Elementary Steps and Rate Laws

A complete chemical reaction may occur in one or more elementary steps, each having its own rate law. The rate of a single elementary step can be derived directly from its stoichiometric equation, since it is an individual effective collision describing a single bond-breaking or bond-forming event. This explains why the rate law of an overall reaction, potentially involving several steps, does not necessarily correlate to the stoichiometry of its balanced chemical equation. For example:

5 Br^–(aq) + BrO₃^–(aq) + 6 H⁺(aq) → 3 Br₂(ℓ) + 3 H₂O(ℓ)

Rate = k[Br^–][BrO₃^–][H⁺]²

However, the rate law for an elementary step is determined from its molecularity: the number of molecules involved in the single effective collision (Table 17.2 “Elementary Steps and Their Rate Laws”). For the following elementary step:

A → B

Rate = k[A]

This is a unimolecular step, and as the concentration of reactant A molecules increases, the number of effective collisions also increases.

Table 17.2 Elementary Steps and Their Rate Laws

مواضيع ذات صلة

The Molecular Interpretation of Entropy

Spontaneous Change

Biological Catalysts

Homogeneous Catalysts

Heterogeneous Catalysts

The Rate-Determining Step

Multistep Mechanisms

Elementary Steps

Half-Life

Graphical Methods for Determining Reaction Order

Zero-Order Reactions

Second-Order Reactions