Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

5.7Kinetics

Introduction to Reaction Mechanisms — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 10 of 14 1 quiz

📚 AP CHEMISTRY Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

5.7 Introduction to Reaction Mechanisms — AP CHEMISTRY notes: Define a reaction mechanism, apply the criteria for an acceptable mechanism and identify intermediates.Reaction mechanism — the sequence of elementary steps by which a reaction occurs at the molecular levelA one-step m…

  • Proposed, not observed
  • Multiple mechanisms
  • Number of steps
  • Number of intermediates
  • Transition state
  • Rate law criterion

Section 1 of 9What a mechanism is

Reaction mechanism
  • Reaction mechanism the sequence of elementary steps by which a reaction occurs at the molecular level
  • A one-step mechanism is a single elementary reaction.
Example
  • CO + NO2 → CO2 + NO is one step above 600 K but two steps at low temperature
  • The two-step path is NO2 + NO2 → NO3 + NO, then NO3 + CO → CO2 + NO2.

Section 2 of 9Criteria for a proposed mechanism

Criteria for a proposed mechanism
The elementary steps must add up to the overall balanced equation.
Each step must be reasonable, so steps involving three or more particles are unlikely.
The mechanism must agree with the experimentally determined rate law.

Section 3 of 9Status of mechanisms

Proposed, not observed
  • Proposed, not observed mechanisms cannot be seen directly and are models that explain experimental data
Multiple mechanisms
  • Multiple mechanisms several can fit the data, so experiment is needed to support one

Section 4 of 9Intermediates

Intermediate
  • Intermediate a species produced in one elementary step and used up in a later step
  • It does not appear in the overall equation.
  • Cl and CCl3 are intermediates in the chlorination of CHCl3 by Cl2.
Detection
  • Detection intermediates can sometimes be identified or isolated, which supports a mechanism
  • NO3 can be detected from its visible spectrum.

Section 5 of 9Intermediate against transition state

FeatureIntermediateTransition state
Energy profileA trough (minimum)A peak (maximum)
StabilityMay be detected or isolatedAlways unstable and never isolated
OccurrenceBetween two stepsAt the top of each barrier

Section 6 of 9Reading a multistep profile

1Number of steps
  • Number of steps equals the number of peaks
2Number of intermediates
  • Number of intermediates equals the number of troughs between peaks

Section 7 of 9More from the specification

More from the specification
Reactants
  • The substances that react together at the start of a reaction.
Approximation
  • A value close to but not exactly the true value.
Endothermic
  • A reaction that takes in thermal energy from the surroundings.
Pre-equilibrium approximation
  • Assumption that a fast reversible step is at equilibrium.
Activation energy
  • The minimum energy colliding particles need to react.

Section 8 of 9Key terms

TermMeaning
Reaction mechanismSequence of elementary steps giving the overall reaction.
Elementary stepA single molecular event within a mechanism.
IntermediateSpecies formed in one step and consumed in a later step.
Transition stateUnstable peak-energy arrangement that cannot be isolated.
Rate law criterionA valid mechanism must predict the experimental rate law.

Section 9 of 9Exam technique

Avoid these mistakes
Treating the mechanism as known fact — it is a proposal consistent with the data.
Confusing an intermediate with a catalyst — an intermediate is made then consumed, a catalyst is used then regenerated.
Leaving intermediates in the overall equation — they cancel when the steps are added.

Last updated: