Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

5.2.12Additional lessons

Molecularity (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 106 of 172

📚 IB Chemistry HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read

Section 1 of 5Defining Molecularity

Molecularity applies only to individual elementary steps, never to a full multi-step reaction.

Definition
  • Definition Molecularity is the number of reactant particles that collide in a single elementary step.
Unimolecular
  • Unimolecular One reactant particle changes in the step, for example an isomerisation or decomposition.
Bimolecular
  • Bimolecular Two reactant particles collide in the step, the most common type of elementary step.
Termolecular
  • Termolecular Three particles collide simultaneously, which is extremely unlikely because it needs a very rare collision.

Section 2 of 5Molecularity and Rate Equation

1
For an elementary step, the order with respect to each reactant equals its coefficient in that step.
2
A unimolecular step A → products has rate = k[A], so it is first order overall.
3
A bimolecular step A + B → products has rate = k[A][B], second order overall.
4
For two identical molecules, 2A → products, rate = k[A]², also second order.
5
For multi-step reactions the observed order depends on the rate-determining step, not on the overall equation.

Section 3 of 5Applications to Organic Chemistry

Applications to Organic Chemistry
SN1
  • SN1 The slow step involves one species only, so the rate depends on the halogenoalkane concentration and is first order overall.
SN2
  • SN2 The slow step involves both the halogenoalkane and the nucleophile, so the rate is second order overall.
Experimental link
  • Experimental link Measuring orders for these reactions helps decide which mechanism operates.
Reminder
  • Reminder Molecularity is a whole number, while an experimental order can be zero or fractional.

Section 4 of 5Worked Example

Question

  • A mechanism has a slow step A + B → X and a fast step X + B → products.
  • State the rate equation and the molecularity of the slow step.

  1. The slow step is rate-determining and involves one A and one B.
  2. So rate = k[A][B], which is second order overall.
  3. Two particles collide in that step, so it is bimolecular.

Answer

  • rate = k[A][B];
    • the slow step is bimolecular

What the symbols mean: rate (mol dm−3 s−1) · k = rate constant (units depend on the overall order) · [ ] = concentration (mol dm−3) · each power is the order with respect to that reactant

Section 5 of 5Exam Checklist & Tips

5.2.12 – Key Points for the Exam

  • Molecularity The number of particles reacting in one elementary step.
  • Unimolecular An elementary step involving one reactant particle.
  • Bimolecular An elementary step involving two reactant particles.
  • Elementary step A single step in a mechanism that cannot be broken down further.

Common Mistakes

  • Using molecularity to describe an overall multi-step reaction.
  • Assuming that a reaction with three reactants in its overall equation is termolecular.
  • Claiming that molecularity can be zero or fractional.
  • Equating the order of an overall reaction with the coefficients in the overall equation.

Last updated:

\n