Understanding Reaction Mechanisms and Catalysts

Nov 15, 2024

Lecture on Reaction Mechanisms, Catalysts, and Reaction Coordinate Diagrams

Introduction

  • Focus on reaction mechanisms, catalysts, and reaction coordinate diagrams.
  • Learn to derive rate laws from reaction mechanisms.
  • Discuss characteristics of catalysts.
  • Explore reaction coordinate diagrams to understand activation energy.

Reaction Mechanisms

  • Definition: Sequence of elementary steps that cannot be broken down further.
  • Most reactions have multiple steps.
  • Rate Law: Determined by the slowest step (rate-determining step).

Elementary Reactions

  • Three classes:
    • Unimolecular: One reactant molecule.
    • Bimolecular: Two reactant molecules.
    • Termolecular: Three reactant molecules (rare).
  • No reactions with more than three molecules colliding at once.

Mechanisms

  • Composed of elementary steps.
  • Can be summed to determine the overall reaction.
  • Catalysts and Intermediates:
    • Intermediate: Produced and consumed within the reaction.
    • Catalyst: Present at the start and end, but not consumed.

Rate Laws from Mechanisms

  • Elementary Steps: Coefficients of reactants can be used to determine rate law.
  • Slow Step: Determines overall reaction rate.
  • Rate Law Scenarios:
    • Slow step first: Rate law of the first step is the overall rate law.
    • Slow step second: More complex derivation needed, involving equilibrium expressions.

Example Problem

  • Two-step mechanism: O3 breaking down and reacting with another O3.
  • Rate Law Derivation:
    • Start with slow step's rate law.
    • Avoid intermediates in overall rate law by substituting equivalent expressions from equilibrium steps.

Catalysts

  • Function: Speed up reactions by lowering activation energy (Eₐ) through alternative pathways.
  • Not consumed during the reaction.
  • Do not shift equilibrium; only affect the rate.

Reaction Coordinate Diagrams

  • Components:
    • Y-axis: Energy.
    • X-axis: Reaction progress.
  • Shows energy changes from reactants to products.
  • Activation Energy (Eₐ): Energy required to reach the transition state.
  • Transition State: Peak of the energy barrier.

Types of Reactions

  • Endothermic: Products higher in energy than reactants (ΔH positive).
  • Exothermic: Products lower in energy (ΔH negative).

Multi-step Reactions

  • Indicated by multiple peaks/hills in diagrams.
  • Intermediates represented by valleys between peaks.
  • Slow step identified by the highest activation energy.

Summary

  • Catalysts provide alternative mechanisms with lower activation energies.
  • Reaction coordinate diagrams help visualize energy changes and identify kinetic properties.

Final Notes

  • Explore more on Chad's Prep for practice and further study materials.
  • Reaction kinetics problems available in the General Chemistry Master Course.