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)

Obtaining the Final (Derived) Rate Expression for the Production of Nitrogen {+d[N2]/dt}

For the low azomethane concentration case, it can be assumed that

k-1[AZO] << k2

Hence the k-1[AZO] term on the denominator of the rate of production of Nitrogen expression

d[N2]/dt = k2k1[AZO]2 / (k-1[AZO] + k2)
can be neglected.

Thus the expression for the rate of production of Nitrogen can be reduced to

d[N2]/dt = k1[AZO]2

d[N2]/dt = k2[AZO]

d[N2]/dt = k2k1[AZO] / k-1

d[N2]/dt = k2k1[AZO]2