Intermediate Topic 433 of 1102 Kinetics & Equilibrium
Molecularity
The number of species colliding in an elementary step.
Practise Kinetics & Equilibrium All Kinetics & Equilibrium topics
In-depth explanation of Molecularity
Molecularity is the number of molecules that participate in a single elementary step of a mechanism, and it is classified as unimolecular, bimolecular or, very rarely, termolecular. Unlike reaction order, which is determined experimentally for an overall reaction, molecularity is a theoretical property that applies only to elementary steps. A unimolecular step involves the rearrangement or decomposition of a single molecule, while a bimolecular step requires a collision between two molecules. Termolecular steps are rare because the simultaneous collision of three particles is statistically unlikely. For an elementary step the molecularity equals the order with respect to each reactant.
Going deeper: Molecularity
Key ideas: Molecularity is 1, 2 or 3 and only whole numbers are allowed, unlike order. Unimolecular steps give first order rates, bimolecular steps give second order rates, and the rate law of any single step follows its stoichiometry. Reactions that have a molecularity greater than 3 must therefore occur in several steps.
Common mistakes and connections: Students assign molecularity to the overall reaction, which only has an order, or call a first order reaction unimolecular without checking the mechanism. Hydrolysis of an ester may show first order in water but be bimolecular. Molecularity applies only after the mechanism is known, so it links to elementary reactions and rate-determining steps.
Visualization of Molecularity
A bimolecular elementary step needs two particles to collide with enough energy and the right orientation; unimolecular and termolecular steps are the other classes. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A single-step isomerization of one molecule, A -> B, is unimolecular.
Frequently asked questions about Molecularity
What is Molecularity?
The number of species colliding in an elementary step.
Is Molecularity a beginner, intermediate or advanced topic?
Molecularity is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Molecularity?
Study these earlier topics in Kinetics & Equilibrium first: Pseudo First Order Reaction, Method of Initial Rates, Integrated Rate Laws.
What comes after Molecularity?
The next topics in Kinetics & Equilibrium are Elementary Reactions and Reaction Mechanism.
Can I practise Molecularity with quizzes?
Yes. The interactive lesson includes a self-check question on Molecularity, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Molecularity fits in Kinetics & Equilibrium
Molecularity is topic 14 of 91 in Kinetics & Equilibrium.