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Elementary Reactions & Introduction to Reaction Mechanisms

Elementary reactions are single-step reactions with a specific molecular event, while reaction mechanisms describe the sequence of elementary steps leading to the overall reaction.

Key Concepts​

  • Elementary Reaction: A reaction that occurs in a single step with a defined molecularity

    • Unimolecular: Involves one molecule
    • Bimolecular: Involves two molecules
    • Termolecular: Involves three molecules (rare)

  • Reaction Mechanism: Sequence of elementary reactions that explains how an overall reaction occurs

  • Rate-Determining Step (RDS): Slowest step in a mechanism, controls overall reaction rate

Writing Rate Laws from Mechanisms​

  1. Identify the elementary steps in the mechanism
  2. Determine molecularity for each step
  3. Use stoichiometry of the slow step to write the rate law
  4. Check that the sum of elementary steps matches the overall balanced equation

Example​

Overall Reaction: 2 NO₂ → N₂O₄

  • Elementary step: NO₂ + NO₂ → N₂O₄
  • Molecularity: Bimolecular
  • Rate law: Rate = k[NO₂]²

Importance​

  • Mechanisms help predict reaction rates and intermediates
  • Essential for understanding catalysis, kinetics, and complex reactions
  • Provides insight into reaction pathways for chemical synthesis

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