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The Pseudo-rate law is used whenever there is a reaction in which there are multiple reactants affecting the rate of the reaction. For example, if rate=k[A]^N[B]^M[C]^L, then we would need to use the pseudo-rate law in order to calculate the orders of the individual reactants. We do this by saying that two of the reactants (for example B and C) have very large concentrations, so the change in concentration is very little and essentially remains constant. As a result, the pseudo-rate law would be rate =k'[A]^N, where we would find the order, N, of A by plotting the data points versus time. If ln[A] versus time is a straight line, then it's a pseudo-first order reaction, and if 1/[A] versus time is a straight line, then it's a pseudo-second order reaction. We would then do this same process in order to find the orders of the other two reactants. In addition, to find the rate constant, k, we could use the equation k=k'/([B]^M*[C]^L)
You’ll need to use the pseudo-orders when very limited experimental data is given, because you will have to manipulate the concentrations yourself. They will most likely make you find the pseudo-order and give you the data when one reactant’s concentration is small and the others are large and so on.
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