Kinetics

1. By: Olivia Bohnhoff & Zach Feldker Kinetics

2. What is kinetics? • The study of the speed of reactions. • It’s based on experiments. • We did it first semester, so no one probably remembers anything. (Right?) • RATE LAWS. • Mechanisms • Half-lives • Catalysts! • And lots of graphs. Fun, right?

3. 5 things that affect a reaction • Size of the atom of the reactants - Larger molecules react more slowly than smaller ones. It’s more dense, and therefore harder to get to. The smaller they are, the easier to get to. Get it? • Temperature - The higher the temperature is, the more quickly things will react. The kinetic energy is greater and more molecules are colliding and reacting than at a lower temperature.

4. Five things continued • Concentration - The more concentrated something is, the more likely it is to react. • State of reactants - Liquids react more quickly than solids, gases react the most quickly because the molecules are the tightest in a solid and less so in a liquid and even more less so in a gas. • Catalysts - Catalysts speed up a reaction by reducing the activation energy, the energy necessary to start the reaction, or “getting over the hump.”

5. Fun graph(s)! The reaction with and without a catalyst

6. RATE LAWS • This is probably what we all failed on the final, right? • Now you don’t have to! • Rate law is the equation involving k. Little k. It’s a constant. • It tells us the rate of the reaction based on the concentration. • Rate = k [A]m[B]n • There are three orders to the rate law: first, second, and zero. Let’s take a look at them individually, shall we?

7. First-Order Rate Laws • It depends on only one reactant, and only raised to the first power. • Rate = k[A]1 or Rate = k[A]1[B]1 • If you want to look at it from a logarithmic point of view (which is probably a good idea), the equation looks like this. ln[A]t-ln[A]0= -kt • [A]t = the concentration of A at a specific, designated time. • [A]0 is the original concentration of A. • k is the rate constant. • t is the time elapsed.

8. First-Order Graph This is what a first-order graph looks like. You can always recognize it because it slopes downward and therefore has a negative slope.

9. Second-Order Rate Law • Looks similar to the first, except squared. • Rate = k[A]2 • The real (integrated) rate law: 1/[A]t – 1/[A]0 = kt • Again, [A]t = the concentration of A at time t, your choice. • [A]0 is the original concentration. • k is still the rate constant. • And t is the amount of time.

10. Second-Order Graph A second-order graph is characterized by a straight line with a positive slope.

11. Zero-Order Rate Law • A little bit different than the first two in that it is the exponent is 0, and therefore leaves you with just the bare rate constant. • Rate = k • The graph is a straight line with a negative slope.

12. Look! Mechanisms! • The overall idea of mechanisms is that reactions do not occur in a single step, but rather a bunch of little ones, called elementary steps. • It’s similar to Hess’s Law in that the steps should add up to the overall equation. • It’s dissimilar to Hess’s Law in that it has nothing to do with enthalpy.

13. Key Things to know about Mechanisms • Intermediates- intermediates are substances produced in one elementary step and then used up in a later one, and so they won’t show up in the overall equation. • Catalysts- catalysts are also included in elementary steps. They show up as reactants and then as products, opposite of intermediates. • Types of Steps- Unimolecular (decompisition), Bimolecular (pretty much everything else), and Termolecular (pretty much nonexistent.)

14. More Mechanisms • In a mechanism, the slowest step is the most important because it determines the rate. Cleverly called the rate-determining step. • You can write rate laws for the individual elementary steps, but you can’t put them all together to write the overall equation. You can only take that from the rate-determining step.

15. Mechanism Example • A + 2B E + F • A+B C • C+B D • D E+F What is the rate Determining step for the below reaction? • NO2(g) + CO(g) NO(g) + CO2(g) • Rate Law = k[NO2]2 • NO2(g) + NO2(g) NO3(g) + NO(g) • NO3(g) + CO(g) NO2(g) + CO2(g) Reaction Intermediates

16. Multiple Choice! • A reaction follows the rate law: Rate = k[A]2. Which of the following plots will give a straight line? • 1/[A] versus 1/time • [A]2 versus time • 1/[A] versus time • Ln [A] versus time • [A] versus time

17. Question # 2 • For the following reaction: NO2(g) + CO(g) NO(g) + CO2(g), the rate law is : Rate = k[NO2]2. If a small amount of gaseous carbon monoxide (CO) is added to a reaction mixture that was .10 molar NO2 and .20 molar CO, which of the following statements is true? • Both k and the reaction rate remain the same • Both k and the reaction rate increase • Both k and the reaction rate decrease • Only k increases, the reaction rate remains the same • Only the reaction rate increases; k remains the same.

18. Question #3 • The steps below represent a proposed mechanism for the catalyzed oxidation of CO by O3 Step 1: NO2(g) + CO(g) NO(g) + CO2 Step 2: NO(g) + O3(g) NO2(g) + O2(g) What are the overall products of the catalyzed reaction? • CO2 and O2 • NO and CO2 • NO2 and O2 • NO and O2 • NO2 and CO2

19. Question 4 • The decomposition of ammonia to the element is a first order reaction with a half-life of 200s at a certain temperature. How long will it take the partial pressure of ammonia to decrease from 0.100 atm to 0.00625 atm? • 200s • 400s • 800s • 1000s • 1200s