IB Chemistry HL

Revision Notes

13.1.4 Catalytic & Magnetic Properties

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

N2 (g) + 3H2 (g) 2NH3 (g)

    • A homogeneous catalyst is in the same physical state (phase) as the reactants

Further Examples of transition metal catalysts

  • 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

Catalytic-Converters, IGCSE & GCSE Chemistry revision notes

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 bloodHaemoglobin, downloadable AS & A Level Biology revision notes

The structure of haemoglobin

 

Haemoglobin, downloadable AS & A Level Chemistry revision notes

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

Diamagnetism electron configuration example, downloadable IB Chemistry revision notes

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

Atomic Structure Electron in Box Notation, downloadable AS & A Level Chemistry revision notes

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 diagram for d block, downloadable IB Chemistry revision notes

 

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

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