Molecularity (HL) (HL IB Chemistry)

What is meant by the term molecularity?

• The molecularity of an elementary step is the number of reacting particles taking part in that step.
• Reactions can be classified as unimolecular, bimolecular or termolecular depending on how many reactant particles are involved:
  • Unimolecular: one reactant particle is involved in the elementary step
  • Bimolecular: two reactant particles are involved in the elementary step
  • Termolecular: three reactants particles are involved in the elementary step
• For example, consider the reaction below:

CH₃Br + OH^– → CH₃OH + Br^–

• It has been shown that the reaction mechanism involves two elementary steps:

Step 1: CH₃Br + OH^– → CH₃OHBr^–

Step 2: CH₃OHBr^– → CH₃OH + Br^–

• CH₃OHBr^– is an intermediate produced in step 1 that subsequently reacts in step 2
• Step 1 involves two reactant particles so is a bimolecular reaction
• Step 2 involves one reactant particle so is a unimolecular reaction