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Reaction Mechanism and Rate Law

The relationship between a reaction mechanism and its rate law allows chemists to understand how individual steps control the overall reaction rate.

Key Concepts​

  • Reaction Mechanism: Sequence of elementary steps leading to the overall reaction
  • Elementary Step: Single-step reaction with a defined molecularity
  • Rate-Determining Step (RDS): Slowest step that controls the reaction rate
  • Rate Law: Expresses how the rate depends on reactant concentrations

Linking Mechanism and Rate Law​

  1. Identify the elementary steps of the reaction
  2. Determine the slowest step (RDS)
  3. Use the stoichiometry of the RDS to write the rate law
  4. Verify that the sum of all steps reproduces the overall balanced equation

Example​

Overall reaction: 2 NO₂ → N₂O₄

  • Mechanism:
    1. NO₂ + NO₂ → N₂O₄ (RDS, bimolecular)
  • Rate law: Rate = k[NO₂]²

Importance​

  • Predicts reaction rates under different conditions
  • Provides insight into intermediates and catalysts
  • Essential for kinetics studies, industrial processes, and chemical design

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