Announcements

1
Announcements

• Homework
• Chapter 4
• 8, 11, 13, 17, 19, 22
• Chapter 6
• 6, 9, 14, 15

• Exam
• Thursday

2
4-8

• Meaning of Confidence interval?
• Is an interval around the experimental mean that
  most likely contains the true mean (m).

3
Homework

• 4.11

4
Question 4-13.

• 4-13. A trainee in a medical lab will be
  released to work on her own when her results
  agree with those of an experienced worker at the
  95 confidence interval. Results for a blood
  urea nitrogen analysis are shown .
• What does abbreviation dL refer to?
• dL deciliter 0.1 L 100 mL
• b) Should the trainee work alone?

5
Comparison of Means with Students t
Is there a significant difference?
First you must ask, is there a significant
difference in their standard deviations?
f-test
YES
NO
6

• 4-13. dL deciliter 0.1 L 100 mL

Ftable 6.26
No difference
Find spooled and t
7
ttable 2.262
No significant difference between two workers
Therefore trainee should be Released
8
Homework

• 4-17. If you measure a quantity four times and
  the standard deviation is 1.0 of the average,
  can you be 90 confident that the true value is
  within 1.2 of the measured average

Yes
9
Homework

• 4-19. Hydrocarbons in the cab of an automobile
  Do the results differ at 95 CL? 99 CL?

Ftable 1.84
No Difference
Find spooled and t
10
Homework
The table gives t for 60 degrees of freedom,
which is close to 62. ttable 1.671 and 2.000 at
the 90 and 95 CL, respectively.
The difference IS significant at both confidence
levels.
11

• 4-22. Q-test, Is 216 rejectable?
• 192, 216, 202, 195, 204

Qtable 0.64
Retain the outlier 216
12
Chapter 6

• Chemical Equilibrium

13
Chemical Equilibrium

• Equilibrium Constant
• Equilibrium and Thermodynamics
• Enthalpy
• Entropy
• Free Energy
• Le Chateliers Principle
• Solubility product (Ksp)
• Common Ion Effect
• Separation by precipitation
• Complex formation

14
Example

• The equilibrium constant for the reaction
• H2O H OH-

Kw 1.0 x 10-14
KNH3 1.8 x 10-5
K3 ?
15
Equilibrium and Thermodynamics

• A brief review

16
Equilibrium and Thermodynamics

• enthalpy gt H
• enthalpy change gt DH
• exothermic vs. endothermic
• entropy gt S
• free energy
• Gibbs free energy gt G
• Gibbs free energy change gt DG

17
Equilibrium and Thermodynamics

• DGo DHo - TDSo
• DGo -RT ln (K)
• K e-(DGo/RT)

18
Equilibrium and Thermodynamics

• The case of HCl
• HCl H Cl-

K?

• DHo -74.83 x 103 J/mol
• DS0 -130.4 kJ/mol
• DGo DHo - TDSo

• DGo (-74.83 kJ/mol) (298.15 K) (-130.4
  kJ/mol)
• DGo -35.97 kJ/mol

19
Equilibrium and Thermodynamics

• The case of HCl
• HCl H Cl-

K?

• DGo (-74.83 kJ/mol) (298.15 K) (-130.4
  kJ/mol)
• DGo -35.97 kJ/mol

20
Predicting the direction in which an equilibrium
will initially move

• LeChateliers Principle and Reaction Quotient

21
Le Chatelier's Principle

• If a stress, such as a change in concentration,
  pressure, temperature, etc., is applied to a
  system at equilibrium, the equilibrium will shift
  in such a way as to lessen the effect of the
  stress.
• Stresses
• Adding or removing reactants or products
• Changing system equilibrium temperature
• Changing pressure (depends on how the change is
  accomplished

22
Consider

• 6CO2 (g) 6 H2O(g) C6H12O6(s) 6O2(g)

Predict in which direction the equilibrium moves
as a result of the following stress
Increasing CO2
23
Consider

• 6CO2 (g) 6 H2O(g) C6H12O6(s) 6O2(g)

Predict in which direction the equilibrium moves
as a result of the following stress
Increasing O2
24
Consider

• 6CO2 (g) 6 H2O(g) C6H12O6(s) 6O2(g)

Predict in which direction the equilibrium moves
as a result of the following stress
Decreasing H2O
25
Consider

• 6CO2 (g) 6 H2O(g) C6H12O6(s) 6O2(g)

NO CHANGE
Predict in which direction the equilibrium moves
as a result of the following stress
Removing C6H12O6(s)
K does not depend on concentration of solid
C6H12O6
26
Consider

• 6CO2 (g) 6 H2O(g) C6H12O6(s) 6O2(g)

Predict in which direction the equilibrium moves
as a result of the following stress
Compressing the system
System shifts towards the direction which
occupies the smallest volume. Fewest moles of
gas.
27
Consider

• 6CO2 (g) 6 H2O(g) C6H12O6(s) 6O2(g)

DH 2816 kJ
Predict in which direction the equilibrium moves
as a result of the following stress
Increasing system temperature
System is endothermic heat must go into the
system (think of it as a reactant)
28
Consider this

• CoCl2 (g) Co (g) Cl2(g)
• When COCl2 is 3.5 x 10-3 M, CO is 1.1 x 10-5
  M, and Cl2 is 3.25 x 10-6M is the system at
  equilibrium?
• Q Reaction quotient

K2.19 x 10-10
29
Compare Q and K
Q 1.02 x 10-8 K 2.19 x 10-10 System is not at
equilibrium, if it were the ratio would be
2.19x10-10 When
QgtK TOO MUCH PRODUCT TO BE AT
EQUILIRBIUM Equilibrium moves to the left
QltK TOO MUCH REACTANT TO BE AT
EQUILIRBIUM Equilibrium moves to the Right
QK System is at Equilibrium
30
Solubility Product

• Introduction to Ksp

31
Solubility Product

• solubility-product
• the product of the solubilities
• solubility-product constant gt Ksp
• constant that is equal to the solubilities of
  the ions produced when a substance dissolves

32
Solubility Product

• In General
• AxBy ltgt xAy yB-x
• Ayx B-xy
• K ------------
• AxBy
• AxBy K Ksp Ayx B-xy

33
Solubility Product

• For silver sulfate
• Ag2SO4 (s) ltgt 2 Ag(aq) SO4-2(aq)

• Ksp Ag2SO4-2

34
Solubility of a Precipitatein Pure Water

• EXAMPLE How many grams of AgCl (fw 143.32)
  can be dissolved in 100. mL of water at 25oC?
• AgCl ltgt Ag Cl-
• Ksp AgCl- 1.82 X 10-10 (Appen. F)
• let x molar solubility Ag Cl-

35
EXAMPLE How many grams of AgCl (fw 143.32)
can be dissolved in 100. mL of water at 25oC?

• AgCl(s) Ag (aq) Cl- (aq)

Initial Some - -
Change -x x x
Equilibrium -x x x
(x)(x) Ksp AgCl- 1.82 X 10-10
x 1.35 X 10-5M
36
EXAMPLE How many grams of AgCl (fw 143.32)
can be dissolved in 100. mL of water at 25oC?
x 1.35 X 10-5M

• How many grams is that in 100 ml?
• grams (M.W.) (Volume) (Molarity)
• 143.32 g mol-1 (.100 L) (1.35 x
  10-5 mol L-1)
• 1.93X10-4 g 0.193 mg

37
The Common Ion Effect
38
The Common Ion Effect

• common ion effect
• a salt will be less soluble if one of its
  constituent ions is already present in the
  solution

39
The Common Ion Effect

• EXAMPLE Calculate the molar solubility of
  Ag2CO3 in a solution that is 0.0200 M in Na2CO3.
• Ag2CO3 ltgt 2 Ag CO3-2
• Ksp Ag2CO3-2 8.1 X 10-12

40
EXAMPLE Calculate the molar solubility of
Ag2CO3 in a solution that is 0.0200 M in Na2CO3.

• Ag2CO3 ltgt 2 Ag CO3-2

Initial Solid - 0.0200M
Change -x 2x x
Equilibrium Solid 2x 0.0200x

• Ksp Ag2CO3-2 8.1 X 10-12

Ksp(2x)2(0.0200M x) 8.1 X 10-12
4x2(0.0200M x) 8.1 X 10-12
41
EXAMPLE Calculate the molar solubility of
Ag2CO3 in a solution that is 0.0200 M in Na2CO3.

• 4x2(0.0200M x) 8.1 X 10-12
• no exact solution to a 3rd order equation, need
  to make some approximation

• first, assume the X is very small compared to
  0.0200 M

4X2(0.0200M) 8.1 X 10-12
4X2(0.0200M) 8.1 X 10-12
X 1.0 X 10-5 M
42
EXAMPLE Calculate the molar solubility of
Ag2CO3 in a solution that is 0.0200 M in Na2CO3.

• X 1.0 X 10-5 M
• (1.3 X 10-4 M in pure water)

Second check assumption

• CO3-2 0.0200 M X 0.0200 M

0.0200 M 0.00001M 0.0200M
Assumption is ok!
43
Separation by Precipitation
44
Separation by Precipitation

• Complete separation can mean a lot we should
  define complete.
• Complete means that the concentration of the less
  soluble material has decreased to 1 X 10-6M or
  lower before the more soluble material begins to
  precipitate

45
Separation by Precipitation

• EXAMPLE Can Fe3 and Mg2 be separated
  quantitatively as hydroxides from a solution that
  is 0.10 M in each cation? If the separation is
  possible, what range of OH- concentrations is
  permissible.
• Two competing reactions

46
EXAMPLE Separate Iron and Magnesium?

• Ksp Fe3OH-3 2 X 10-39
• Ksp Mg2OH-2 7.1 X 10-12
• Assume quantitative separation requires that the
  concentration of the less soluble material to
  have decreased to lt 1 X 10-6M before the more
  soluble material begins to precipitate.

47
EXAMPLE Separate Iron and Magnesium?

• Ksp Fe3OH-3 2 X 10-39
• Ksp Mg2OH-2 7.1 X 10-12

Assume Fe3 1.0 X 10-6M
What will be the OH- required to reduce the
Fe3 to Fe3 1.0 X 10-6M ?

• Ksp Fe3OH-3 2 X 10-39

48
EXAMPLE Separate Iron and Magnesium?

• Ksp Fe3OH-3 2 X 10-39

(1.0 X 10-6M)OH-3 2 X 10-39
49
Add OH-
Mg2
Mg2
Fe3
Fe3
Fe3
Fe3
Mg2
Mg2
Mg2
Fe3
Mg2
Fe3
Mg2
Fe3
Mg2
Fe3
Fe3
Fe3
Mg2
Mg2
Mg2
Fe3
Fe3
50
Mg2
Mg2
Mg2
Mg2
Mg2
_at_ equilibrium
Fe3
Mg2
What is the OH- when this happens

Mg2
Mg2
Is this OH- (that is in solution) great enough
to start precipitating Mg2?
Mg2
Mg2
Mg2
Fe(OH)3(s)
51
EXAMPLE Separate Iron and Magnesium?

• Ksp Fe3OH-3 2 X 10-39

(1.0 X 10-6M)OH-3 2 X 10-39
52
EXAMPLE Separate Iron and Magnesium?

• What OH- is required to begin the precipitation
  of Mg(OH)2?
• Mg2 0.10 M

• Ksp (0.10 M)OH-2 7.1 X 10-12

• OH- 8.4 X 10-6M

53
EXAMPLE Separate Iron and Magnesium?
_at_ equilibrium

• OH- to completely remove Fe3
• 1.3 X 10-11 M

OH- to start removing Mg2 8.4 X 10-6M
All of the Iron will be precipitated b/f any of
the magnesium starts to precipitate!!