ANNOUNCEMENTS Welcome back! me this week if your Exam #2 score was <70 FYI- Exam #3 is one week from Friday (April 23 rd )

Review- The rate law tells us about the concentration dependence of a reaction Rate = k [R 1 ] x [R 2 ] y Two methods to determine concentration dependence ◦ Initial rate method ◦ Graphical method (using integrated rate law) Radioactive decay is a first order process ◦ For a 1 st order process:

EXAMPLE: Carbon-14 dating of an artifact The Carbon-14 activity of an artifact in a burial site is found to be 8.6 counts per minute per gram. Living material has an activity of 12.3 counts per minute per gram. How long ago did the artifact die? t 1/2 = 5730 years

Other things besides concentration can affect reaction rates Collision theory: Reactions occur when ◦ Molecules collide… ◦ In the correct orientation… ◦ With enough energy

A reaction coordinate diagram shows energy changes throughout a process

The reaction coordinate diagram for reaction with more than one step

Simulation: Thermodynamics and the Equilibrium Constant What happens to the rate of a reaction when we change the activation energy (E a ) barrier?

Which reaction is faster? (  E is the same for both A and B, but B has a higher E a ) 1. A 2. B 3. The rates are the same for A and B. P R EaEa A P R EaEa B

Which reaction is faster? (E a is the same for both A and B, but B has a larger  E) 1. A 2. B 3. The rates are the same for both A and B. P R EaEa A P EaEa R B

The Arrhenius equation gives the temperature and E a dependence of the rate Rate Law Arrhenius equation

The Boltzmann plot shows the fraction of collisions that have enough energy to overcome E a

What happens to k as Ea increases (R and T are constant)? 1. Increases 2. Decreases 3. Stays the same

What happens to k as T increases (R and Ea are constant)? 1. Increases 2. Decreases 3. Stays the same.

The Arrhenius equation is used to determine the activation energy (E a ) from experimental data Graphical Form Two point form

Using the graphical method to find E a

Example: A reaction triples its rate when the temperature increases from 25 o C to 37 o C. What is the activation energy?

Example: A reaction has Ea = 75 kJ/mol. If k = 1.50 x at 25 o C, what is k at 50 o C?