Download SECTION B Part 2

CHEM210 - TEXTBOOKS
 Inorganic Chemistry by Housecroft and
Sharpe, 4th Ed., Pearson
 Inorganic Chemistry by Miessler and
Tarr, 3rd Ed., Prentice Hall
 Inorganic Chemistry by Shriver and
Atkins, 4th Ed., Oxford
 Basic Inorganic Chemistry by Cotton,
Wilkinson and Gaus, John Wiley & Sons
CHEM210
Part B

Inorganic Chemistry by
Housecroft and Sharpe,
4th Ed., Pearson
Classes of Bonding
 Ionic, metallic, covalent, van der Waals
• Dr V.O. Nyamori
 The Structures and Energetics of Ionic Solids

H&S Chapter 6 p 148‐180; C&W Chapter 4
• Dr V.O. Nyamori
 Descriptive Chemistry: Aspects of the chemistry of Groups 14‐16.

H&S Chapter 14,15,16
•
Dr V.O. Nyamori
Group 14
Carbon Group
Group 14 - The Carbon Group
• The nonmetal carbon exists as an element in
several forms.
• You’re familiar with two of them, i.e. diamond and graphite.
Carbon nanotube stabilizers in Tennis rackets
increase torque and flex resistance
6
Tour de France ‐ cyclists use a bike with a frame containing carbon nanotubes. Swiss manufacturer BMC claims that the frame of its "Pro Machine" weighs less than 1 kg and has excellent stiffness and strength.
7
Carbon Family
• Carbon family ‐ Group 14
• Elements included in this group are Carbon,
Silicon, Germanium, Tin, and Lead
• Carbon ‐ atomic number is 6
• Atomic symbol is C
• Melting point = 3,550 °C
• Boiling point= 3,800 °C
Group 14 ‐ The Carbon Group
•
Carbon also is found in all living things
•
Carbon is followed by the metalloid silicon, an
abundant element contained in sand
•
Sand contains ground up particles of minerals such
as quartz, which is composed of silicon and oxygen
•
Glass is an important product made from sand
Group 14 ‐ The Carbon Group
•
Silicon and its Group 14 neighbor, germanium, are metalloids.
•
They are used in electronics as semiconductors. •
A semiconductor doesn’t conduct electricity as well as metal, but does conduct electricity better than a nonmetal.
Group 14 - The Carbon Group
•
Tin and lead are the two heaviest elements in
Group 14
•
Lead is used to protect your torso during dental X
rays
•
It also is used in car batteries, low‐milting alloys,
protective shielding around nuclear reactors, and
containers used for storing and transporting
radioactive materials.
•
Tin is used in pewter, toothpaste, and the coating
on steel cans used for food.
How Carbon Effects our lives?
When carbon is mixed with oxygen
 Green houses gases are produced
into the air causing the ozone to
dissipate.
 Also carbon is produced through
factories, cars, and others.
 Depletion of forests are causing
the carbon cycle to change.
Group 14 ‐ General trends
 Chemistry Group 14 shows a vary obvious transition from a
non‐metal to increasingly metallic elements going down the
group, ending in true metals
 Carbon is a classic example of a non‐metal
 Silicon and Germanium are semi‐metals
 Tin and Lead are metals
 Group 14 gives perhaps the most obvious example of the
difference in properties between elements of Period 2 and
higher periods
 The elements from silicon to lead show a nice transition of
properties towards increasingly metallic character
 +4 and +2 oxidation states are common. +2 becomes
more stable down the group
 Reactivity of compounds increases down the group due
to weaker bond energies and larger size of atoms
 Multiple bonding decreases down the group due to
poorer overlap between the orbitals, weaker
element‐element bonding
 Higher coordination numbers down the group
 Hypervalency due to low lying d‐orbitals, e.g. [SiF6]-2
“Second row anomalies”
 2nd row (Li‐Ne) vs. 3rd row (Na‐Ar) elements
2nd row
 Greater stability for element‐element bonds
• (increased allotropy e.g. C vs.Si)
 Greater stability of multiple bonds
• (strong N2 vs. weak P2)
 Octet rule generally obeyed
• (CF4 but no CF62‐ vs. both SiF4 and SiF62‐ are stable)
 Generally maximum coordination number of four
• (BF3.NH3 but no BF3.2NH3 vs. AlF3.2NH3 stable)
 Lower reactivity of compounds
• (CCl4 vs. SiCl4)
Two reasons
1.
The 2nd row elements have only a 1s2 core shell shielding the outer
electrons
• This leads to high Zeff and IE therefore small radii and contracted
atomic orbitals
• Also, the 2s and 2p orbitals are closer in energy and size
compared to the 3s and 3p orbitals. Hence, very efficient overlap
of orbitals between 2nd row elements ‐ strong bonds (allotropes,
multiple bonding)
2. No low lying d orbitals for 2nd row elements
• The effects: limits oxidation number and coordination numbers to
maximum of 4
• Limits reactivity since no coordination sites available in compounds
What do you understand by the term low lying d-orbitals?
1. It could reference the d-orbitals of a lower energy level than
the outermost energy level. For instance, the valence
electrons of Br are found in the 4s and 4p sublevels, the 3d
sublevel might be described as "low lying" since it is lower in
energy.
2. The d-orbitals are arranged in such a way that the electrons
found in d-orbitals come closer to the nucleus than do the
electrons of the outermost p-orbitals, for instance.
Therefore, "low lying" may refer to the "deeper penetration"
of the d-orbitals.
Methane: CH4
 Hydrogens are in a tetrahedral arrangement around the sp3
hybridized carbon atom.
 Hydrogens bond to the carbon sp3
orbitals with 1s orbitals.
sp3 Hybridization
carbon
Energy
2p
2s
Hybridization
1s
Multiple π Bonding
• Full ∏‐bonds (double, triple) are common in
period 2 (C, O, N) using 2p orbitals.
• e.g. C=C, C=O, O2, N2, N=O
• 2s/2p orbitals are similar in size and
energy and therefore “hybridize” well
• Mixing of 2s/2p orbitals on adjacent atoms
is highly efficient (small and localized due
to high Zeff) and form strong bonds
• Not for period 3 and below which have
larger, more diffuse orbitals
• So only very weak Si=Si, As=As etc.
sp3
19
Example of the strange
arrangements:
Tin has three allotropes:
 α‐tin (gray tin): non‐metallic, stable below 13°C, atoms
bonded in diamond lattice ‐ʺTin diseaseʺ
 β‐tin (white tin): the common, metallic form, stable from
18°C ‐161°C
 γ‐tin (rhombic tin): atoms are bonded in an orthorhombic
lattice, brittle, stable above 161°C to the melting point of
232°C
α‐Tin (gray tin)
Sn atoms are bonded tetrahedrally to four other Sn atoms where
Sn‐Sn bond = 2.81 Å and I‐I bond length = 2.72 Å
N.B. Cu‐Cu bond length = 2.56 Å
Perceived as a non‐metallic network of covalent bonds
β‐Tin (white tin)
The Sn‐Sn bond length changes: 4 x close atoms with a distance
of 3.02 Å and 2 x further atoms at a distance of 3.18 Å generates
a distorted octahedron
γ‐tin (rhombic tin)
Atoms are bonded in an orthorhombic lattice, brittle, stable above
161°C to the melting point of 232°C
Compound
Conductivity ohm‐1 cm‐1x 106
Diamond
10‐12
α‐tin
10‐10
β‐tin
0.092
lead
0.048
copper
0.596
Oxidation States
• Common ox. states: +4, +2, e.g. SnCl4, CO2, PbCl2, SnO
• The oxidation state of carbon
• The “inert pair effect” leads to the lower oxidation state
becoming progressively more stable down the group
• ns2 electrons are “retained” in elements further down the
group – explanation is “small bond energies and lattice
energies” associated with the larger atoms and ions are not
sufficiently great to offset the ionization energies of the ns2
electrons”
• +2 is favoured in lead over +4
Oxides
• In group 14 there is a stark contrast between CO/CO2 and SiO2
gases versus hard polymeric solid
• As mentioned previously, the strong multiple bonding between C
and O leads to molecular species
• GeO is similar to SiO2 (as expected since they possess similar size
and electronegativities)
• SnO2 and PbO2 are polymeric but each metal has six nearest
neighbours (larger atoms can accommodate more neighbours)
The lower oxidation states SnO and PbO show a
movement towards more ionic character they both
consist of sheets of oxygens, where a square of oxygen
atoms is capped by metal atoms
Structures (“Inorganic Chemistry” Housecroft and Sharpe, Ch. 13)
• The “cluster” chemistry of Si to Pb is very different
from that of carbon
• (graphite, C60) due to the large atomic radii which
allows variation in bond angles
• Silicon forms silicides with alkali‐earth and
transition metals e.g.[Si4]4‐ (isoelectronic with P4)
• Ge, Sn and Pb do not form stable binary
compounds but Zintl ions diamagnetic Zintl ions
include [M4]4‐
NaSn
M = Ge, Sn or Pb
4Na+ + 4Sn−
polyanion is tetrahedral (Sn4)4−
• diamagnetic/paramagnetic species are known
(see diagram)
Silicon (“Chemistry of the Elements” N.N. Greenwood and A. Earnshaw)
 ~27 % of the earth’s crust (second most abundant to oxygen)
 FCC – room temperature Si‐Si distance 2.32 Å. No allotropes except at
high pressure
 Denser form observed when the tetrahedral bond angles distort to give
three at 99 °and three 108 °
 Si‐Si bond is weaker than C‐C
Properties:
• Solid silicon not very reactive to acids (except HF)
• Dissolves in hot aqueous soln. (SiO44‐)
• Rapidly oxidizes metals to form SiO2 (Df~ 900 KJ mol‐1)
• SiO2 reacts with halides (F at room temp., Cl at ~ 300 °)
• Si does not form binary compounds with heavier elements of the group
(Ge, Sn, Pb). But does react with carbon to form silicon carbide (SiC)
Germanium, Tin & Lead - Trends in Reactivity
Germanium
•
More reactive and more electropositive than silicon
•
It dissolves slowly in hot conc. sulfuric and nitric acids but does not
react with water or dilute acids/bases
•
Ge is oxidized to GeO2 in air at “red heat” – and reacts with H2S to
form GeS2
•
Heating in HCL yields GeCl4 – reaction with alkyl halides gives
organogermanium halides
Tin
•
More reactive and more electropositive than germanium but still has
an amphoteric nature – reacts with steam to form SnO2 and H2
•
hot conc. sulfuric yields SnSO4 and SO2
•
Hot Conc. HCl gives SnCl2 and H2
• Dilute acids have little reaction except nitric as Sn(NO3)2 and
ammonium sulfate is formed
• All of these compounds give tin(II) compounds with hot aqueous
bases complexes are formed
Sn + 2KOH → 4H2OK2[Sn(OH)6] + 2H2
• Tin reacts with chlorine and bromine (cold) and fluorine and Iodine
(hot) to give SnX4
• Reacts vigorously with heated sulfur forming Sn(II) and Sn(IV) species
Lead
• Finely powdered lead is pyrophoric it usually has a thin oxide or
other anionic layer that reduces its reactivity
• Reacts with HCl and nitric acid to yield PbCl2 and Pb(NO3)2
• can react with organic acids to form Pb(OAc)2
Germanium
• Hydrides of the general formula Gen H2n+2 are known and are
colourless gases or liquids for n = 1‐5 (less volatile than silanes
and less reactive)
• Chemical and physical properties are similar to silanes
• GeH4 does not ignite on contact with air and can behave like an
acid in liquid ammonia forming NH4+ and GeH3‐
• MGeH3 can be formed with M = alkali metals
• Germanium halides are more stable than silicon following the
trend:
• CX2 << SiX2 < GeX2 < SnX2< PbX2
• GeF2 is a white solid
Tin
• It is a more abundant element than germanium – used in
solder (Pb) and bronze
• Sn(II) fluorides structure is interesting as tin tends to
polymerize into larger units
• the first and second ionization energies are similar to
magnesium
• The 5S electrons can act as “donors” and coordinate to
any “vacant” 5p or 5d orbitals “acceptors” – adducts are
thus formed e.g. SnF4 is composed of Sn4F8 tetramers
interlinked with weaker Sn‐F weaker interactions
Organotin compounds
• Organotin compounds have been widely used in industry
• They were used as stabilizers in PVC’s – prevents photo or aerobic
oxidation (brittle) or “vulcanizers” for silicone
• Employed also as agricultural biocides and marine anti‐fouling
agents (a number of synthesise employed)
• Problems have been observed as the compounds get into the food
chain by tissue absorption – organotins are toxic e.g. tributyltin
oxide
• Sn‐C bond not as strong as the Si‐C bond
Lead
• Most abundant as PbS (galena) found in over fifty countries
• Galena is processed by froth floatation then roasting PbO +
CPb(liquid) + CO/CO2
• Impurities are present:
• Cu removed by liquidation (held just above f.p. of lead
– cu rises then is skimmed off)
• Sn, As and Sb are removed by fluxing in molten
NaCl/NaNO3 (Harris process)
• Zn is removed when the solution is cooled from
480°‐420°C and the “crust” is skimmed off
• Ag, Au removed during vacuum distillation
• Bi and final purification by electrolysis with Pb cathodes
Hydrides
• PbH4 is the least well characterized of the group 14 hydrides
• The remainder are not very stable. Pb‐H is not a stable bond (why?)
• Me3PbH decomposes above ‐30 °C
Halides
• Stability : PbX2 > PbX4 (PbF4) is the only stable example
• PbCl4 is a yellow oil and at 50 °C it decomposes to PbCl2
• PbX2 where X = F mp = 818 °C
Cl mp = 500 °C
Br mp = 367 °C
I mp = 400 °C
• Mixed halides do occur PbFCl, PbFBr
Cs4PbX6 exists so does CsPbX3 and it has a similar structure to
perovskite (calcium titanium oxide mineral ‐ CaTiO₃)
Organometallics
•
The Pb‐C bond is not as stable as the others in the group but
ore found as PbCO3
•
The most important commercially has been the use of Et4Pb
in petroleum fuels
Group 15
Nitrogen Group