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Transition metals as catalysts
A catalyst is a substance that speeds up reactions
by providing an alternative reaction route with lower
activation energy.
Transition metals are good catalysts for two reasons:

they show variable oxidation states. This allows them
to act as intermediates in the exchange of electrons
between reacting species.

they provide a surface for reactions to occur. The metal
forms weak bonds to the reacting species, holding them
in place.
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Types of catalysts
There are two types of catalysts: homogeneous and
heterogeneous.
Homogeneous catalysts are in the
same phase as the reaction species,
e.g. two miscible liquids.
Heterogeneous catalysts are in a
different phase to the reaction species,
e.g. two immiscible liquids.
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Heterogeneous catalysts
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Improving catalyst efficiency
Catalysts are often very expensive. Maximising the efficiency
of catalysts minimizes the cost. One method of increasing
efficiency is to increase the surface area of the catalyst.
In a catalytic converter, a
ceramic honeycomb
structure is coated with
finely divided rhodium and
platinum. The ceramic
support medium is inert but
it increases the surface area
of the catalyst and reduces
the amount needed.
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The Haber process
Many industrial processes use heterogeneous catalysts.
Catalysts increase the rate of a chemical reaction,
although the equilibrium position is unchanged.
The Haber Process produces ammonia from hydrogen and
nitrogen gases. It uses a heterogeneous iron catalyst.
iron catalyst
N2(g) + 3H2(g)
2NH3(g)
Over several years, the iron catalyst becomes poisoned by
impurities such as sulfur compounds. When the efficiency of
the catalyst is greatly reduced, it must be replaced.
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The Contact process
Sulfuric acid is produced by the Contact Process using a
heterogeneous catalyst of vanadium(V) oxide.
2SO2 + O2
2SO3
There are two steps in the reaction.
SO2 + V2O5
2V2O4 + O2
SO3 + V2O4
2V2O5
The oxidation number of vanadium changes from +5 to +4
to +5 again over the course of the reaction.
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Producing methanol
Methanol is produced by two consecutive reactions.
Synthesis gas, a mixture of carbon monoxide and hydrogen,
is first produced from the reaction of methane and steam.
Step 1
CH4(g) + H2O(g)
CO(g) + 3H2(g)
This gas is then used to produce methanol. The reaction is
sometimes catalysed by chromium(III) oxide, (Cr2O3).
Step 2
CO(g) + 2H2(g)
synthesis gas
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CH3OH(g)
methanol
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Heterogeneous catalysts
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Homogenous catalysis
Homogeneous catalysis occurs when the catalyst and
reactants are in the same phase.
It often involves a change in
oxidation state of transition
metal ions.
The transition metal ion
forms an intermediate, then
a further reaction occurs to
regenerate the original
transition metal ion.
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Reaction of I2 and S2O82–
The uncatalysed reaction between iodine and peroxodisulfate
ions is very slow even though it is thermodynamically
favourable.
S2O82– + 2I–
I2 + 2SO42–
Both reactant ions are negatively charged and are likely to
repel each other. Adding aqueous Fe2+ ions provides an
alternative reaction pathway which is much faster.
S2O82– + 2Fe2+
2Fe3+ + 2I–
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2SO42– + 2Fe3+
2Fe2+ + I2
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Autocatalysis
Autocatalysis is when one of the products of
a reaction acts as a catalyst for the reaction.
Initially the rate of an
autocatalysed reaction
is very slow, but as the
product increases, the
reaction rate increases.
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Oxidation of ethanedioic acid
Ethanedioic acid is oxidized by acidified potassium
manganate(VII) ions. The reaction reduces the oxidation
state of manganese from +7 to +2. Mn2+ acts as a catalyst
for the reaction.
2MnO4– + 16H+ + 5C2O42–
2Mn2+ + 10CO2 + 8H2O
Once Mn2+ has been formed, it provides an alternative,
faster reaction pathway via an Mn3+ intermediate.
4Mn2+ + MnO4– + 8H+
2Mn3+ + C2O42–
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5Mn3+ + 4H2O
2CO2 + 2Mn2+
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Industrial catalysts
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Development of new catalysts
Catalysts create high atom economy and this is important in
‘green chemistry’. Developing new catalysts is a priority for
chemical research.
Methanol and carbon monoxide react to produce ethanoic
acid in the presence of a metal catalyst.
CH3OH + CO
CH3COOH
Cobalt was initially used with an iodide co-catalyst. It was
replaced within the same decade by a rhodium-based
catalyst, which works at lower temperatures and pressures.
Further research has shown that an iridium/ruthenium
mixture is cheaper, more stable and more easily recycled. It
also reduces the need for water in the production process.
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Industrial catalysts: true or false?
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Redox titrations
In a titration, the concentration of a solution is determined
by titrating with a solution of known concentration.
In redox titrations, an oxidizing
agent is titrated against a reducing
agent. Electrons are transferred
from one species to the other.
Indicators are sometimes used to
show the endpoint of the titration.
However, most transition metal
ions naturally change colour when
changing oxidation state.
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Oxidation states of manganese
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Potassium manganate(VII) titration
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Potassium manganate(VII) calculation
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Potassium dichromate(VI) titrations
Potassium dichromate(VI) (K2Cr2O7) is an oxidizing agent
used in titrations. The oxidation state of the chromium ion is
reduced from +6 to +3.
The colour change in the titration
is not very visible, so an indicator
of sodium diphenylaminesulfonate
is used. This turns from colourless
to purple at the endpoint.
The solution being titrated against
must be acidified with excess dilute
sulfuric acid.
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Potassium dichromate(VI) equation
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Potassium dichromate(VI) calculation
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Iodine and thiosulfate
A redox reaction occurs between iodine and thiosulfate ions:
2S2O32–(aq) + I2(aq)
2I–(aq) + S4O62–(aq)
The light brown/yellow colour of the iodine turns paler as it
is converted to colourless iodide ions. When the solution is
a straw colour, starch is added to clarify the end point. The
solution turns blue/black until all the iodine reacts, at which
point the colour disappears.
This titration can be used to determine the concentration of
an oxidizing agent, which oxidizes iodide ions to iodine
molecules. The amount of iodine is determined from titration
against a known quantity of sodium thiosulfate solution.
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Iodine and thiosulfate calculation
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Catalytic converters
Most cars in the UK are fitted with catalytic converters. These
convert pollutants such as carbon monoxide, nitrogen oxides
and unburnt hydrocarbons into carbon dioxide, nitrogen and
water; gases which are found naturally in our atmosphere.
Catalytic converters contain an
inert honeycomb structure coated
with the catalyst – platinum and
rhodium. The exhaust gases
enter through the holes and react
on the catalyst surface.
Catalytic converters are easily poisoned, especially by antiknock additives. They do not work when cold and reduce
fuel economy by 2-10%.
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Polychromic sunglasses
The lenses of polychromic sunglasses contain silver halide
nanoparticles. These particles are transparent in artificial
light. A photochemical reaction occurs on exposure to UV
radiation, found in sunlight.
UV radiation changes the shape of
the nanoparticles. They absorb
some of the visible light, so the
lenses appear darker. Without UV
radiation, the molecules return to
their original shape and the lenses
appear colourless again.
Glass blocks the UV light responsible for this reaction, so
polychromic lenses will not darken in a car or when looking
out of a window.
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Chemotherapy drugs
Platin is a platinum complex that forms cis–trans
stereoisomers. The cis isomer, cisplatin, is used as an anticancer drug. The trans isomer, transplatin, doesn't have the
same effect and is not used in chemotherapy.
cisplatin
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transplatin
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How does cisplatin act?
Cisplatin is administered intravenously. It is very useful in
treating solid tumours.
For a cell to replicate, the double helix DNA molecule must
unwind. Cisplatin prevents it from unwinding by forming
coordinate bonds with the DNA bases. Nitrogen atoms in the
bases displace the ammonia ligands in the cisplatin complex.
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Risks of cisplatin
Cisplatin is an important drug used to prolong the life of
cancer patients. However, there are some risks associated
with its usage.

Cisplatin also prevents
normal cells in the body
from replicating.

Patients may experience
side-effects, ranging from
nausea and vomiting to lifethreatening complications
such as kidney damage.

Patients can become resistant to cisplatin.
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Paints and dyes
Most transition metal compounds are coloured and many are
used in paints and dyes.
Copper compounds produce very
vibrant blue colours.
Phthalocyanine blue is a copper
complex used in paint dyes. It is
very stable and insoluble in water.
Titanium dioxide is a white
solid at room temperature.
Nanoparticles of titanium oxide
are used to whiten paper and
as a white pigment in paint.
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Alloys
An alloy is a solid mixture of two or more metals, that can
also contain other non-metal elements. Alloys often have
properties that are very different to their constituent metals.
Carbon is added to iron to make
an alloy of steel, which is much
stronger than iron.
Chromium can also be added to
make stainless steel, which is
resistant to corrosion.
Copper is used in many different alloys,
such as brass, bronze and coinage metals.
The copper content of an alloy can be
estimated by titration with I2/S2O32–.
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Match the use to the property
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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