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5.4Kinetics

Elementary Reactions — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 7 of 14 1 quiz

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Lesson summary

5.4 Elementary Reactions — AP CHEMISTRY notes: Distinguish elementary steps from overall reactions, classify them by molecularity and write their rate laws.Overall equation — shows reactants and products but not the molecular stepsElementary reaction — a single molecular event in…

  • Reaction mechanism
  • Intermediate
  • Unimolecular, A → products
  • What the symbols mean
  • Bimolecular, A + B → products
  • Bimolecular, 2A → products

Section 1 of 8Overall and elementary reactions

Overall equation
  • Overall equation shows reactants and products but not the molecular steps
Elementary reaction
  • Elementary reaction a single molecular event involving the collision of a small number of particles
  • Most reactions occur as a series of elementary steps.
Reaction mechanism
  • Reaction mechanism the set of elementary reactions that add up to the overall equation
  • For CO + NO2 → CO2 + NO below 600 K: NO2 + NO2 → NO3 + NO, then NO3 + CO → CO2 + NO2.

Section 2 of 8Intermediates

  • Intermediate
    • a species formed in one elementary step and consumed in a later one
    • It appears in the steps but cancels from the overall equation.
    • NO3 in the CO/NO2 mechanism is an example.

Section 3 of 8Molecularity

TypeNumber of reactant particlesFrequency
UnimolecularOneCommon
BimolecularTwoCommon
TermolecularThreeRare, because three-body collisions are improbable

Section 4 of 8Rate laws for elementary steps

1Unimolecular, A → products
  • Unimolecular, A → products rate = k[A]
    What the symbols mean: rate (mol dm−3 s−1) · k = rate constant · [ ] = concentration (mol dm−3) · each power is the order with respect to that reactant
    • (units depend on the overall order)
2Bimolecular, A + B → products
  • Bimolecular, A + B → products rate = k[A][B]
3Bimolecular, 2A → products
  • Bimolecular, 2A → products rate = k[A]2
4General rule
  • General rule for an elementary step the order of each reactant equals its coefficient in that step

Section 5 of 8Limits of the coefficient rule

1Elementary steps only
  • Elementary steps only rate laws are written from coefficients only when the equation is an elementary step
2Overall equations
  • Overall equations never write the rate law from the coefficients of the overall balanced equation

Section 6 of 8More from the specification

More from the specification
Conjugated double and single chemical bonds
  • Conjugated double and single chemical bonds alternating single and double covalent bonds allow delocalisation of pi electrons across extensive aromatic systems.
Activation energy
  • The minimum energy colliding particles need to react.
Activated complex
  • Unstable high-energy species at the transition state.
Collision model
  • Reactions occur when particles collide with enough energy and correct orientation.
Arrhenius equation
  • K = Ae^−Ea/RT, relating the rate constant to temperature and activation energy.
Maxwell-Boltzmann distribution
  • A graph showing how particle energies are spread at a given temperature.
Carbon monoxide
  • A toxic gas that binds to haemoglobin.
Steric factor
  • Fraction of collisions with the correct orientation.
Temperature
  • A measure of the average kinetic energy of the particles.
Fraction
  • A group of hydrocarbons with similar chain length and boiling points.
Theory
  • A tested hypothesis that is widely accepted.

Section 7 of 8Key terms

TermMeaning
Elementary reactionA single molecular event; one step of a mechanism.
Reaction mechanismSequence of elementary steps summing to the overall reaction.
IntermediateSpecies produced in one step and consumed in a later step.
MolecularityNumber of reactant particles in an elementary step.
UnimolecularElementary step with one reactant particle.
BimolecularElementary step with two reactant particles.

Section 8 of 8Exam technique

Avoid these mistakes
Writing the rate law from an overall equation — orders equal coefficients only in an elementary step.
Counting an intermediate as a product — intermediates are consumed later and do not appear in the overall equation.
Thinking termolecular steps are common — they are rare, so mechanisms use bimolecular steps.

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