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2 Mar 2008 17:27:57 IST

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explain meaning of pseudo first order reaction and

how to determine a rate law for certain reaction??

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LAMPARD

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Joined: 1 May 2007

Posts: 777

2 Mar 2008 17:34:51 IST

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To calc. rate of 2nd order reacn. is difficult.So,If in a second order reaction,conc. of one of the reactants is kept constant,then reacn can be considered as pseudo 1st order bcoz it depends on conc. of 1 reactant only.
If for example [B] remains constant then:

$r = k[A][B] = k'[A]$

where

k' = k [B] 0 One way 2 obtain above condn is takin large conc. of 1 reactant(

[B]>>[A])

...so as reaction progresses,a small amt. of excess reactant is consumed and so its conc. can be taken as constant..hope u got it...

Aatish Pandit

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Joined: 3 Feb 2007

Posts: 1200

2 Mar 2008 17:45:31 IST

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A Pseudo First Order Reaction goes like this.......

Let me take an example.....hydrolysis of ester....

              CH₃COOCH₃ + H₂O ----> CH₃COOH + CH₃OH
initial
conc.             0.01             10                    0                 0

final
conc.               0               9.99                0.01            0.01

so here we see that [H₂O] does not change considerably.....I mean it remains almost constant during the course of rxn....

so when we write the RATE LAW EXPRESSION for this rxn then we have........

Rate = k [CH₃COOCH₃] [H₂O]

Now k[H₂O] can be taken as another constant k'

so Rate = k'[CH₃COOCH₃]

So the reaction which seemed to be of second order, now behaves as a first order rxn.......

This is Pseudo first order rxn......

and so all the calculations related to such rxns can be done in the same way as with first order rxn........

hope this helps you......

best of luck!!!!!!!

Gaurav |spideyunlimited| Ragtah

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Joined: 16 Dec 2006

Posts: 3373

2 Mar 2008 17:55:49 IST

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Example of pseedo first order reaction is hydrolysis of sucrose to give fructose and glucose... experimentally observed that concentration of water has little effect on the rate of reaction once it is available.

dev_22oct

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Joined: 27 Feb 2008

Posts: 426

2 Mar 2008 23:07:45 IST

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well !!!..............

psedo fst order reactions are the one whose rate does not depend upon the conc of reactants..

lemme explain this to u..

CH₃COOCH₃ + H₂O ----> CH₃COOH + CH₃OH

the rate of this reaction is independent of the conc of reactants...it depends upon the catalyst .

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