Composite Reactions & the Steady State Approximation

DECOMPOSITION OF AZOMETHANE

Review
Overall ReactionAZO → C2H6 + N2
Experimental Rate
Law (at low [AZO])		+d[N2]/dt = ka[AZO]2
Proposed Mechanism
[Azomethane Mechanism (s1)]
[Azomethane Mechanism (s2)]
[Azomethane Mechanism (s3)]
Overall Rate of
Production of
(Azomethane)*		d[AZO*]/dt = + k1[AZO]2 - k-1[AZO][AZO*] - k2[AZO*]
From the Steady
State Approximation		[AZO*]SS = k1[AZO]2 / (k-1[AZO] + k2)
Overall Rate
of Production
of Nitrogen		d[N2]/dt = k2k1[AZO]2 / (k-1[AZO] + k2)

Simplifying the Rate Expression for the Production of Nitrogen {+d[N2]/dt}

Having eliminated the [AZO*] term from the rate expression for the production of Nitrogen

d[N2]/dt = k2k1[AZO]2 / (k-1[AZO] + k2)
the equation must be analyzed to see if it can be simplified further.

In the introduction to the Decomposition of Azomethane Example, it was stated that:

Experimental observations have shown that the rate law for this reaction, at low azomethane concentrations is: +d[N2]/dt = ka[AZO]2
Examining the derived rate expression
d[N2]/dt = k2k1[AZO]2 / (k-1[AZO] + k2)
for the low azomethane concentration case,

it can be assumed that

k-1[AZO] >> k2

k-1[AZO] ~ k2

k-1[AZO] << k2