Chemical Kinetics Chapter 15

1
Goals Dependence of rate on conc. Distinguish
different rates How T affects rate How
catalysts affect rate Determine simple
mechanisms Suggested Problems 1, 7, 9, 11,
13, 15, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37,
41, 45, 47, 49, 53, 55, 57, 59, 61, 65
2
Relative Rates of Change
Your Turn Show the relative rates of change for
the reaction N2O5(solvent) --gt 2 NO2(solvent)
½ O2(g)
Hint First, convert to whole number
stoichiometric coefficients.
3
Relative Rates of Change
Not Given
Problem Show the relative rates of change for
the reaction N2O5(solvent) --gt 2 NO2(solvent)
½ O2(g) 2 N2O5(solvent) --gt 4 NO2(solvent)
O2(g)
4
Deriving Rate Laws

• Your Turn Derive rate law and k for
• CH3CHO(g) ---gt CH4(g) CO(g)
• from experimental data for rate of disappearance
  of CH3CHO Rate of rxn - k CH3CHO?
• Expt. CH3CHO Disappear of CH3CHO (mol/L) RA
  TE (mol/Lsec)
• 1 0.10 0.020
• 2 0.20 0.081
• 3 0.30 0.182
• 4 0.40 0.318

5
Deriving Rate Laws
Not Given

• Rate of rxn k CH3CHO2
• Here the rate goes up by FOUR when initial conc.
  doubles. Therefore, we say this reaction is
  Second order.
• Now determine the value of k. Use expt. 3 data
• 0.182 mol/Ls - k (0.30 mol/L)2
• k - 2.0 (L / mols)
• Using k you can calc. rate at other values of
  CH3CHO at same T.

6
Deriving Rate Laws
Note In this example the rate of removal
depends on the conc. in soln. Increase conc. a
increase removal Go back to the ethanol problem,
rate is independent of the presence of
ethanol. So what will happen?
7
Your Turn Determine the rate order expression
for the following data set
8
Concentration/Time Relations

• Tylenol dose is 650 mg
• Human body has 5L of blood. Thus, Initial
  Tylenol Co 8.61 x 10-4 M (confirm this!)
• Rate of removal of Tylenol is k 0.21 hr-1
• What is conc. when you need to take another dose
  in 4 hours?

Acetaminophen (Tylenol) C8H9O2N
How long will it take to drop the conc. by 90 of
Co (10 is left or 8.61 x 10-5 M)?
9
Concentration/Time RelationshipsRate of removal
of Tylenol k Tylenol, k 0.21 hr-1. If
initial Tylenol 8.61 x 10-4 M What is the
blood conc. when you take another dose in 4 hrs
How long will it take to drop the blood conc. by
90 of Co or to 8.61 x 10-5 M?

• Use the first order integrated rate law

10
Concentration/Time RelationshipsRate of removal
of Tylenol k Tylenol, k 0.21 hr-1. If
initial Tylenol 8.61 x 10-4 M What is the
blood conc. when you take another dose in 4 hrs
How long will it take to drop the blood conc. by
90 of Co or to 8.61 x 10-5 M?
Use the first order integrated rate law
11
Your Turn

• If k 0.1 min-1 what is half-life?
• If half-live 1000 years, what is k?
• THERE IS NO ANSWER TO THIS PROBLEM ON THE WEB

12
Pharmacology / Forensic Science
Your Turn You are working as a technician
for the State crime lab and you receive a urine
sample for cocaine analysis. The sample is from
a driver who fled the scene of an accident and
was apprehended 3 hours later. It has been 5
hours after the accident when you obtain
the sample and you find a concentration of 6.04 x
10-7 M. The average removal rate of cocaine
from the human body is 0.8 hr-1. The court
decides that a concentration above 3.3 x 10-6 M
will impair the driver. What was the cocaine
concentration in the driver at the time of the
accident?
13
Your Turn You are working as a technician
for the State crime lab and you receive a urine
sample for cocaine analysis. The sample is from
a driver who fled the scene of an accident and
was apprehended 3 hours later. It has been 5
hours after the accident when you obtain
the sample and you find a concentration of 6.04 x
10-7 M. The average removal rate of cocaine
from the human body is 0.8 hr-1. The court
decides that a concentration above 3.3 x 10-6 M
will impair the driver. What was the cocaine
concentration in the driver at the time of the
accident?
14
Natural Formation of Radioactive Elements 146C

• N2 in the atmosphere is bombarded by cosmic
  radiation. One type of radiation is neutrons.
• 147N 10n gt 146C 11H
• 146C formed in this reaction, goes on to form
  radioactive CO2which is taken up by
  plants.which is eaten by animals.which are
  eaten by more animals. With time 146C decays.
• 146C gt 0-1b 147N

15
Half-Life Problem
You are working as an anthropologist, and you
find a well-preserved burial ground containing
wooden artifacts. You have some of these dated
using a 14C dating technique and the results show
that 35 of the original 14C remains. Given
that the half-live of 14C is 5730 years, what is
the age of the burial site? ln(At/Ao)-kt
Hint calc. k using t½, then calc. age
16
Half-Life Problem
Soln ln(At/Ao)-kt ln(0.5)-k (5735yr) 1.21
x 10-4 /yr k ln(0.35) - (1.21 x 10-4
/yr)t age t 8686 years