Reaction Mechanisms and Rate Laws

Introduction to Reaction Mechanisms

• Reaction Mechanism Definition: Step-by-step pathway by which a particular reaction occurs.
• Elementary Reactions: Individual steps in a reaction mechanism.

Example of a Reaction Mechanism

• Step 1: A + B → C + D
• Step 2: D + E → F + B
• Reaction Mechanism: Series of steps above.
• Rate Law Expressions:
  • Step 1: Rate = K1[A][B]
  • Step 2: Rate = K2[D][E]

Identifying Catalysts and Intermediates

• Catalyst: Consumed first, produced later.
• Intermediate: Produced first, consumed later.
• Example: In given reactions, B is the catalyst and D is the intermediate.

Characteristics of a Catalyst

• Speeds up chemical reactions by providing an alternative reaction pathway.
• Lowers the activation energy.

Writing Overall Reaction

• Cancel intermediate and catalyst species.
• Example: A + E → C + F

Molecularity of Reactions

• Unimolecular Reaction: Single reactant, first order.
  • Example: A → B
  • Rate = K[A]
• Bimolecular Reaction: Two reactants, second order.
  • Example 1: A + A → B
  • Example 2: A + B → C
  • Rate = K[A]^2 or K[A][B]
• Termolecular Reaction: Three reactants (rare), third order.
  • Example 1: A + B + C → D
  • Rate = K[A][B][C]

Example Problem: O3 and NO2 Reaction

• Steps:
  • 1st Step: O3 + NO2 → NO3 + O2 (slow)
  • 2nd Step: NO3 + NO2 → N2O5 (fast)
• Overall Reaction: O3 + 2NO2 → N2O5 + O2
• Rate Law for Overall Reaction: Based on slow step, Rate = K[O3][NO2]

Practice Problem: Hydrogen Peroxide Decomposition

• Reaction Mechanism:
  • Step 1: H2O2 + I^- → H2O + IO^- (slow)
  • Step 2: IO^- + H2O2 → I^- + H2O + O2 (fast)
• Overall Reaction: 2H2O2 → 2H2O + O2
• Molecularity:
  • Step 1: Bimolecular
  • Step 2: Bimolecular
• Rate Law Expression for Overall Reaction: Rate = K[H2O2]

These notes capture the key points about reaction mechanisms, rate laws, and how to determine molecularity and identify intermediates and catalysts.