• Catalysts?increase the rate of reaction by providing an alternative pathway which has a lower?activation energy
• Catalysts can be either?homogeneous?or?heterogeneous
• Homogeneous catalysts are those that are in the same phase as the reaction mixture
• For example, in the esterification of ethanoic acid (CH₃COOH) with ethanol (CH₃CH₂OH) to form ethyl ethanoate (CH₃COOCH₂CH₃) under acidic conditions

• The H⁺?is a homogeneous catalyst as like the reactants and product it is in the?aqueous?phase

• Heterogeneous catalysts are those that are in a different phase to the rest of the reaction mixture
• For example, in the Born-Haber process to form ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂) an iron (Fe) catalyst is used

• The Fe catalyst is a heterogeneous catalyst as it is in?the?solid phase whereas the reactants and products are all in the?gas?phase

• 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

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

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 (NO_y) 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

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

• 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 (S_2,O₈^2-?) ions?oxidise?the?iodide?to?iodine

S₂O₈^2-?(aq) + 2I^-?(aq) → 2SO₄^2-?(aq) + I₂?(aq)

• Since both the S₂O₈^2-?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 Fe²⁺?by? I^-

2Fe³⁺?(aq) + 2I^-?(aq) → 2Fe²⁺?(aq) + I₂?(aq)

• • Then, Fe²⁺?is?oxidized?back to Fe³⁺?by S₂O₈^2-

2Fe²⁺?(aq) + S₂O₈^2-?(aq) → 2Fe³⁺?(aq) + 2SO₄^2-? (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⁺

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 sulfur dioxide which oxidises in air to sulfur trioxide, and then?dilute sulfuric acid?(H₂SO₄) is formed by reaction with water. The result is acidification of 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) + ? 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