1 of 40© boardworks ltd 2010. 2 of 40© boardworks ltd 2010

1 of 40 © Boardworks Ltd 2010


Transition metals as catalysts

A catalyst is a substance that speeds up reactions by providing an alternative reaction route with lower

activation energy.

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.

Transition metals are good catalysts for two reasons:


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.


Heterogeneous catalysts


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.


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.

The Haber process

The Haber Process produces ammonia from hydrogen and nitrogen gases. It uses a heterogeneous iron catalyst.

N2(g) + 3H2(g) 2NH3(g)

iron catalyst

Many industrial processes use heterogeneous catalysts. Catalysts increase the rate of a chemical reaction, although the equilibrium position is unchanged.


The Contact process

2SO2 + O2

Sulfuric acid is produced by the Contact Process using a heterogeneous catalyst of vanadium(V) oxide.

There are two steps in the reaction.

The oxidation number of vanadium changes from +5 to +4 to +5 again over the course of the reaction.

2SO3

SO2 + V2O5 SO3 + V2O4

2V2O4 + O2 2V2O5


Producing methanol

Methanol is produced by two consecutive reactions.

Step 1

Step 2

CH4(g) + H2O(g) CO(g) + 3H2(g)

CO(g) + 2H2(g) CH3OH(g)

synthesis gas methanol

This gas is then used to produce methanol. The reaction is sometimes catalysed by chromium(III) oxide, (Cr2O3).

Synthesis gas, a mixture of carbon monoxide and hydrogen, is first produced from the reaction of methane and steam.


Heterogeneous catalysts


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.


Reaction of I2 and S2O82–

The uncatalysed reaction between iodine and peroxodisulfate ions is very slow even though it is thermodynamically favourable.

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– + 2I– I2 + 2SO4

2–

S2O82– + 2Fe2+ 2SO4

2– + 2Fe3+

2Fe3+ + 2I– 2Fe2+ + I2


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.


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+ + 5C2O4

2– 2Mn2+ + 10CO2 + 8H2O

Once Mn2+ has been formed, it provides an alternative, faster reaction pathway via an Mn3+ intermediate.

4Mn2+ + MnO4– + 8H+ 5Mn3+

+ 4H2O

2Mn3+ + C2O42– 2CO2 + 2Mn2+


Industrial catalysts


Methanol and carbon monoxide react to produce ethanoic acid in the presence of a metal catalyst.

Development of new catalysts

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.

Catalysts create high atom economy and this is important in ‘green chemistry’. Developing new catalysts is a priority for chemical research.

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.

CH3OH + CO CH3COOH


Industrial catalysts: true or false?


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.


Oxidation states of manganese


Potassium manganate(VII) titration


Potassium manganate(VII) calculation


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 solution being titrated against must be acidified with excess dilute sulfuric acid.

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.


Potassium dichromate(VI) equation


Potassium dichromate(VI) calculation


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.


Iodine and thiosulfate calculation


Catalytic converters

Catalytic converters are easily poisoned, especially by anti-knock additives. They do not work when cold and reduce fuel economy by 2-10%.

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.

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.


Polychromic sunglasses

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.

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.

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.


Chemotherapy drugs

Platin is a platinum complex that forms cis–trans stereoisomers. The cis isomer, cisplatin, is used as an anti-cancer drug. The trans isomer, transplatin, doesn't have the same effect and is not used in chemotherapy.

cisplatin transplatin


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.


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 life-threatening complications such as kidney damage.

Patients can become resistant to cisplatin.


Paints and dyes

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.

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.

Most transition metal compounds are coloured and many are used in paints and dyes.


Alloys

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/S2O3

2–.

Carbon is added to iron to make an alloy of steel, which is much stronger than iron.

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.

Chromium can also be added to make stainless steel, which is resistant to corrosion.


Match the use to the property


Glossary


What’s the keyword?


Multiple-choice quiz

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