12.6 Reaction Mechanisms - Chemistry 2e | OpenStax

Learning Objectives

• Distinguish net reactions from elementary reactions (steps).
• Identify the molecularity of elementary reactions.
• Write a balanced chemical equation for a process given its reaction mechanism.
• Derive the rate law consistent with a given reaction mechanism.

Key Concepts

• Reaction Mechanism

  • A step-by-step process by which a chemical reaction occurs.
  • Reveals the sequence of elementary reactions leading to a net chemical reaction.

• Elementary Reactions

  • Occur exactly as represented in the reaction equations.
  • Must sum up to yield the balanced chemical equation.
  • Intermediates are species produced in one step and consumed in another.

• Rate Laws

  • Can be derived directly from elementary reactions, unlike overall reactions which require experimental data.

Types of Elementary Reactions

1. Unimolecular Elementary Reactions

• Involves a single reactant entity.
• Example: O₃ → O₂ + O
• Rate law: First order rate = k[A]

2. Bimolecular Elementary Reactions

• Involves two reactant entities.
• Example:
  • A + B → products
  • 2A → products
• Rate law:
  • For A + B: Second order overall rate = k[A][B]
  • For 2A: rate = k[A]²

3. Termolecular Elementary Reactions

• Involves three reactant entities.
• Rare due to low probability of simultaneous collisions.
• Example: 2NO + O₂ → 2NO₂
• Rate law: rate = k[NO]²[O₂]

Reaction Mechanisms and Rate Laws

• Rate-Determining Step:

  • The slowest step in a mechanism limits the overall reaction rate.

• Deriving Rate Laws:

  • Must be determined experimentally for overall reactions.
  • Mechanisms deduced from experimental data.

• Example Reaction:

  • NO₂ and CO reaction:
    • High temperature: rate = k[NO₂][CO]
    • Low temperature: rate = k[NO₂]²
    • Different rate laws suggest different reaction mechanisms.

Example Problem - Deriving a Rate Law

Given Mechanism

• Step 1: NO + Cl₂ → NOCl₂ (fast)
• Step 2: NOCl₂ + NO → 2NOCl (slow)

Solution

• Overall reaction: 2NO + Cl₂ → 2NOCl
• Rate law for Step 2:
  • Intermediate NOCl₂ needs substitution:
  • Assume step 1 is at equilibrium: rate₁ = k₁[NO][Cl₂] = k₋₁[NOCl₂] [NOCl₂] = (k₁/k₋₁)[NO][Cl₂]
  • Substitute into Step 2 rate law: rate = (k₂k₁/k₋₁)[NO]²[Cl₂]

Conclusion

Reaction mechanisms are crucial for understanding the detailed steps of chemical reactions. They allow for the derivation of rate laws from elementary steps, providing insights into how reactions proceed at the molecular level. Understanding these mechanisms and their rate laws are essential for predicting reaction behavior under different conditions.