ANNOUNCEMENTS Answer key posted on web site in “Exam” section See me the week after break if <70

Review:There are two methods for determining rate law variables 1. Initial rate method ◦ Look at how fast reactant disappears at the beginning of the reaction (initial rate) ◦ Technique= vary [R], look at initial rate  Double [R], if rate  Stays the same = Zero order  Doubles = First order  Quadruples = Second order

Review:There are two methods for determining rate law variables 2. Graphical method  [R] vs. time  ln[R] vs. time  1/[R] vs. time ◦ Linear plot determines order ◦ Caveat: This method is only useful when only one reactant changes ◦ BUT: We can play with reactant concentrations to get around this

Both the graphical and initial rate methods were used in the dye kinetics lab Pseudo-order kinetics

What is the order of this reaction? 1. First 2. Second 3. Third

What is the rate constant (k) for this reaction?

The integrated rate law yields a relationship between [R] 0, [R] t, k, and t Zero order reaction: [R] 0 - [R] t = kt First order reaction: [R] t = [R] 0 e -kt andln([R] t /[R] 0 )= -kt Second order reaction: 1/[R] t – 1/ [R] 0 = kt

Summary of rate laws and graphical method Reaction Order Integrated Rate Law Characteristic Kinetic Plot Slope of Kinetic Plot Units of Rate Constant Zero[R] 0 - [R] t = kt[R] vs. t-kmole L -1 sec -1 First [R] t = [R] 0 e -kt ln([R] t /[R] 0 )= -kt ln[R] vs. t-ksec -1 Second1/[R] t – 1/ [R] 0 = kt1/[R] vs. tkL mole -1 sec -1

Half life is the time it takes for one half of something to “go away” (react, decay, etc.) Half life = t 1/2 For a reaction= time for [R] to drop to half its original value

We can use the integrated rate law to derive a relationship between t 1/2 and k For a first order reaction

Radioactive decay is a 1 st order process Always 1 st order Independent of changes in T, P, and [R] ◦ N 0 = # radioactive atoms ◦ N t = # radioactive atoms after time t Measure in counts per minute (cpm) cpm  N

Radioactive Decay: Example 1 Radioactive gold-198 is used in the diagnosis of liver problems. The half-life of this isotope is 2.7 days. If you begin with a 5.6-mg sample of the isotope, how much of this sample remains after 1.0 day?

Carbon-14 dating can be used to date objects that are up to 60,000 years old

Radioactive Decay: Example 2 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

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