AP Chemistry 1.5 Chemical Reaction Rates
AP Chemistry 1.5 Chemical Reaction Rates. What is the rate law for the reaction?
|AP Chemistry||Chemical Reaction Rates|
|Test Prep||AP Chemistry|
which step in the mechanism limits the rate of the overall reaction, which is called the
“rate limiting step.” …Creativity isn’t exactly a chemist's [A girl awarded a least creative trophy]
strong suit… Anyway, the rate limiting step is kind of
like your grandpa at the grocery store. You can’t walk down the aisle any faster than [Grandpa walks slowly down an aisle and blocks people]
grandpa… And neither can anyone else. Good thing grocery carts don’t have horns.
Just like your grandpa limits your rate of walking, the slowest step in a chemical reaction
mechanism limits the rate of the overall reaction. In this case, we know that the second step,
C reacts to form D, is the slow step. The rate of the overall reaction will be equal
to the rate of this second step. So the second step is a first order reaction [Chemist pouring a substance into an erlenmeyer flask]
with only one reactant, C. That means the rate law for this step is
The rate equals the rate constant times the concentration of C
And that’s the rate of our overall reaction! Ta-da!
But wait, that isn’t one of the final answer choices! What kind of a cheap trick is this? [magician reaches into tophat and hand is bitten by a rabbit]
Well… Remember that C is an intermediate species in this reaction. Our overall rate
law can’t include any intermediate species, so we need to rewrite this equation in terms
of the reactants, species A and B.
The first step in the reaction mechanism, an equilibrium between A and B
and C, is fast. We can assume this step is at equilibrium, so the rate at which A and [A+B on one side of see-saw and C on the opposite]
B are consumed is equal to the rate at which C is formed: In equation form, that means:
the rate constant times concentration of a times concentration of b equals the rate
constant times the concentration of c. Equality for all As, Bs, and Cs! [A, B, C and D letters in the street and D runs away]
Ds can get lost. Anyway, we already figured out that the overall
reaction rate = the rate constant times the concentration of C.
Now we know that the rate constant times the concentration of C equals
the rate constant times the concentration of a times the concentration of b. That means
our overall reaction rate is the rate constant times concentration of a times
concentration of b! Long, concentration-filled story short, that
means C is the correct answer. So next time you’re eating some delicious [Kid jumps up on a kitchen counter for cookies]
fresh-baked cookies, maybe you should make sure the rate limiting step is your dignity
and not how quickly your mom can work that frosting knife.