## Pseudo first order rxn

$\frac{d[R]}{dt}=-k[R]; \ln [R]=-kt + \ln [R]_{0}; t_{\frac{1}{2}}=\frac{0.693}{k}$

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kennedyp
Posts: 56
Joined: Tue Nov 13, 2018 12:18 am

### Pseudo first order rxn

Can someone explain exactly what a pseudo 1st order rxn is? The book was kind of confusing for me.

BeylemZ-1B
Posts: 95
Joined: Thu Jul 25, 2019 12:17 am

### Re: Pseudo first order rxn

i found this video really helpful since the textbook barely explained this concept.
https://youtu.be/0BTyxuVRLf8

VPatankar_2L
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Joined: Thu Jul 25, 2019 12:17 am
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### Re: Pseudo first order rxn

A pseudo first-order reaction is one that is not normally first order but can be changed to a first-order reaction by significantly increasing or decreasing the concentration of one of the reactions. By doing so, you would be measuring the effect that the reactant whose concentration you are not changing, has on the rate of the reaction. Thus, you would write the rate in terms of the reactant whose concentration is not changing by substituting k' for k[reactant whose concentration is changing].
Last edited by VPatankar_2L on Sun Mar 15, 2020 7:08 am, edited 1 time in total.

BeylemZ-1B
Posts: 95
Joined: Thu Jul 25, 2019 12:17 am

### Re: Pseudo first order rxn

Basically, if there is a reaction that appears to be 2nd order, we can assume that there is excess reactant in a pseudo first order. The concentration of excess reactant is treated as constant since the change in the amount of excess reactant is negligible as the reaction proceeds. So now we treat the reaction is first order

sarahsalama2E
Posts: 164
Joined: Fri Aug 30, 2019 12:16 am

### Re: Pseudo first order rxn

Hi, I attached some helpful information that I wrote regarding pseudo ordered reactions. Essentially, pseudo ordered reactions are used when you have more than one reactant and one (or two) of the reactants is in excess. Because they are in excess, we can assume their rates are essentially constant, as seen in the work I have written.
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BeylemZ-1B
Posts: 95
Joined: Thu Jul 25, 2019 12:17 am

### Re: Pseudo first order rxn

sarahsalama2E wrote:Hi, I attached some helpful information that I wrote regarding pseudo ordered reactions. Essentially, pseudo ordered reactions are used when you have more than one reactant and one (or two) of the reactants is in excess. Because they are in excess, we can assume their rates are essentially constant, as seen in the work I have written.

This was extremely helpful thank you so much!

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