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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

Part 1 of 6Overall 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.

Part 2 of 6Intermediates

  • 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.

Part 3 of 6Molecularity

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

Part 4 of 6Rate 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

Part 5 of 6Limits 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

Part 6 of 6More 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.

Key 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.

Avoid these mistakes

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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