Elementary Reactions and Molecularity

Definition

• Elementary Reaction: A reaction where reactants convert to products in a single step.

Molecularity

• Molecularity: Refers to the number of molecules participating in an elementary reaction.
  • Unimolecular Reaction: Involves a single reactant molecule.
  • Bimolecular Reaction: Involves two reactant molecules.
  • Termolecular Reaction: Involves three reactant molecules (rare).

Rate Laws

• Rate Law: Expresses the rate of a reaction as a function of the concentration of reactants.
  • Rate = k [Reactant]^n
  • k: Rate constant
  • [Reactant]: Concentration of the reactant
  • n: Exponent derived from the coefficient in the balanced equation (only for elementary reactions)

Examples

Unimolecular Reaction

• Reaction: A → Products
  • Molecularity: Unimolecular
  • Rate Law: Rate = k [A]^1
  • Order: First order in A

Bimolecular Reaction

1. Reaction: A + B → Products
   • Molecularity: Bimolecular
   • Rate Law: Rate = k [A]^1 [B]^1
   • Order: First order in A and B
2. Reaction: A + A → Products (or 2A → Products)
   • Rate Law: Rate = k [A]^2
   • Order: Second order in A

Termolecular Reaction

• Reaction: A + B + C → Products
  • Molecularity: Termolecular
  • Rate Law: Rate = k [A]^1 [B]^1 [C]^1
  • Note: Rare due to the difficulty of three molecules colliding simultaneously.

Important Notes

• For elementary reactions, coefficients can be directly used as exponents in the rate law.
• Cannot apply this method to complex reactions with detailed mechanisms; such rate laws need to be determined experimentally.