UNIT: 5.11 Notes on Catalysis and Reaction Mechanisms

  • Objective of Catalysts:

    • Catalysts speed up chemical reactions without being consumed in the process.
    • They don't appear in the overall balanced reaction.
    • In reaction mechanisms, catalysts appear as reactants in initial steps and are produced later.

  • Catalysts vs. Enzymes:

    • The term "enzyme" is often used in biological contexts but refers to the same concept as a catalyst.
    • Enzymes are typically proteins that act as catalysts in biological settings.

  • Types of Catalysts:

    • Homogeneous Catalysts:
    • These are in the same phase as the reactants (solid, liquid, gas, aqueous).
    • Example: All reactants and catalyst are in an aqueous solution.
    • Heterogeneous Catalysts:
    • These are in a different phase from the reactants.
    • Example: Platinum in catalytic converters, where the reactants (gases) react with the solid platinum catalyst.

  • Activation Energy:

    • Catalysts lower the activation energy required for a reaction to proceed, creating an alternate pathway for the reaction.
    • Even with a catalyst, the overall change in enthalpy (ΔH) remains the same, as it is only the activation energy that is affected.

  • Mechanisms of Action:

    • Catalysts may create a more stable activated complex or transition state, facilitate better molecular orientation, and increase collision frequency between reactants.

  • Example Problem:

    • Problem Statement: Determine if the rate of the first mechanism for ozone decomposition is faster than the second due to having fewer steps.
    • Clarification:
    • Both mechanisms have the same overall reaction but differ in the number of elementary steps.
    • Mechanism 1 has no catalyst, while Mechanism 2 includes nitrogen monoxide as a catalyst.
    • Conclusion:
    • Disagreement with the student's claim: presence of a catalyst (Mechanism 2) allows for a faster reaction due to lower activation energy, not simply the number of steps.

  • Final Remarks:

    • The study of catalysis is key to understanding chemical kinetics and reactions, marking the conclusion of Unit 5.