Catalytic Properties
- Transition metals are often used as catalysts in the elemental form or as compounds
- The ability of transition metals to form more than one stable oxidation state means that they can accept and lose electrons easily
- This enables them to catalyse certain redox reactions. They can be readily oxidised and reduced again, or reduced and then oxidised again, as a consequence of having a number of different oxidation states of similar stability
- There are two types of catalyst:
- A heterogeneous catalyst is in a different physical state (phase) from the reactants
- The reaction occurs at active sites on the surface of the catalyst
- An example is the use of iron, Fe, in the Haber process for making ammonia
- A heterogeneous catalyst is in a different physical state (phase) from the reactants
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
- A homogeneous catalyst is in the same physical state (phase) as the reactants
- The hydrogenation or reduction of alkenes makes use of a nickel catalyst
CH2=CH2 (g) + H2 (g) → CH3CH3 (g)
- The same reaction is used in the hydrogenation of vegetable oils to form polyunsaturated fats
- The decomposition of hydrogen peroxide is a common reaction in the study of chemical kinetics and uses manganese(IV) oxide as the catalyst
2H2O2 (g) → 2H2O (aq) + O2 (g)
Catalytic converters
- Catalytic converters are used in car exhaust boxes to reduce air pollution. They usually consist of a mixture of finely divided platinum and rhodium supported on a ceramic base
Diagram of a catalyst on an inert support medium in a vehicle catalytic converter
- Carbon monoxide, nitrogen dioxide and unburnt hydrocarbons are sources of pollution in car exhaust
- The transition metal catalysts facilitate the conversion of these pollutants into less harmful products
2NO (g) + 2CO (g) → N2 (g) + 2CO2 (g)
CH3CH2CH3 (g) + 5 O2 (g) → 3CO2 (g) + 4H2O (g)
- Some of the transition metals are precious metals so they can be very expensive
- In order to minimise the cost and maximise the efficiency of the catalyst the following measures can be taken:
- Increasing the surface area of the catalyst
- Coating an inert surface medium with the catalyst to avoid using large amounts of the catalyst
- This is achieved by spreading the catalyst over a hollow matrix such as a honeycomb-like structure
Biological catalysts
- Many of the enzyme catalysed reactions in the body make use of homogeneous transition metal catalysts
- An example of this is haemoglobin, abbreviated to Hb, which transports oxygen around the blood
The structure of haemoglobin
The structure of haem
- The iron(II) ion is in the centre of a large heterocyclic ring called a porphyrin
- The iron has a coordination number of four, is square planar and can bind to one oxygen molecule
- The Hb molecule contains four porphyrin rings so each Hb can transport four oxygen molecules
Magnetic Properties
- Materials are classified as diamagnetic, paramagnetic or ferromagnetic according to their behaviour when placed in an external magnetic field
- Diamagnetism is a property of all materials and produces a very weak opposition to an applied magnetic field
- It arises from the repulsion of electrons to the applied magnetic field
- Paramagnetism only occurs in substances which have unpaired electrons
- It produces magnetisation proportional to the applied field and in the same direction
- Transition metal complexes show paramagnetism
- Ferromagnetism has the largest effect and produces magnetisation greater than the applied field
Diamagnetism
- The atoms of diamagnetic materials have paired electrons
- Spinning electrons create a tiny magnetic dipole
- The paired electrons orientate themselves so that the magnetic field they create opposes the external field
- This result is a very weak repulsion force
Argon is diamagnetic with the electron configuration 1s2 2s2 2p6 3s2 3p6
- Many molecules are diamagnetic since all the electrons are paired up in bonds
- It is very hard to demonstrate diamagnetism but it is possible by suspending a sample of the material from a sensitive force meter and lowering it into a strong horseshoe magnet - a slight change in the force should be seen
Paramagnetism
- Paramagnetic materials are attracted to an external magnetic field
- The unpaired electrons can be temporarily aligned to the magnetic field causing attraction into the field
The electrons in titanium are arranged in their orbitals as shown. The unpaired electrons can be temporarily aligned in an external field.
- Most of the transition metals and their ions are paramagnetic as they have unpaired electrons
- Paramagnetism increases with the number of unpaired electrons, so it generally increases across the d-block up to a maximum with chromium and then decreases
- Zinc has no unpaired electrons so is not paramagnetic
Orbital diagrams for the first row d-block showing the number of unpaired electrons increasing up to chromium
Ferromagnetism
- The metals iron, cobalt and nickel show the unusual property of ferromagnetism
- The alignment of the unpaired electrons in an external field in ferromagnetic materials can be retained so the material becomes permanently magnetised
- If ferromagnetic materials are heated and cooled in a magnetic field, the magnetic field of the electrons remains
- Magnetic regions within the metal that are aligned magnetically are know as domains
- Banging or heating a permanent magnet will weaken the magnetism
Ferromagnetic materials are used to make permanent magnets which produce characteristic magnetic field lines