PChem Teaching Lab | Halogenation 2p2

Composite Reactions & the Steady State Approximation

COMPOSITE REACTIONS AND THE STEADY STATE APPROXIMATION

Review

A → B
B + C → D
D → B + C

is an example of a combination of Consecutive and Reversible Reactions.

General Strategy for dealing with Composite Reactions

Step Procedure

A Determine if the reaction is elementary or composite by examining the experimental rate law.

B If the reaction is composite, propose a reaction mechanism consistent with the experimental data.

C Experimentally confirm the validity of the proposed reaction mechanism.

D Derive a theoretical rate expression for the proposed reaction mechanism, which is consistent with the experimentally determined rate law.

Review Question

The rate law for the reaction

2NO₂ + F₂ → 2NO₂F

has been experimentally determined as
r = k[NO₂][F₂]
Therefore

The reaction is elementary
The reaction is not elementary

Step	Procedure
A	Determine if the reaction is elementary or composite by examining the experimental rate law.
B	If the reaction is composite, propose a reaction mechanism consistent with the experimental data.
C	Experimentally confirm the validity of the proposed reaction mechanism.
D	Derive a theoretical rate expression for the proposed reaction mechanism, which is consistent with the experimentally determined rate law.