Cracking the AP Chemistry Exam
Part IV
Content Review for the AP Chemistry Exam
Chapter 6
Big Idea #4: Chemical Reactions and their Rates
REACTION MECHANISMS
Many chemical reactions are not one-step processes. Rather, the balanced equation is the sum of a series of individual steps, called elementary steps.
For instance, the hypothetical reaction
2A + 2B → C + D Rate = k[A]2[B]
could take place by the following three-step mechanism:
|
I. A + A |
(fast) |
|
II. X + B → C + Y |
(slow) |
|
III. Y + B → D |
(fast) |
Species X and Y are called reaction intermediates, because they are produced but also fully consumed over the course of the reaction. Intermediates will always cancel out when adding up the various elementary steps in a reaction, as shown below.
I. A + A 
X
II. X + B → C + Y
III. Y + B → D
_____________________________________________________________________
A + A + X + B + Y + B → X + C + Y + D
Cancel species that appear on both sides.
2A + 2B → C + D
By adding up all the steps, we get the balanced equation for the overall reaction, so this mechanism is consistent with the balanced equation.
As in any process where many steps are involved, the speed of the whole process can’t be faster than the speed of the slowest step in the process, so the slowest step of a reaction is called the rate-determining step. Since the slowest step is the most important step in determining the rate of a reaction, the slowest step and the steps leading up to it are used to see if the mechanism is consistent with the rate law for the overall reaction.
The rate for an elementary step can be determined using by taking the concentration of the reactants in that step and raising them to the power of any coefficient attached to that reactant. So, for the current reactions:
The rate law for the entire reaction is equal to that of the slowest elementary step, which is step II. However, step II has an intermediate (X) present in it. Looking at step 1, we can also see that [X] is equivalent to [A]2 (as the sides are in equilibrium). If [A]2 is substituted in for [X], the rate law for step II becomes rate = k[A]2[B], which is the overall rate law for this reaction.
It is important to emphasize that you can only use the coefficient method to determine the rate law of elementary steps. You CANNOT use it to determine the rate law of an overall reaction. Overall rate laws can only be determined via experimental data.