Reaction Mechanisms and Rate Laws

Definition of a Mechanism

• A mechanism is the sequence of elementary steps by which a reaction proceeds.
• Example reaction: Nitrogen dioxide (NO2) + Carbon monoxide (CO) → Nitric oxide (NO) + Carbon dioxide (CO2).
• Reactions don't occur in one step; they proceed through a sequence of elementary steps.

Example Mechanism

• Possible mechanism involves two elementary steps:
  1. Step 1: NO2 + NO2 → NO + NO3
  2. Step 2: NO3 + CO → NO2 + CO2
• Elementary steps must add up to give the overall reaction.
  • Reactants: NO2 + NO2 + NO3 + CO
  • Products: NO + NO3 + NO2 + CO2
  • Cross out common terms (NO2 and NO3), left with: NO2 + CO → NO + CO2

Intermediates

• Intermediates are species made in one step and consumed in another.
  • Example: NO3 is made in Step 1 and consumed in Step 2.
  • Intermediates help in determining possible mechanisms.

Rate Law Consistency

• A possible mechanism must be consistent with the experimental rate law.
• Overall reaction: NO2 + CO → NO + CO2
• Experimental rate law: Rate = k [NO2]^2
  • Second order in NO2, zero order in CO.

Explanation of Rate Law

• Mechanism Step Analysis:

  • Step 1: Slow, formation of intermediate (NO3).
    • Slow step = Rate determining step.
    • Rate of overall reaction determined by this slow step.
  • Step 2: Fast, product formation.

• Rate Determining Step:

  • Effective rate of reaction determined by slow step.
  • Rate of reaction approximation: Rate = rate of rate determining step.

Rate Law for Elementary Reaction

• For rate determining step (Step 1):
  • Rate = k1 [NO2][NO2] = k1 [NO2]^2
  • This matches the experimental rate law form, confirming mechanism's consistency.

Significance of Carbon Monoxide

• Reaction is zero order in CO.
• CO only appears in Step 2, not the rate determining step.

Rate Constant

• k1 for the rate determining step should equal k for the overall reaction.

Summary

• Understanding the mechanism helps explain the experimental rate law.
• The rate of an overall reaction is approximated by the slow step of the mechanism.
• Detecting intermediates aids in elucidating reaction mechanisms.