Consecutive elementary reaction (Series reaction):
Consider the following series reaction scheme In this, the reactant A decays to four intermediate I, and this intermediate undergoes subsequent decay resulting in the formation of product P. The above series is elementary first order reaction.
Then the rate law expression is :
Let only the reactant A is present at t = 0 such that
then the rate law expression is
The expression for [A] is substituted into the rate law of I resulting in
This differential equation has a standard form and after setting
[I]0 = 0, the solution is
The expression for [P] is
[A]0 = [A] + [I] + [P]
[P] = [A]0 – [A] – [I]
Case I. Let kA >> kI
kI – kA ≈ –kA
and e-kAt ≈ 0
∴[P] = [A]0 – [A] – [I]
The rate of formation of product can be determined by slowest step.
[A] = [A]0 e-kAt ............(1)
The graph representation for case I i.e. when kA >> kI.
Case II. kA >> kI
kI – kA ≈ kI
The graph representation of case II i.e. when kA >> kI
The Steady-State Approximation.
The steady-state approximation assume that, after an initial induction period, an interval during which the concentration of intermediate ‘I’ rise from zero, and during the major part of the reaction, the rates of change of concentration of all reaction intermediate are negligibly small.
Problem. Consider the following reaction
assuming that only reactant A is present at t = 0, what is the expected time dependence of [P] using the steady state approximation ?
Sol. The differential rate expression for this reaction are:
Applying the steady state for I we get
This is expression for [P].
Problem. Using steady state approximation find the rate law for for the following given equation.
I1 & I2 are intermediate & apply steady state approximation on intermediate, we get
Problem. Using steady state approximation, derive the rate law for the decomposition of
N2O5. 2N2O5(g) → 4NO2(g) + O2(g)
On the basis of following mechanism.
Sol. The intermediate are NO & NO3.
The rate law are:
= kb[NO2[[NO3] – kc [NO][N2O5] = 0
= ka[N2O5] – ka’ [NO2][NO3] – kb[NO2][NO3] = 0
= –ka[N2O5] + ka’[NO2][NO3] – kc[NO][N2O5]
and replacing the concentration of intermediate by using the equation above gives