[kelvinLTR]

In the reaction 2C :rarrow:D, the following mechanism is suggested:

2C C + C*  (forward rate constant k₁ and backward rate constant k₂) C* is a reactive intermediate

C*  D  (rate constant k₃)

two questions are asked

1. What is the condition for the reaction to be pseudo-first order
2. What is the condition for the reaction to be 2nd order

I found d[D]/dt=k₁k₃[C]/k₂ using the steady state approximation for [C*]
But got stuck at those two questions.

My argument is for the reaction to be pseudo first order, [C*] formed must disappear much faster than it is formed, thus k₂>>k₁

am I right? and can someone help me with the 2nd part

thanks in advance 

[Corribus]

I found d[D]/dt=k₁k₃[C]/k₂ using the steady state approximation for [C*]

Granted, it's been quite a while since I solved rate law problems, but I believe your difficulty may be related to this.

[kelvinLTR]

in the question, in an earlier part, it is asked to derive the rate of formation of [D] in terms of [C]

[Corribus]

And I'm saying I didn't get the same answer you did.

[kelvinLTR]

mmm I got :

d[D]/dt = k₃[C*]
d[C*]/dt=k₁[C]²-k₂[C][C*]=0 in steady state
therefore [C*]=k₁[C]/k₂

substituting d[D]/dt=k₁k₃[C]/k₂

[Corribus]

d[C*]/dt=k₁[C]²-k₂[C][C*]=0 in steady state

You are forgetting to include the loss of the intermediate C* to form your final product via rate k₃.

[kelvinLTR]

oh silly me  thanks a lot. I'll try again

[kelvinLTR]

figured it out. Thank you very much 

[Corribus]

No problem.  Should be easy to find the conditions of your limits now.