Copyright

Determining Rate Equation, Rate Law Constant & Reaction Order from Experimental Data

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: Practical Application: Determining Rate Equation, Rate Law Constant & Reaction Order

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
 Replay
Your next lesson will play in 10 seconds
  • 0:04 Rate Law
  • 2:00 Reaction Order
  • 2:24 Rate Constant
  • 3:09 Some Practice
  • 4:57 Lesson Summary
Save Save Save

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Log in or Sign up

Timeline
Autoplay
Autoplay
Speed Speed
Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Is your head spinning from rate laws, reaction orders and experimental data? Don't worry, this lesson will help you become a pro at all of these things by walking through two comprehensive problems that address all of these topics.

Rate Law

Buckle your seatbelt, we are about to do a whole lot of chemistry! Let's get started right away with rate laws, sometimes called rate equations, which are equations that relate the concentrations of reactants with the reaction speed. Remember, when you look at a chemical reaction, the reactants are on the left side of the arrow and the products are on the right side.

Take a look at this rate law equation:

RateLaw

Where:

  • A and B are the concentrations of compounds or molecules
  • x and y are the reaction orders
  • r is the rate
  • k is the rate constant

Consider the reaction where NO and H2 are the reactants and N2 and H2 O are the products.

2NO(g) + 2H2 (g) → N2 (g) + H2 O (g)

Look at the experimental data table that relates the concentrations of reactants (note: M is molarity) with the speed of the reaction (note: M/s is molarity per second).

Experiment Concentration of NO (M) Concentration of H2 (M) Initial Rate (M/s)
1 0.0050 0.0020 1.25 x 10-5
2 0.0100 0.0020 5.00 x 10-5
3 0.0100 0.0040 1.00 x 10-4

We can use this data to help us figure out x and y and write a rate law equation for this reaction. In order to find x, we need to look at the data and determine what happens to NO when H2 remains the same (Experiments 1 and 2). Notice that NO doubles from Experiment 1 to Experiment 2.

How does this doubling impact the rate? The rate increases by a factor of four. In order to figure out x, we can relate the doubling and the increase by a factor of four in the following way: 2x = 4. Doing some math, we know that x = 2.

Next, we need to figure out y. Looking the table, you can see that Experiments 2 and 3 hold NO constant, while H2 is doubled. You can also see that when H2 is doubled, the rate is doubled. We can relate these two factors in the following way to determine y: 2y = 2. So we know y = 1.

Using our rate law equation, we can plug in A, B, x and y.

Example1

Reaction Order

It's safe to say we are cruising on through this! Are you ready for more? Now that we have determined the rate law, we can determine the reaction order, which tells us if and how the concentration of reactants impacts the rate.

In order to determine the reaction order, add the exponents x and y together. In our previous example, we would add 2 + 1 = 3. This tells us that we have a third order reaction.

Rate Constant

We have one more piece to bring everything together. Remember k from earlier? This is the rate constant, which relates the concentration of reactants to the rate of a reaction. We can use our rate law and our experimental data to determine k for our equation.

We can plug data from our experiment and then solve for k. We can use any experiment, but let's use the data from Experiment 2.

Plug in the molarity for NO and H2 and the rate from the data table. Now solve for k, which gives you 250.

Constant

The units are a little tricky, but if you do it right, you should end up with 1/ M2 s.

To unlock this lesson you must be a Study.com Member.
Create your account

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back
What teachers are saying about Study.com
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 200 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it risk-free for 30 days!
Create an account
Support