16.1 Rate expression and reaction mechanism

Rate equation (Rate law)

  • Consider reaction:
    • Reaction
    • Where x, y, p, q are stoichiometry coefficients
  • The rate equation is:
    • Rate Equation
  • In general the rate law is expressed as such:
    • Rate Law
    • Where m and n are the orders with respect to their reactants
    • Order of reaction = m + n
    • DO NOT USE STOICHIOMETRY COEFFICIENTS AS “m” AND “n” IN THIS EQUATION
  • Rate equations can only be determined experimentally because orders can only be deduced empirically

Molecularity and rate-determining step (slow-step) of a reaction

  • Reaction mechanism: The sequence of steps outlining the reaction pathway from reactants to products
  • Elementary steps: Any individual step in the reaction mechanism
  • Molecularity: the number of molecules or atoms involved as reactants in the elementary reaction
    • Unimolecular: Single molecule or atoms involved in an elementary step
    • Bimolecular: Two molecules or atoms involved in an elementary step
  • Each elementary step has its own rate constant, k, and its own activation energy Ea
  • The rate of reaction depends on the slow step or the rate-determining step (RDS)
  • Catalysts alter reaction mechanism by introducing a step with a lower activation energy

Deduction of a rate equation

  1. Decide on which step is the RDS
  2. Deduce rate equation for the RDS

Graphical representations of reactions

Concentration vs Time Graphs

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