2.2.8 (HL)—Molecularity

Molecularity is the number of reacting particles in one elementary step: one is unimolecular, two is bimolecular and three is termolecular.

Count particles in the individual elementary step, not coefficients in the overall reaction equation.

A step A + 2B → products is termolecular because three reacting particles meet in that elementary event. Molecularity is always a positive whole-number description of one proposed step; reaction order is experimental and can be zero, fractional or unrelated to overall stoichiometric coefficients.

Representative question

[Maximum number: 1]

Identify the molecularity of the rate-determining step in this reaction.

Retrieve the route: measure a tangent rate, explain effective collisions, map rate factors, read Ea and energy profiles, evaluate mechanisms, determine molecularity and orders, calculate k, then use Arrhenius gradient and intercept for Ea and A.

Check tangent versus average slope, energy versus orientation, barrier labels, intermediate versus transition state, one-variable trial comparisons, order-dependent units, kelvin temperature and the signs of gradient and Ea.

2.2.8 (HL)—Molecularity

Molecularity of an Elementary Step

Identifying Molecularity

Assessment in practice

Rate and Mechanisms Summary

Question 1