Molecularity (HL) (DP IB Chemistry): Revision Note

Molecularity

What is molecularity?

• The molecularity of an elementary step is the number of reacting particles taking part in that step

• Reactions can be classified by the number of reacting particles in their elementary steps:

  • Unimolecular: one reactant particle reacts

  • Bimolecular: two reactant particles collide and react

  • Termolecular: three reactant particles must collide simultaneously and react

• For example, consider the reaction below:

CH₃Br + OH^– → CH₃OH + Br^–

• 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, and it 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