DECOMPOSITION OF AZOMETHANE

Review

Overall Reaction	AZO → C2H6 + N2
Experimental Rate Law (at low [AZO])	+d[N2]/dt = ka[AZO]2
Proposed Mechanism
Overall Rate of Production of (Azomethane)*	d[AZO*]/dt = + k1[AZO]2 - k-1[AZO][AZO*] - k2[AZO*]
Rate of Production of Nitrogen in the 1st Elementary Step	(d[N2]/dt)1 = 0
Rate of Production of Nitrogen in the 2nd Elementary Step	(d[N2]/dt)2 = 0
Rate of Production of Nitrogen in the 3rd Elementary Step	(d[N2]/dt)3 = +k2[AZO*]

Overall Rate of Production of Nitrogen {+d[N₂/dt}

The overall rate of production of (azomethane)* was calculated by summing the production rates in the elementary steps. In the same way the overall rate of production of Nitrogen can be found by summing the Nitrogen production rates in the individual steps:

d[N₂]/dt = 0 + 0 + k₂[AZO*] = k₂[AZO*]

Overall Rates of the other Species

By using the same technique the overall rate of consumption of azomethane {-d[AZO]/dt}, and the overall rate of production of ethane {+d[C₂H₄]/dt} can be determined as:

-d[AZO]/dt = + k₁[AZO]² - k_-1[AZO][AZO*]

+d[C₂H₄]/dt = k₂[AZO*]

Application of the Steady State Approximation

Comparing the results, it can be seen that the expression derived from the proposed mechanism

d[N₂]/dt = k₂[AZO*]

d[N₂]/dt = k_a[AZO]²

A (model) plot of the concentration profiles of the species involved in the decomposition of azomethane is given below.

Given this, if the Steady Approximation is to be used, it should be applied to

d[C₂H₆]/dt

d[AZO]/dt

d[AZO*]/dt

d[N₂]/dt