ShmoopTube

Where Monty Python meets your 10th grade teacher.

Search Thousands of Shmoop Videos

AP Chemistry 1.5 Chemical Reaction Rates 42 Views


Share It!


Description:

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.

Up Next

AP Chemistry 1.3 Chemical Reaction Rates
189 Views

AP Chemistry 1.3 Chemical Reaction Rates. What is the overall order of the reaction?

Related Videos

AP Chemistry 1.4 Chemical Reaction Rates
43 Views

AP Chemistry 1.4 Chemical Reaction Rates. What are the correct units for a second order rate constant?

AP Chemistry 3.2 Laws of Thermodynamics
12 Views

AP Chemistry 3.2 Laws of Thermodynamics. What is the value for ΔG?

AP Chemistry 3.1 Laws of Thermodynamics
18 Views

AP Chemistry 3.1 Laws of Thermodynamics. What is the change in enthalpy of this reaction?