Catalysts in Chemical Reactions

Catalysts and Rate of Reaction

• Activation energy (Ea) is 245 kJ mol-1 for uncatalysed reactions and 121 kJ mol-1 for catalysed reactions.
• Catalysts have a big effect on the rate of reaction.

Types of Catalysts

• Homogeneous catalysts: reactants and catalyst are in the same phase (e.g., gas/gas or liquid/liquid).
• Heterogeneous catalysts: reactants and catalyst are in different phases (e.g., liquid/gas, solid/gas, or solid/liquid).

Advantages and Disadvantages

• Homogeneous catalysts:
  • Can control concentration precisely.
  • Difficult to remove catalyst.
• Heterogeneous catalysts:
  • Can be easily filtered away from the reaction mixture.
  • Can be poisoned by substances binding to active sites.

Heterogeneous Catalysts

• The catalyst must adsorb the reactant strongly enough to enable it to react.
• The catalyst must allow the product to desorb, otherwise, the active sites will become blocked.

Transition Metal Catalysts

• Transition metals are good redox catalysts because they have variable oxidation states and can be repeatedly oxidized and reduced.

Increasing the Rate of Reaction

• Requirements for a reaction to take place:
  • Particles collide.
  • Particles collide with sufficient energy to react.
  • Particles collide with the correct orientation.
• Ways to increase the rate of reaction:
  • Increase concentration.
  • Increase pressure.
  • Increase surface area.
  • Stir the solution.
  • Increase temperature.
  • Add a catalyst.

Effect of Catalysts

• A catalyst provides an alternative route for the reaction with a lower activation energy.
• A catalyst does not lower the activation energy.
• A catalyst can:
  • Increase the rate of the reaction.
  • Increase the probability of a successful collision.
  • Increase the rate constant for a reaction.
  • Help the system reach equilibrium more rapidly.

Rate Equations

• The rate equation gives information about how a reaction takes place and the most efficient method of increasing the rate of reaction.
• The overall order of the reaction is found by adding up the individual orders with respect to all reactants.

Determining the Order of a Reaction

• To determine the order of a reaction, you need to measure the rate of reaction and see how it changes as you alter the concentration of each reagent.
• Applications:
  • Working out how a reaction takes place (e.g., within the body).
  • Finding the best way to optimize a reaction (e.g., industrially).
  • Determining how much time has passed since a reaction started.