Why are you in this class

1
Why are you in this class?

• using chemistry to understand
• identify important factors
• quantitative calculations
• predict changes
• improve lab skills
• learn to work as team

2
Imagine a world without hypothetical situations

• hypothetically, what do you expect out of this
  class?

3
Can you make Learning your goal rather than just
a good grade?
4
Have you looked at the problems
5
http//keats.admin.virginia.edu
6
Equilibrium

• The equilibrium constant for the following
  esterfication reaction carried out in an aqueous
  solution with a little sulfuric acid present is
  slow
• CH3OH CH3COOH CH3COCH3 H2O

7
(No Transcript)
8
Equilibrium

• methanol acetic acid methyl acetate water
• The equilibrium constant for this reaction

9
Equilibrium
If you start by mixing equal volumes of a 0.5 M
solution of methanol and 0.5 M solution of acetic
acid and then check the concentrations of
each substance after a week, what will each of
the concentrations in the solution be?
10
Equilibrium

• The following reaction is for the hydrolysis of
  the ester methyl acetate
• H2O CH3COCH3 CH3OH CH3COOH

11
Equilibrium

• water methyl acetate methanol acetic acid
• The equilibrium constant for this reaction

12
Equilibrium

• If you start with a 0.25 M solution of methyl
  acetate, add a small amount of sulfuric acid, and
  then wait for a week, what will be the
  concentration of each of the three species? Would
  these results change if you added the enzyme
  esterase?

13
Equilibrium - Unique Conditions

• consider hydrolysis of 0.25 M methyl acetate
• mix equal volumes of 0.5 M methanol and 0.5 M
  acetic acid
• same mixture with the enzyme esterase
• eventually the same concentrations

14
Equilibrium

• If the enzyme esterase were added to the
  solutions when they were mixed how would the
  concentrations be changed ?

15
Thermodynamic equilibrium

• results are not always accurate
• temperature dependence
• we use concentrations rather than activities
• common ions
• competing reactions

16
Salt in Water

• An ionic crystal dissolves and produces ions in
  solution

17
NaCl in Water

• Na cation interacts with oxygen end of water
  molecules
• Cl anion interact with hydrogen end of water
  molecules
• Ions and water form a COMPLEX
• Equilibrium established
• Saturation with large equilibrium constant

18
What about insoluble compounds?

• AgCl is insoluble
• WHY?
• Energy solid vs ions in solution
• Some ions in solution / heterogeneous equilibrium
  (2 phases)
• Equilibrium constant (a for solid is 1 or 0)
• What happens when concentrations exceed constant?

19
http//ehs.virginia.edu
20
Consider chloride salts

• NaCl - soluble
• AgCl - insoluble Ksp 1x e-10
• PbCl2 - insoluble Ksp 1.6 x e-5
• differences are a function of energy -
  thermodynamics

21
Gravimetric analysis

• weigh a precipitate - a solid of known composition

22
When does precipitation begin?

• Theoretically when the solubility constant is
  exceeded

23
When does precipitation begin?

• Theoretically when the solubility constant is
  exceeded
• When the product of concentrations is less than
  or equal to the Ksp, the crystals do not form

24
When does precipitation begin?

• Theoretically when the solubility constant is
  exceeded
• Imagine the problem of individual ions finding
  the necessary counterion
• one ion pair does not make a crystal

25
(No Transcript)
26
Precipitation process

• Temporary supersaturation
• Microcrystals
• Crystal growth
• Impurities
• Ions in solution may not be of equal
  concentrations (limiting reagent)

27
Precipitation

• Supersaturation
• temperature dependence
• other ions
• competing reactions

28
Best to keep supersaturation low

• dilute solutions
• mix slowly and stir
• start warm and cool
• solubility usually greater in acid so start at
  low pH and increase
• homogeneous precipitation - generate reagent in
  solution

29
Selectivity

• you must know what you weigh
• precipitate only what you want to weigh
• choose compounds that have low solubility
• control conditions to make sure that you get only
  the reaction you want

30
Can you precipitate one ion in the presence of an
interfering ion?

• chloride in presence of bromide

31
Precipitation of AgCl

• Thermodynamics
• Kinetics
• Supersaturation
• Nucleation - the higher the degree of saturation,
  the faster the nucleation of more small crystals
• Crystal growth - faster growth if more saturated

32
(No Transcript)
33
Problems with AgCl

• chloride is a common contaminant
• silver ions form many insoluble compounds
• carbon dioxide in air forms carbonate which
  precipitates with Ag ion
• heating Cl ion with nitric acid oxidizes Cl- with
  loss of Cl2 gas
• photodecomposition of AgCl

34
Incorporation of of impurities and defects

• if the surface area is great and the growth is
  rapid

35
Impurities in precipitates

• Occlusion - ions or molecules trapped as crystals
  grow rapidly

36
Impurities in precipitates

• Surface adsorption - ions in excess are attracted
  to charges on surface and create a net charge

37
Impurities in precipitates

• Isomorphous replacement - ions of similar size
  and character substitute , especially if they
  form compounds of limited solubility

38
Impurities in precipitates

• Post precipitation - other compounds precipitate
  using crystal as nucleus

39
ERRors

• Why isnt phonetic spelled the way it sounds?

40
Indeterminant ERRORS

• most time consuming
• not understood
• random
• related to repeatability - precision
• can average to improve accuracy
• statistical treatment
• possibility of bias

41
Determinant ERRORS

• most important
• can be understood, corrected, or avoided
• instrumental - calibration, tests
• method - blanks, interferences, serious
• reagent impurities
• operator - YOU - improves with experience and
  good records

42
Determinant ERRORS

• some are constant and some are proportional to
  sample size

43
Instrumental errors

• check calibration, zero, analyze known samples

44
Reagent errors

• usually proportional to sample size

45
Calculations

• keep records clearly identified, be aware of bias
  in choosing what seems to be the best answer

46
Standard deviation

• Replicate samples (assume no errors)
• Calculate mean (average)
• Difference between each value and the mean is a
  deviation
• Root mean square or standard deviation eliminates
  the sign of the deviation
• Random errors obey Gaussian Statistics

47
ERRORS 1 MILE PLUS 8 inchs About 30 of 568112
48
(No Transcript)
49
TRY...Do the best you can
50
(No Transcript)
51
(No Transcript)
52
(No Transcript)
53
(No Transcript)
54
(No Transcript)
55
(No Transcript)
56
(No Transcript)
57
(No Transcript)
58
(No Transcript)
59
(No Transcript)
60
(No Transcript)
61
(No Transcript)
62
(No Transcript)
63
(No Transcript)
64
(No Transcript)
65
(No Transcript)
66
(No Transcript)
67
(No Transcript)
68
(No Transcript)
69
(No Transcript)
70
(No Transcript)
71
(No Transcript)
72
(No Transcript)
73
(No Transcript)
74
(No Transcript)
75
(No Transcript)
76
(No Transcript)
77
(No Transcript)
78
(No Transcript)
79
(No Transcript)
80
(No Transcript)
81
(No Transcript)
82
(No Transcript)
83
(No Transcript)
84
(No Transcript)
85
(No Transcript)
86
(No Transcript)
87
(No Transcript)
88
(No Transcript)
89
(No Transcript)
90
(No Transcript)
91
(No Transcript)
92
(No Transcript)
93
(No Transcript)
94
(No Transcript)
95
(No Transcript)
96
(No Transcript)
97
O n e Red G
98
(No Transcript)
99
(No Transcript)
100
(No Transcript)
101
(No Transcript)
102
Nernst equation

• Define free energy (G)
• Change in free energy
• Delta G related to Keq

103
(No Transcript)
104
Measurement of standard potential
105
(No Transcript)
106
(No Transcript)
107
(No Transcript)
108
(No Transcript)
109
(No Transcript)
110
(No Transcript)
111
(No Transcript)
112
(No Transcript)
113
(No Transcript)
114
(No Transcript)
115
(No Transcript)
116
(No Transcript)
117
(No Transcript)
118
(No Transcript)
119
Potentiometric Measurements

• 1st kind Ag
• 2nd kind AgCl
• 3rd kind AgEDTA

120
(No Transcript)
121
(No Transcript)
122
(No Transcript)
123
(No Transcript)
124
(No Transcript)
125
(No Transcript)
126
MnO4-2 ---gt MnO2 MnO4- (alkaline solution)b)
MnO4- H2S ---gt Mn2 Sc) SbH3 Cl2O ---gt
H4Sb2O7 HCld) FeS NO3- ---gt Fe3 NO2 S
(acid solution)e) Al NO3- ---gt AlO2- NH3f)
FeAsS ClO2 ---gt Fe3 AsO4-3 SO4-2 Cl-
(acid solution)g) K2NaCo(NO2)6 MnO4- ---gt K
Na Co3 NO3- Mn2 (acid solution)
127
(No Transcript)
128
Beers Law

• Aabc
• Aabsorbance
• aabsorbtivity (depends on species and
  wavelength)
• bpathlength in sample
• cconcentration of absorbing species

One million photons impinge on a sample in a
UV-vis spectrometer and 800,000 of the photons
pass through to the detector, the remaining
photons having been absorbed. How many photons
will pass through the sample if the concentration
is doubled?
129
(No Transcript)
130
(No Transcript)
131
(No Transcript)
132
(No Transcript)
133
(No Transcript)
134
(No Transcript)
135
(No Transcript)
136
(No Transcript)
137
(No Transcript)
138
(No Transcript)
139
(No Transcript)
140
(No Transcript)
141
(No Transcript)
142
(No Transcript)
143
(No Transcript)
144
(No Transcript)