely poisonous and in large doses, can cause death.
SULFIDE COMPOUNDS
The only sulfide of Al, Al2S3, which is prepared by direct reaction of the elements
at elevated temperatures: 2 Al(s) + 3 S(s) → (Δ) Al2S3(s). It is rapidly
hydrolyzed in aqueous solution; Al2S3(s) + 6H2O(l) → 2Al(OH)3(s) + 3H2S(g)
Al2S3, exists in three different forms – α, β and γ forms. The structure of α and
β forms are based on wurtzite structure in α–Al2S3 the S2- ions are hexagonal
close packed and the Al3+ ions occupy two-thirds of the tetrahedral sites
randomly. The γ-form adopts the same structure as γ-Al2O3. The sulfides of Ga,
Tl and In are numerous and varied than those of Al and adopt many different
structural types.
Al, In and Ga react with P, As and Sb to form materials that are semiconductors
Sulfide Structure
Material
Band gap
(Eg; eV)
GaS
Layer structure with Ga-Ga
bonds
GaAs
1.35
α-Ga2S3
Defect wurtzite structure
(hexagonal)
GaSb
0.67
γ-Ga2S3
Defect sphalerite structure
(cubic)
InAs
0.36
InS
Layer structure In-In bonds
InSb
0.163
β-In2S3
Defect spinel (γ-Al2O3)
Si
1.107
TlS
Chains of edge-shared TlIIIS4
tetrahedra
Tl4S3
Chains of TlIIIS4 and TlITlIIIS3
tetrahedra