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5.4 Elementary Reactions

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Elementary Reactions and the Rate Law

As you have seen, the rate law does not always match the stoichiometric coefficients in a balanced chemical equation. The coefficients of reactants do not necessarily equal the reaction order. However, elementary reactions are an exception. In elementary reactions, the stoichiometry directly reflects the rate law. These are single-step reactions that represent one collision event and together make up the overall reaction mechanism.

For example, consider the reaction:

2N2O→2N2+O22N_2O \rightarrow 2N_2 + O_2

If the rate-determining elementary step is:

N2O→N2+ON_2O \rightarrow N_2 + O

then the rate law is:

Rate=k[N2O]\text{Rate} = k[N_2O]

Future lessons focus on identifying which step in a mechanism is the rate-limiting step.

In this unit

11 lessons

  1. 5.1Reaction RatesPreview lesson
  2. 5.2Introduction to Rate LawPreview lesson
  3. 5.3Concentration Changes Over TimePreview lesson
  4. 5.4Elementary ReactionsThis lesson
  5. 5.5Collision ModelPreview lesson
  6. 5.6Reaction Energy ProfilePreview lesson
  7. 5.7Introduction to Reaction MechanismsPreview lesson
  8. 5.8Reaction Mechanism and Rate LawPreview lesson
  9. 5.9Pre-Equilibrium ApproximationPreview lesson
  10. 5.10Multistep Reaction Energy ProfilePreview lesson
  11. 5.11CatalysisPreview lesson

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