6.6 Reaction Mechanisms

• Series of elementary steps that describe how a chemical reaction occurs.
• These steps add together to be equivalent to the balanced chemical equation (Hess’ Law).
• Most elementary steps involve only 1 or 2 reactants.
• The slowest step is called the rate-determining step. A reaction can only proceed as fast as the slowest step. The rate law equation can be determined from this step.
• Entities that are produced in one step then use in another step and do not appear in overall balanced chemical equation are called intermediates.

Examples of elementary steps

• A → products
  • rate: k[A]
  • example: CO2 → C + O2
• A + A → products
  • rate: k[A]2
  • example: N + N → N2
• A + B→ products
  • rate: k[A][B]
  • example: NO + O2 → NO2 + O

Requirements

• Sum of elementary steps must give the overall balanced chemical equation (Hess’ Law).
• Reaction mechanism must agree with experimentally determined rate law (rate law from rate determining step must equal experimental rate law).