AP Chemistry Unit 5.11 Catalysis

Catalysis

• Catalyst: chemical agent that speeds up a reaction without being consumed by the reaction → can be used over and over again
  • Do not affect the free energy/enthalpy of a reaction!
  • Reactions with a catalyst will typically require at least 2 steps
• Catalysts allow reactions to occur with a lower AE → lower activation energy means that more collisions will have enough energy to overcome AE and form a product → reaction rate increased
• Homogeneous catalyst: is present in the same phase as the reacting molecules
• Heterogeneous catalyst: exists in a different phase (usually as a solid; a catalytic converter is one type)
  • 4 steps of heterogeneous catalysis

1. 1. 1. Adsorption (collection of one substance on the surface of another substance) and activation of the reactants
      2. Migration of the adsorbed reactants on the surface
      3. Reaction of the adsorbed substances
      4. Escape (desorption) of the products

How Catalysts Speed Up Reactions

• A catalyst increases the rate constant and lowers Ea Barrier by…
  • Forming a more stable activated complex
  • Increased collision frequency
  • Improved orientation effects
• Allow chemical reactions to occur at lower temperatures
• Speeds up natural reactions ≠ cause them
• Reactions can occur without catalysts, but would be slower & cost a lot more energy