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# Consider the reaction: 2A + B $\rightarrow$ 2C + D. What is the overall rate law given these intermediate steps:

A + B $\rightarrow$ E + C (Fast)

A+E $\rightarrow$ C + D (Slow)

$\begin{array}{1 1}k[A][E][C] \\ k[C]^2[B] \\k[A]^2[B] \\K[A][B][C]\end{array}$

Rate law = k[A]$^2$[B]
The Rate Determining Step is the slowest step of a chemical reaction that determines the speed (rate) of the overall reaction
Step-2 is the slowest: A+E $\rightarrow$ C + D (Slow)
$\Rightarrow$ Rate = k[A][E]
However, [E] is an intermediate product in step 1 and an intermediate reactant in step 2 that will not show up in the overall reaction. It needs to be replaced with the substances that make up the intermediate in a previous step: A + B $\rightarrow$ E + C (Fast)
$\Rightarrow$ Rate = k[A][A][B] = k[A]$^2$[B]
edited Jul 25, 2014