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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

R2.2 - How fast? The rate of chemical change

2.2.1 Rate of Reaction 2.2.2 Collision Theory 2.2.3 Factors Affecting Reaction Rate 2.2.4 Activation Energy and Temperature 2.2.5 Catalyst and Activation Energy 2.2.6 Reaction Mechanism and Intermediates (AHL) 2.2.7 Energy Profile and Rate Determining Step (AHL) 2.2.8 Molecularity in Reaction Mechanism (AHL) 2.2.9 Rate Equations and Experimental Data (AHL) 2.2.10 Reaction Orders and Graphs (AHL) 2.2.11 Rate Constant, K (AHL) 2.2.12 Arrhenius Reaction and Temperature (AHL) 2.2.13 Arrhenius Factor and Activation Energy (AHL)

Molecularity of Elementary Steps HL Only

Specification Reference R2.2.8

Quick Notes

  • Molecularity refers to the number of reactant particles involved in a single elementary step in a mechanism.
  • Types of molecularity:
    • Unimolecular: 1 particle.
    • Bimolecular: 2 particles.
    • Termolecular: 3 particles.
  • Termolecular steps are rare due to the low probability of three particles colliding simultaneously with correct orientation and energy.

Full Notes

RECAP: Many chemical reactions don’t happen in one step, they occur through a sequence of elementary steps:

IB Chemistry schematic showing a reaction mechanism as a sequence of elementary steps that sum to the overall reaction.

Each step involves the breaking or forming of a small number of bonds. The overall reaction is the sum of all elementary steps that occur.

How the steps link together is referred to as the ‘reaction mechanism’.

What Is Molecularity?

Molecularity describes how many reactant species are involved in an elementary step of a reaction mechanism.

It applies only to individual steps, not to the overall reaction.

Types of Molecularity

There are three main examples of molecularity - unimolecular, bimolecular and trimolecular.

IB Chemistry diagram showing unimolecular, bimolecular, and termolecular elementary step examples.

Unimolecular:

Involves 1 particle.

Example: A → product or intermediate.

Common in decomposition reactions.

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Unimolecular steps usually involve just one reacting species, but they can still be triggered by collisions with non-reactive particles, like solvent molecules. These collisions may supply energy to break bonds, but because the solvent doesn’t change chemically, it isn’t considered a reactant and its concentration doesn’t affect the rate.

Bimolecular:

Involves 2 particles.

Example: Example: A + B → product or intermediate.

Most common type in collisions and substitution reactions.

Termolecular:

Involves 3 particles colliding simultaneously.

Example: A + B + C → product or intermediate.

Note that termolecular processes are very rare, as it is statistically unlikely for three particles to collide with the proper orientation and energy at the same time.

Summary