Transcript Lecture 9

Regulations for American Pupils and
Middle School Students
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Always refer to a teacher by title and last name
Get to class on time
Raise your hand when you want to ask a question
You may speak to the teacher from your desk while
you are seated
When you are absent, you must make up the work you
have missed. Ask either the teacher or a classmate for
the work
If you expect to be away from school because of an
emergency, tell your teacher in advance and ask for
the work you will miss
All assignments you hand in must be your own work
Never cheat on a test
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Transition elements: the facts I
Transition elements: the facts I
DEFINITIONS
d block element: any element with its highest
energy electron in a d orbital
Transition element: those elements having
ions with electrons in an incomplete d shell
i.e. from titanium to copper
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Transition elements: the facts I
TYPICAL PHYSICAL PROPERTIES:
all metals
high m.p. chromium 2160K, iron 1800K compared with sodium 371K
hard
dense
but not titanium
similar atomic and ionic sizes and ionization energies.
Unlike elements in the s and p blocks, there is little change in
atomic and ionic radii as the d block is crossed. This is because the
additional electrons are going into an inner d sub-shell. This also
results in only a small increase in ionization energy across the d
block. Although each successive nucleus has one more proton, this
extra positive charge is partly shielded from the outer 4s electrons
by the extra d electron in an underlying shell.
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Transition elements: the facts I
TYPICAL CHEMICAL PROPERTIES
1. Variable valency
Transition elements show many oxidation states; these fall into two
kinds:
● higher oxidation states: the covalently bonded oxo-compounds
e.g. CrO42-; Cr2O72-; MnO4-; MnO42● lower oxidation states: the atomic ions
e.g. Cr3+; Cr2+; Mn3+; Fe3+; Fe2+; Cu2+; Cu+
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Transition elements: the facts I
2. Colored compounds
Many of the compounds of the transition elements are colored.
common examples:
CrO42--yellow;
Cr2O72--orange; Cr3+-green; Cr2+-blue;
chromate(VI)
dichromate
MnO4--purple;
MnO42--green;
Mn2+-pale pink
(per)manganate(VII) manganate (VI)
Fe3+-yellow; Fe2+-green
Co2+-pink in water, blue when dry
Cu2+-blue
●
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Transition elements: the facts I
3. Catalytic properties
Transition metals and their compounds can be:
heterogeneous catalysts, for example:
iron in the Haber process
V2O5 in the Contact process
● homogeneous catalysts, for example:
Mn2+ in the reaction between ethanedioate and manganate(VII)
Fe2+/Fe3+ in the reaction between iodide ions and
peroxydisulphate (VI).
●
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Transition elements: the facts II
Transition elements: the facts II
Magnetic properties
Some of the transition elements are ferromagnetic which means
that they can be magnetized, e.g. iron, cobalt, and nickel.
Some of their compounds are paramagnetic which means that
they move in a strong magnetic field.
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Transition elements: the facts II
TYPICAL CHEMICAL PROPERTIES
4. Complex ion formation
A complex ion consists of a central ion or atom surrounded by
other particles called ligands. A ligand is a particle (ion or
molecule) with a lone pair which forms a dative covalent bond to
the central particle. The ligands are said to be coordinated to the
central particle.
Transition metal ions form many complex ions which vary in
charge, shape, color, and stability.
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Transition elements: the facts II
Shape
Ligands differ in size and this means that the number which can fit
around the central cation changes. Ammonia and water are
relatively small ligands and six of each can fit around a cobalt or
copper ion forming octahedral complexes, while only four of the
larger chloride ion can fit around either the cobalt or copper ions.
Silver is unusual in forming
linear complexes.
[NC—Ag – CN][H3N—Ag –NH3]+
[O3S2—Ag – S2O3]39
Transition elements: the facts II
Charge
The charge of the complex depends on the relative charges of the
central ion or atom and the ligands, and on the number of ligands
around it. Complex ions may be cations or anions
cationic complexes
anionic complexes
Cu(NH3)4(H2O)22+(aq),
CuCl42-(aq),
FeCNS2+(aq)
Fe(CN)63-(aq)
Color
The color of the complexes is affected by the nature of the ligand
and the number of ligands around the central cation.
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Transition elements: the theories I
Transition elements: the theories I
PHYSICAL PROPERTIES
The physical properties are dominated by the fact that the electrons
with the highest energies go into an inner 3d orbital rather than the
outer 4s orbital. These electrons in an underlying d orbital increase
the electron repulsion on the outer 4s electrons.
The elements are hard and have high m.p.s because they have high
lattice energies.
The lattice energies are high because the effective nuclear charge of
the cations in the lattice is high, because the electrons in the d
orbitals are bad at shielding the nuclear charge.
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Transition elements: the theories I
The sizes of the atoms and ions do not decrease much as the block is
crossed. Although the atoms of each successive element have one
more proton in the nucleus, increasing the attraction on the outer 4s
electrons, there is increased repulsion on these outer electrons caused
by the new electron in the inner d shell.
The first ionization energies do not increase very much for the same
reason. There are more protons attracting the electrons as one crossed
the d block, but also more inner electrons repelling the outer electrons.
The two effects almost cancel out.
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Transition elements: the theories I
CHEMICAL PROPERTIES
1. Variable valencies
In higher oxidation states the transition elements form molecules
and molecule ions because of the availability of vacant d and p
orbitals, which can accept electrons from the surrounding atoms.
The lower oxidation states happen because the transition elements
have successive ionization energies which are similar in value to the
size of hydration energies. This is not the case with the s block
metals.
e.g. compare sodium and iron
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Transition elements: the theories I
Sodium
Iron
So the hydration energy in solutions (and the lattice energy in solids)
makes up for the slightly higher ionization energies in the higher
oxidation states of iron, but not the very much higher ionization
energies in the likes of sodium.
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Transition elements: the theories I
2. Color
Light falling on transition element compounds interacts with the d
electrons. Some of the wavelengths in the light are absorbed leaving
the complementary colors to be seen.
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Transition elements: the theories I
3. Catalytic properties
Heterogeneous catalysis
The fact that the d block elements have 3d as well as 4s electrons
helps them to form bonds with gaseous particles and so adsorb them
onto the catalyst surface. This adsorption weakens the bonds in the
gas particle, so lowering the activation energy of the reaction (see
page 41 on catalysis).
Homogeneous catalysis
The fact that d block elements can exist in so many oxidation states
is a crucial factor in making them such good homogeneous catalysts
(see page 41 on catalysis)
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Transition elements: the theories II
Transition elements: the theories II
CHEMICAL PROPERTIES
4. Complex formation
d electrons are not as good at shielding the positive charge of the
nucleus as either s or p electrons. This means that transition metal
atoms and ions have greater polarizing power than the atoms and
ions from the s block.
The poorly shielded nucleus
attracts lone pairs of
electrons strongly and dative
covalent bonds are formed
between the central atom and
the ligands, forming a
complex ion—or complex for
short.
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Transition elements: the theories II
There is often more than one kind of ligand in a solution and, if so,
the different ligands will compete for the cation. For example,
aqueous ammonia contains both water and ammonia ligands. The
better a ligand is at competing for cation, the more stable will be the
complex formed. A stable complex has a large Kstab value, so a table
of Kstab figures allows you to predict whether one ligand will replace
another.
e.g.
Kstab(Cu(NH3)42+) = 1.4×1013dm12mol-4 but
Kstab(CuCl42- ) = 4×105dm12mol-4
So ammonia ligands will replace chloride ligands around copper if
the concentrations of the two are the same.
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Transition elements: the theories II*
Chelates
A ligand with more than one lone pair can form more than one
dative bond with the central atom if the lone pairs are the right
distance apart. Because a ligand with two lone pairs looks
rather like a claw, it is called a chelate from the Greek wordmeaning claw. If the ligand has two lone pairs it is called a
bidentate chelate. Hemoglobin(血色素) contains a tetradentate
chelate while the ion ethanediaminotetraethanoate (acetate) is a
hexdentate chelate known by its initials EDTE(EDTA).
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Transition elements: the facts and theories
words
Words and Expressions
Titanium; vanadium; chromium; manganese; iron; cobalt;
nickel; copper; zinc
ionic radius; ionic radii
octahedral; tetrahedral; linear
ferromagnetic; paramagnetic
vacant orbitals; filled orbitals; incomplete orbitals
heterogeneous catalysis; homogeneous catalysis
adsorption; adsorb; absorb; absorption
chelate; claw
bidentate; tetradentate; hexadentate
Hemoglobin(血色素)
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