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.

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Final Notes

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