Outline: 2/2/07 Chem Dept Seminar - today @ 4pm Exam 1 – two weeks from today…

Outline Quiz #3 Chapter 15 - Kinetics (cont’d): - determining reaction order - integrated rate law calcs

Quiz #3

Please put away all books and papers.Please put away all books and papers. If you don’t have a calculator, just set If you don’t have a calculator, just set

up the equations correctly…up the equations correctly…

Quiz #3

Please turn your quiz over and pass it Please turn your quiz over and pass it to your right.to your right.

Quiz #3 Reaction was endothermic…Reaction was endothermic…

= = H H productsproducts - - HHreactantsreactants

Reaction increased entropy…Reaction increased entropy…

= = S S productsproducts - - SSreactantsreactants

GG = = HH - T - TSS spontaneous when spontaneous when G = 0G = 0

= = GG + RT ln + RT ln QQ

Quiz #3 Reaction was endothermic…Reaction was endothermic…

= = H H productsproducts - - HHreactantsreactants

Reaction increased entropy…Reaction increased entropy…

= = S S productsproducts - - SSreactantsreactants

GG = = HH - T - TSS

spontaneous when spontaneous when G = 0G = 0

= = GG + RT ln + RT ln QQ

When do you use G = G + RT ln Q ?

Calculate Calculate GGreactionreaction for the reaction @ 25 for the reaction @ 25°°C:C:

CC22HH55OHOH((ll)) + 3O + 3O2(g)2(g) 2CO 2CO2(g)2(g) + 3H + 3H22OO((ll))

if if pp=0.25 bar for each gaseous substance.=0.25 bar for each gaseous substance.

Grxn = Gorxn RT ln Q

Gorxn 1325 kJ/mol (from appendix D)

R kJ/mol K

T K

Problem 14.40 (c):

Calculate Calculate GGreactionreaction for the reaction @ 25 for the reaction @ 25°°C:C:

CC22HH55OHOH((ll)) + 3O + 3O2(g)2(g) 2CO 2CO2(g)2(g) + 3H + 3H22OO((ll))

if if pp=0.25 bar for each gaseous substance.=0.25 bar for each gaseous substance.

What is Q?

Q = (0.25)2/(0.25)3

Q

Q 3

2

2

2

O

CO

p

p

Grxn = -1325 2.48 ln 4 = -1322

How many problems of this type have you already worked out on your own....

20%

20%

20%

20%

20% 1.1. I have done dozens or more…I have done dozens or more…

2.2. I have done many, but I still miss some…I have done many, but I still miss some…

3.3. I have done a couple from the book & CAPAI have done a couple from the book & CAPA

4.4. I have done the CAPA ones…I have done the CAPA ones…

5.5. Never seen this type of problem before..Never seen this type of problem before..

11 22 33 44 55

Practice Problems: Chapter 14

14.11, 14.15, 14.17, 14.19, 14.23, 14.25, 14.11, 14.15, 14.17, 14.19, 14.23, 14.25, 14.27, 14.31, 14.35, 14.37, 14.38, 14.41, 14.27, 14.31, 14.35, 14.37, 14.38, 14.41, 14.43, 14.49, 14.51, 14.53, 14.55, 14.57, 14.43, 14.49, 14.51, 14.53, 14.55, 14.57, 14.61, 14.65, 14.67, 14.71, 14.75, 14.77, 14.61, 14.65, 14.67, 14.71, 14.75, 14.77, 14.79, 14.81, 14.91, 14.101, 14.10314.79, 14.81, 14.91, 14.101, 14.103

Back to Chapter 15…

The rate equation cannot be predicted, it can only be measured empirically.

Calculate k from initial rates Use the integrated form of the rate eqn.

to solve for concentration (Section 15.4)

There are two forms to know:

First order: ln[A] = ln[A]o k t

Second order: 1/[A] = 1/[A]o k t

15-3

15-5

Can use data to find kk and reaction order

How do you find

the reaction order?

Plot data

Plot both….1st Order Plot

R2 = 0.8389-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

0 100 200 300

Time (s)

Ln

[NO

2]

Series2

Linear(Series2)

2nd Order Plot

R2 = 0.9949

0.0

100.0

200.0

300.0

400.0

0 100 200 300

Time (s)

1/[N

O2]

Series2

Linear(Series2)

ln[A] = ln[A]o k t

1/[A] = 1/[A]o k t

Only one will be truly linear….

Rate = k [NO2]2

slope

Example: 1st or 2nd order decay?

Try both plots….

2nd order plot

R2 = 0.9643

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 1 2 3

time

1/[c

is] Series1

Linear(Series1)

1st order plot

R2 = 1

0

0.5

1

1.5

2

2.5

3

0 1 2 3

time

ln[c

is]

Series1

Linear(Series1)

Rate = k [cis-AB]

Rate = k pNO2 pCl2

0.000040 atm/s = k (0.1 atm)2 (0.1 atm)

k = 0.040 1/atm2 s

Know the “order” from the isolation experiments: e.g. double pNO and see what happens to rate…

Don’t forget to practice lots of problems from the back of the chapter in the textbook

Worksheet #4

Summary: Can build a rate law from observed data:

Rate = k [A]m [B]n

m, n depend only on the chemical reaction under consideration….

Can use integrated rate laws to predict rates, concentrations at various times, etc.

15-3

15-5

There are two forms to know:

First order: ln[A] = ln[A]o k t

Second order: 1/[A] = 1/[A]o k t

(initial rates)

The integrated forms of the rate laws are important:

Can predict [concentration] as a function of time!

Example: The decomposition reaction of

NO2 is second- order in [NO2], with a rate constant of 1.51 Ms.

If the initial concentration is 0.041M, when will the concentration = 0.010 M?

“Rate Law” of a reaction: Rate = k [NO2]2

The integrated solution: 1/[NO2] 1/[NO2]o = k t

1/0.010 1/0.041 = 1.51 M-1s-1 t t = ? 50 sec