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Let's say we had a second order rate law that was equal to k[A][B]. To render this a pseudo-first order reaction, one could ensure that the concentration of A was so high relative to the concentration of B (lets say .85M and .003M, respectively) that the concentration of A effectively stayed constant throughout the course of the reaction. In this case, the constant k[.85] could be written as k', and the rate law for our pseudo first order reaction is equal to k'[B]. I understand that we've eliminated a species and simplified the equation somewhat, but since we still need k and the initial concentration of A, how does creating a pseudo-first order reaction help me specifically in the scope of this course? Is this only meant to be useful in a laboratory setting, and just be a conceptual thing that we should know about? Thanks.
I think that it is mainly conceptual, since it was mentioned in class how we can use large concentrations of most reactants and a small concentration of one reactant to determine the effect of that one reactant on the rate law. That being said, you may be given k' and the large concentration of one of the reactants (A) so you can do k' divided by that concentration and get the value of k (the rate constant), which you can then use to multiply by the small concentration of the other reactant (B) to get the rate of consumption of B.
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