DECOMPOSITION OF AZOMETHANE

Introduction

Azomethane, (CH₃)₂N₂, decomposes in the gas phase to give ethane and nitrogen:

(CH₃)₂N₂ → C₂H₆ + N₂

Experimental observations have shown that the rate law for this reaction, at low azomethane concentrations, is:

+d[N₂]/dt = k_a[(CH₃)₂N₂]²

A proposed mechanism for this reaction is:

In this worked example: An appropriate rate expression will be derived from the proposed mechanism. This rate expression will be compared with the experimental rate law in order to determine whether or not the proposed mechanism and the experimental rate law are consistent.

Note: For the remainder of this example (CH₃)₂N₂ will be represented by AZO and ((CH₃)₂N₂)* will be represented by AZO*, i.e.

Overall Reaction	AZO → C2H6 + N2
Experimental Rate Law	+d[N2]/dt = ka[AZO]2
Proposed Mechanism

Composite or Elementary?

The first step is to determine whether or not the reaction is elementary.

If the reaction

AZO → C₂H₆ + N₂

+d[N₂]/dt = k[C₂H₆]

+d[N₂]/dt = k[C₂H₆][N₂]

+d[N₂]/dt = k[AZO][C₂H₆][N₂]

+d[N₂]/dt = k[AZO]

+d[N₂]/dt = k[AZO]²