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AP Chemistry 1.5 Chemical Reaction Rates 42 Views


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AP Chemistry 1.5 Chemical Reaction Rates. What is the rate law for the reaction?

Language:
English Language

Transcript

00:03

And here’s your Shmoop du jour, brought to you by limits, know ‘em, love ‘em, [Students listening in class]

00:08

and maybe test ‘em every once in a while… But you didn’t hear that from us.

00:15

Assume that a reaction occurs by the mechanism given below. What is the rate law for the

00:20

reaction?

00:21

And here are your potential answers: To figure out the rate law, we need to consider [Spoiler alert sign and Pause button sign post]

00:28

which step in the mechanism limits the rate of the overall reaction, which is called the

00:32

“rate limiting step.” …Creativity isn’t exactly a chemist's [A girl awarded a least creative trophy]

00:36

strong suit… Anyway, the rate limiting step is kind of

00:40

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]

00:45

grandpa… And neither can anyone else. Good thing grocery carts don’t have horns.

00:49

Just like your grandpa limits your rate of walking, the slowest step in a chemical reaction

00:54

mechanism limits the rate of the overall reaction. In this case, we know that the second step,

01:01

C reacts to form D, is the slow step. The rate of the overall reaction will be equal

01:07

to the rate of this second step. So the second step is a first order reaction [Chemist pouring a substance into an erlenmeyer flask]

01:12

with only one reactant, C. That means the rate law for this step is

01:17

The rate equals the rate constant times the concentration of C

01:22

And that’s the rate of our overall reaction! Ta-da!

01:25

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]

01:31

Well… Remember that C is an intermediate species in this reaction. Our overall rate

01:36

law can’t include any intermediate species, so we need to rewrite this equation in terms

01:41

of the reactants, species A and B.

01:44

The first step in the reaction mechanism, an equilibrium between A and B

01:49

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]

01:56

B are consumed is equal to the rate at which C is formed: In equation form, that means:

02:01

the rate constant times concentration of a times concentration of b equals the rate

02:08

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]

02:14

Ds can get lost. Anyway, we already figured out that the overall

02:17

reaction rate = the rate constant times the concentration of C.

02:23

Now we know that the rate constant times the concentration of C equals

02:28

the rate constant times the concentration of a times the concentration of b. That means

02:32

our overall reaction rate is the rate constant times concentration of a times

02:36

concentration of b! Long, concentration-filled story short, that

02:39

means C is the correct answer. So next time you’re eating some delicious [Kid jumps up on a kitchen counter for cookies]

02:44

fresh-baked cookies, maybe you should make sure the rate limiting step is your dignity

02:49

and not how quickly your mom can work that frosting knife.

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