CIE A Level Chemistry (9701) 2019-2021

Revision Notes

5.5.8 Homogeneous & Heterogeneous Catalysts

Heterogeneous Catalysis

  • In heterogeneous catalysis, the molecules react at the surface of a solid catalyst
  • The mode of action of a heterogeneous catalyst consists of the following steps:
  • Adsorption (or chemisorption) of the reactants on the catalyst surface
    • The reactants diffuse to the surface of the catalyst
    • The reactant is physically adsorbed onto the surface by weak forces
    • The reactant is chemically adsorbed onto the surface by stronger bonds
    • Chemisorption causes bond weakening between the atoms of the reactants
  • Desorption of the products
    • The bonds between the products and catalyst weaken so much that the products break away from the surface
  • For example, the adsorption of hydrogen molecules onto a palladium (Pd) surface

Reaction Kinetics - Mode of Action Heterogeneous Catalysis, downloadable AS & A Level Chemistry revision notes

The reactants are adsorbed on the catalyst surface causing bond weakening and eventually desorption of the products

Iron in the Haber process

  • In the Haber process ammonia (NH3) is produced from nitrogen (N2) and hydrogen (H2)
  • An iron catalyst is used which speeds up the reaction by bringing the reactants close together on the metal surface
  • This increases their likelihood to react with each other
  • The mode of action of the iron catalyst is as follows:
    • Diffusion of the nitrogen and hydrogen gas to the iron surface
    • Adsorption of the reactant molecules onto the iron surface by forming bonds between the iron and reactant atoms
      • These bonds are so strong that they weaken the covalent bonds between the nitrogen atoms in N2 and hydrogen atoms in H2
      • But they are weak enough to break when the catalysis has been completed
    • The reaction takes place between the adsorbed nitrogen and hydrogen atoms which react with each other on the iron surface to form NH3
    • Desorption occurs when the bonds between the NH3 and iron surface are weakened and eventually broken
    • The formed NH3 diffuses away from the iron surface

Reaction Kinetics - Iron Catalyst (1), downloadable AS & A Level Chemistry revision notes

Reaction Kinetics - Iron Catalyst (2), downloadable AS & A Level Chemistry revision notes

Iron brings the nitrogen and hydrogen closer together so that they can react and hence increases the rate of reaction

Heterogeneous catalyst in catalytic converters

  • Heterogeneous catalysts are also used in the catalytic removal of oxides of nitrogen from the exhaust gases of car engines
  • The catalysts speed up the conversion of:
    • Nitrogen oxides (NOy) into harmless nitrogen gas (N2)
    • Carbon monoxide (CO) into carbon dioxide (CO2)
  • The catalytic converter has a honeycomb structure containing small beads coated with platinum, palladium, or rhodium metals which act as heterogeneous catalysts
  • The mode of action of the catalysts is as following:
    • Adsorption of the nitrogen oxides and CO onto the catalyst surface
    • The weakening of the covalent bonds within nitrogen oxides and CO
    • Formation of new bonds between:
      • Adjacent nitrogen atoms to form N2 molecules
      • CO and oxygen atoms to form CO2 molecules
    • Desorption of N2 and CO2 molecules which eventually diffuse away from the metal surface

Reaction Kinetics - Catalytic Converters, downloadable AS & A Level Chemistry revision notes

The metals in catalytic converters speed up the conversion of nitrogen oxides and CO into N2 and CO2 respectively

Homogeneous Catalysis

  • Homogeneous catalysis often involves redox reactions in which the ions involved in catalysis undergo changes in their oxidation number
    • As ions of transition metals can change oxidation number they are often good catalysts
  • Homogeneous catalysts are used in one step and are reformed in a later step

The iodine-peroxydisulfate reaction

  • This is a very slow reaction in which the peroxydisulfate (S2,O82- ) ions oxidise the iodide to iodine

S2O82- (aq) + 2I (aq) → 2SO42- (aq) + I2 (aq)

  • Since both the S2O82- and I ions have a negative charge, it will require a lot of energy for the ions to overcome the repulsive forces and collide with each other
  • Therefore, Fe3+ (aq) ions are used as a homogeneous catalyst
  • The catalysis involves two redox reactions:
    • First, Fe3+ ions are reduced to Fe3+ by  I

2Fe3+ (aq) + 2I (aq) → 2Fe2+ (aq) + I2 (aq)

    • Then, Fe2+ is oxidized back to Fe3+ by S2O82-

2Fe3+ (aq) + S2O82- (aq) → 2Fe3+ (aq) + 2SO42-  (aq)

  • By reacting the reactants with a positively charged Fe ion, there are no repulsive forces, and the activation energy is significantly lowered
  • The order of the two reactions does not matter
    • So, Fe2+ can be first oxidised to Fe3+ followed by the reduction of Fe3+ to Fe2+

Reaction Kinetics - Energy Level Diagram Homogeneous Catalyst, downloadable AS & A Level Chemistry revision notes

The catalysed reaction has two energy ‘humps’ because it is a two-stage reaction

Nitrogen oxides & acid rain

  • As fossil fuels contain sulfur, burning the fuels will release dilute sulfuric acid (H2SO4) into the atmosphere which can cause acidic rain

SO3(g) + H2O(l) → H2SO4(aq)

  • Nitrogen oxides can act as catalysts in the formation of acid rain by catalysing the oxidation of SO2 to SO3

NO2(g) + SO2(g) → SO3(g) + NO(g)

  • The formed NO gets oxidised to regenerate NO2

NO(g) + ½ O2(g) → NO2(g)

  • The regenerated NO2 molecule can again oxidise another SO2 molecule to SO3 which will react with rainwater to form H2SO4 and so on

Enzymes

  • Enzymes are biological catalysts that are made up of proteins
  • They increase the rate of reactions that take place in cells
  • The active sites of enzymes are very specific and will only bind molecules (also called substrates) with a similar shape
  • This is also known as the lock-and-key model
    • The substrate molecule fits in the enzyme’s active site like a key fits in a lock
  • Because of this, enzymes are highly specific and will only speed up specific reactions
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