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How do you know the rate determining step?

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Al E. · Truong-Son N.

Dec 30, 2017

There are many ways. However, in most general chemistry classes, this will suffice.

It's basically like the "bottleneck" of the reaction. For instance,

If there is a slow, first step, that is the rate-determining step.
If there is a fast, reversible first step, but a slow second step, then it's a tad more complicated. Here's a simple demonstration

$A + B ⇌ C$ $A + B rightleftharpoons C$ $(fast)$ $("fast")$
$C + D \to E$ $C + D to E$ $(slow)$ $("slow")$
$E + D \to F$ $E + D to F$

$k_{1} [A] [B] = k_{- 1} [C]$ $k_1[A][B] = k_(-1)[C]$
$[C] = \frac{k_{1} k_{2}}{k_{- 1}} [A] [B]$ $[C] = (k_1k_2)/(k_-1)[A][B]$

Therefore the rate law is,

$R(t) = \frac{k_{2} k_{1}}{k_{- 1}} [A] [B] [D]$ $"R(t)" = (k_2k_1)/(k_-1)[A][B][D]$

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