## Pseudo First Order Reaction

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

Sarah Clemens 1B
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### Pseudo First Order Reaction

Can someone explain when we would need to use a pseudo first order reaction?

Max Mazo 2C
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### Re: Pseudo First Order Reaction

Expanding on this, what is a pseudo first order reaction?

Emily Oren 3C
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### Re: Pseudo First Order Reaction

We use pseudo first order reactions when we are studying a reaction with multiple reactants with concentrations that could affect the rate law. For example if the reaction in question is A+B-->P we would design an experiment to find out how the rate changes with the concentrations of each reactant. The rate law=k' * [A]^n * [B]^m. To find [A], the [B] is made very high so that it remains relatively constant and the [A] is varied. We can then analyze the data like we did earlier in the section in order to calculate n. If n is 1 then the reaction is pseudo first order. Since we do not know the order for B, it is only pseudo first order not first order. We can then repeat the process for the other reactants.

Lizzie Zhang 2C
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### Re: Pseudo First Order Reaction

For example, if we have a equation telling us that rate of consumption of I- = k [S2O8 2-][I-]. Because the rate equation involves more than one species, it would be hard to analyze the change of rates. Then we can just set persulfate ions concentration really high, turning an actual second order reaction into a first order reaction. Then rate of consumption of I- equals to k'[I] while k' = k [S2O8 2-].

Now the reaction rate depends only on one species, and we can analyze one reactant at a time using pseudo first order reaction method.

Abby Ellstrom 1I
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### Re: Pseudo First Order Reaction

You use a pseudo first order reaction when there is more than one reactant by making one of the reactant's concentrations very large so you can test the rate while manipulating the other reactant.

Victoria Draper 1G
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### Re: Pseudo First Order Reaction

A pseudo first order reaction is when a reaction is a second order reaction but is treated as though it is a first order reaction. This occurs when the concentrations of one of the reactants is so large and remains constant throughout the reaction that it does not play a significant role in the rate law. Therefore the new rate law can be written as Rate=k' * [A] where k' is equal to the value of k*[reactant that was present in excess] and [A] is the substance that still plays a relevant role in the overall rate of the reaction.

Nicole Jacobs 1C
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### Re: Pseudo First Order Reaction

do we have to know how to apply a psuedo first order reaction or just understand it conceptually?

Cali Rauk1D
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Joined: Fri Sep 29, 2017 7:03 am

### Re: Pseudo First Order Reaction

Basically a pseudo first order reaction is when you change a second order into a first order by taking away the intermediate. It makes it easier to solve