3.5.2 Catalysis

• A catalyst is a substance that increases the rate of reaction by providing the particles with an alternative mechanism with a lower activation energy
  • Catalysts do not have to be directly involved in the chemical reaction
  • They can provide a surface for the reaction to occur on, e.g. iron in the Haber process
  • Alternatively, they can provide a particle (often an ion) to start the reaction but this particle is regenerated at the end so the catalyst appears unaltered, e.g. chlorine radicals in ozone depletion

A catalyst increases the rate of a reaction by providing an alternative pathway which has a lower activation energy

• Catalysts can be divided into two types:
  • Homogeneous catalysts
  • Heterogeneous catalysts

• Homogeneous means that the catalyst is in the same phase as the reactants
  • For example, the reactants and the catalysts are all in solution

• Heterogeneous means that the catalyst is in a different phase to the reactants
  • For example, the reactants are gases, but the catalyst used is a solid

Benefits of catalysts

• Catalysts speed up the rate of reaction, meaning the use of a catalyst may mean lower temperatures and pressures can be used
  • This can save energy costs as there is reduced energy demand for providing high temperatures and less electrical pumping costs for producing the high pressures usually required
  • This can mean fewer CO₂ emissions from burning fossil fuels
• Catalysts can also enable different reactions to be used, with better atom economy and with reduced waste, or fewer undesired products or less use of hazardous solvents and reactants
• Catalysts are often enzymes, generating very specific products, and operating effectively close to room temperature and pressure