Transcript Document

Marvellous Metals
Nyholm Lecture 2002
Professor Tony Baker &
Dr Linda Xiao
Faculty of Science, UTS
Sir Ronald Nyholm 1917-1971
Coordination Chemist
Inspiring Chemical Educator
Leader of the Profession
Sponsorship
The Royal Australian Chemical
Institute (RACI)
www.chem.unsw.edu.au/raci
Crown Scientific
APS
Marvellous Metals: the Lecture
Redox Chemistry
Spectra and Spectroscopy
Coordination Chemistry
Redox Chemistry
• Many reactions can be classified
as redox reactions.
• These are reactions in which the
oxidation numbers of the elements
involved change
Example: Redox Chemistry
• An acidified solution of
permanganate ions reacts with
hydrogen peroxide to give
dioxygen gas:
2 MnO4- + 6 H+ + 5 H2O2 
2 Mn2+ + 8 H2O + 5 O2
Mn +7  +2; O (in peroxide) –1  0
Vanadium
• Vanadium is a transition element
that displays a maximum oxidation
state of +5 (eg in the oxide V2O5).
• Named after Vanadis, the Norse
goddess of beauty because of the
beautiful colours in solution
• Used in high strength steels
Vanadium reduction: demo
Initial: solid NH4VO3
Acidification:
VO3- + 2 H+  VO2+ + H2O
Reduction (Zn as reductant):
VO2+ + 2 H+ + e-  VO2+ + H2O
VO2+ + 2 H+ + e-  V3+ + H2O
V3+ + e-  V2+
Vanadium Application
• Sulfuric Acid Manufacture:
SO2 (g) + ½ O2 (g)  SO3 (g)
• Vanadium(V) oxide catalysts are
used in this process.
• Sulfuric acid: 150 million tonnes
produced each year.
Other redox processes
The rusting of iron
Batteries
Electrolysis to purify metals
Using reductants to liberate metals
from ores
Photoreduction: Blueprint
• Blueprints (an early form of
copying) were first made around
1840
2 [Fe(C2O4)3]3-  2 Fe2+ + 2 CO2 + 5 C2O42(K+ +) Fe2+ + [Fe(CN)6]3-  Prussian Blue
• The pigment Prussian Blue has
been known since 1704
More on Prussian Blue
Fe3+ + [Fe(CN)6]4-  Prussian Blue
Fe2+ + [Fe(CN)6]3-  Turnbull’s Blue
Found to have same spectra / XRD.
Colour arises from charge transfer:
Fe3+ + e  Fe2+ (lmax 700nm).
Probable formula:
Fe(III)4[Fe(II)(CN)6]3.15H2O
Spectra and Spectroscopy
• Spectrum: solar spectrum, rainbow
• Plot of radiation intensity vs.
wavelength / frequency
• May be absorption or emission
Uses of Spectroscopy
• Identification
• Quantification
• Study bonding / energy levels
X-ray: inner shell electrons
UV-Vis: outer shell electrons
IR: molecular vibrations
Microwave: rotations
Vanadium check-up
VO2+
VO2+
yellow
blue
V3+
green
V2+
violet
Emission Spectra
E2
h
E1
Emission
Flame tests
Lithium
Sodium
Potassium
Calcium
Strontium
Barium
Copper
Flame tests
• The thermal energy is enough to
shift electrons to higher energy
levels (excited state).
• The electron returns to a lower
energy level with emission of
visible radiation.
Absorption spectra
E2
h
E1
Absorption
Absorption: demonstration
1
Absorbance
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
500
550
600
650
700
Wavelength (nm)
750
800
850
Absorption and colour
• The copper solution appears blue
and absorbs red light.
• Under white light illumination some
wavelengths are absorbed and
some are reflected / transmitted.
• The object / solution has the
complementary colour to the
radiation absorbed.
Atomic absorption
• Atoms in the ground state will
absorb radiation that promotes
electrons to an excited state.
• The amount of radiation absorbed
is proportional to the the number of
atoms present.
• This concept is the basis of Atomic
Absorption Spectroscopy (AAS).
AAS: schematic diagram
Light
source
Flame
E2
E2
h
E1
h
E1
Detector
AAS: Australia’s contribution
• Alan Walsh had worked on
emission spectra and molecular
spectroscopy.
• Demonstrated possibility of AAS in
early 1952.
• Developed commercially by
CSIRO and Australian instrument
manufacturers
AAS: application
• AAS was long considered the best
technique for trace metal analysis.
• Detection Limits (ppb):
Cd 1
Cr 3
Cu 2
Pb 10
V 20
Vanadium: one more time
VO2+
VO2+
yellow
blue
V3+
green
V2+
violet
Coordination Chemistry
….it is correct to say that modern
inorganic chemistry is, especially in
solution, the study of complex
compounds.
Nyholm, The Renaissance of
Inorganic Chemistry, 1956
Dissolution of a salt
• Water binds to ions at edges of
lattice
• When bonds to water are stronger
than bonds to ions, the ion enters
solution
H
+
O
H
Na
O
H
H
Examples
• Nickel(II) ions in solution: Ni2+(aq).
• Species in solution is [Ni(H2O)6]2+.
OH2
H2O
OH2
2+
Ni
H2O
OH2
OH2
• Other examples would include
[Cu(H2O)6]2+, [Fe(H2O)6]3+, etc.
Shapes of Complexes
6-coordinate: Octahedral
4-coordinate: Tetrahedral
Demonstration:
[Co(H2O)6]2+ + 4 Cl- 
[CoCl4]2- + 6 H2O
Changing shapes: demo
[Co(H2O)6]2+ + 4 Cl-  [CoCl4]2- + 6 H2O
pink
blue
OH2
OH2
OH2
2+
Co
OH2
2-
Cl
Co
Cl
OH2
OH2
OCTAHEDRAL
Cl
Cl
TETRAHEDRAL
Coordinate Bond
• Many molecules and ions have
lone pairs of electrons (eg NH3)
and can act as electron pair donors
(Lewis bases).
• Transition metal ions can have
vacant orbitals and can accept
electron pairs (Lewis acids).
Ligands
• The molecules or ions that bind to
a metal ion are known as ligands.
• Many ligands are known ranging
from monoatomic ions such as
chloride to huge protein molecules.
• Examples include NH3, H2O,
NH2CH2CH2NH2 (diaminoethane, a
chelating ligand), SCN(thiocyanate)
Nickel(II) Complexes: Demo
[Ni(H2O)6]2+
[Ni(NH3)6]2+
green
blue
[Ni(NH2CH2CH2NH2)3]2+ blue-purple
[Ni(dmg)2]
red
Colours of Metals Complexes
• In an octahedral complex, the d
orbitals are split into two energy
levels separated by a gap Do.
• The size of Do depends on the
nature of the ligand.
eg
Do
t2g
Differing interactions
• Different metals react in different
ways with the same ligand.
• One example is the difference in
interaction of Ni2+ and Co2+ with
SCN-.
• In the case of cobalt a stable
complex ion is formed [Co(SCN)4]2which is soluble in some organic
solvents.
Demonstration
• A mixture of Ni2+ and Co2+ is
treated with excess SCN-.
• 2-Butanone (CH3COCH2CH3) is
used to extract the reaction
mixture.
• Nickel ions remain in the aqueous
phase and cobalt ions (as
[Co(SCN)4]2-) are extracted into the
organic phase.
Application
• Many extractive metallurgical
processes depend on different
metals interacting in different ways
with ligands.
• Copper can be purified through a
solvent extraction technique.
• Treatment of 107 tonnes per year
of low grade tailings (1%) recovers
a further 105 tonnes of copper.
Thermite: Return to Redox
• The thermite reaction can be used
for such applications as welding in
remote locations and depends on
the activity of aluminium.
• Aluminium powder and iron oxide
are mixed together and the
reaction is started with burning Mg
ribbon.
• Highly exothermic reaction!
Thermite Thermodynamics
Reaction
DH
(kJ mol-1)
2 Al(s) + 3/2 O2(g)  Al2O3(s)
Fe2O3(s)  2 Fe(s) + 3/2 O2(g)
2Al(s) + Fe2O3(s)  Al2O3(s)+ 2Fe(s)
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