Lesson 9: Reaction Mechanism

• Reaction Mechanisms

  A series of steps that make up an overall reaction
• Elementary reaction

  Each step in a reaction mechanism
• Rate law


  k[NO2]^2, squared because the 1st step has reactants of 2 NO2 molecules colliding
• The first step is the slowest, so it controls the rate
• Intermediate

  The product of one elementary step that becomes a reactant in a subsequent step
• Catalyst

  Appears as a reactant in one step but is a product in a subsequent step, and is regenerated so that it can be reused
• Characteristics of an Elementary Reaction

  • Involves a single event, such as a collision between particles
  • Involves the formation of new molecules or ions, and a change in the energy or geometry of the starting particles
  • Cannot be broken down into further steps
• Types of Elementary Reactions

  • Unimolecular: only one molecule or ion reacts
  • Bimolecular: involve two particles colliding and reacting
  • Termolecular: Involve three particles colliding and reacting

  
• Characteristics of a Reaction Mechanism


  • Rate determining step (RDS): The slowest step in the reaction mechanism
  • Reaction Intermediate: The product of one elementary step that becomes a reactant in a subsequent step
  • Catalyst: Appears as a reactant in one step but is a product in a subsequent step, and is regenerated so that it can be reused
• Reaction Mechanism (Un-catalyzed reaction)

  • Step 1: A + B → C (very slow)
  • Step 2: C + D → E (fast)
  • Step 3: E + A → 2 F (fast)
• Rate determining step

  The slowest step in the reaction mechanism
• Reaction intermediate

  The product of one elementary step that becomes a reactant in a subsequent step
• Reaction Mechanism (Catalyzed reaction)

  • Step 1: A + X → G (medium rate)
  • Step 2: G + B → H (fast)
  • Step 3: H + D → E + X (fast)
  • Step 4: E + A → 2 F (fast)
• Because of the catalyst, the rate-determining step occurs faster
• A Proposed Mechanism Must

  • Have elementary steps that will combine to give the overall reaction
  • The proposed elementary reaction steps must be reasonable (i.e. uni- or bimolecular)
  • Mechanism must support the overall, experimentally determined rate law
• Types of Catalysts


  Heterogeneous: a catalyst that is in a different phase than the reactants

  Homogeneous: a catalyst that is in the same phase as the reactants