Transcript SOLUTION

普通化學（二）/分析化學
Week
100 學年 第 2 學期
Spring 2012 (Jan 2012 - May 2012)
Topics
Date
Lecturer
1
Evaluating analytical data and statistics
1/12
王明芳
2
Acids, bases, and buffer
2/02
王明芳
3
Titration and acid-base titration
2/09
王明芳
4
EDTA titration
2/16
王明芳
5
Solubility equilibria
2/23
黃紀榕
6
Precipitation titration
3/01
黃紀榕
7
Electrode potentials & electrode measurements 3/08
黃紀榕
8
Redox titration and electrochemical methods
黃紀榕
9
------- 期中考
10
A deeper look at chemical equilibrium
3/29
王明芳
11
Spontaneity, entropy, and free energy
4/05
王明芳
12
Spectroscopic methods (I)
4/12
王明芳
13
Chromatography (I)
4/19
王正康
14
Spectroscopic methods (II)
4/26
王明芳
15
Chromatography (II)
5/03
王正康
16
Chromatography (III)
5/10
王正康
17
Electrophoresis
5/17
王正康
18
------- 期末考
3/15
所有教師
-------
-------
所有教師
教科書： Daniel C. Harris (2009): Exploring Chemical Analysis, 4th ed.,
W. H. Freeman & Company.
上課時間： A: 週四 13:30 - 15:20, B: 週四 08:00 - 09:50
上課地點： 1 教室
註： 第五週及第十三週各一次小考
1
Exploring Chemical Analysis
Daniel C. Harris (Fourth Edition)
11
Polyprotic Acids and
Bases
國防醫學院
生化學科
王明芳老師
2012-3-29
2
Erosion of carbonate stone
3
Outline

Amino acids are polyprotic

Finding the pH in Diprotic systems

Which is the principal species?

Titration in polyprotic systems
4
Acid dissolves buildings and teeth

The main constituent of limestone and marble, which
are used in many building is calcite, a crystalline form
of calcium carbonate.

This mineral is insoluble in neutral or basic solution
but soluble in acid solution.
CaCO3(s)
Calcite
Ca2+ + CO32Carbonate
CO32- + H+
HCO3Bicarbonate
5
Tooth decay

Tooth enamel contains the mineral hydroxyapatite, a
calcium hydroxy-phosphate.
Ca10(PO4)6(OH)2(s) + 14 H+
10 Ca2+ + 6 H2PO4- + 2H2O
Hydroxyapatite

Bacteria residing on your teeth metabolize sugar into
lactic acid, which lowers the pH below 5 at the surface of
a tooth. Acid dissolves the hydroxyapatite, thereby
crating tooth decay.
6
11-1 Amino Acids Are Polyprotic



Carbonic acid from limestone, phosphoric acid from
teeth, and amino acids from proteins are all
polyprotic acids.
Polyprotic acids have more than one acidic proton.
Amino acids from which proteins are built have an
acidic carboxylic acid group, a basic amino group,
and a variable substituent designated R:
7

Amino acid is a kind of inner salt.

The resulting structure, with positive and
negative sites, is called a zwitterion.

A acid dissociation constants of the 20 common
amino acids are given in Table 11-1.
8
Table 11-1
9
1. Nonpolar side chain: alanine, glycine,
leucine, isoleucine, valine, proline,
methionine, and phenylalanine.
2. Polar but uncharged side chain: serine,
glutamine, threonine, cysteine,
asparagine, tyrosine, and tryptophan.
3. Polar charged side chain: aspartic acid,
glutamic acid, lysine, arginine, and
histidine.
10
Isoelectric point (pI)

One molecule can not move in the electric field at certain
pH (the net charge of that molecule is 0 at this pH), and we
call this pH as the isoelectic point (pI) of that molecule.
pI = (pKa1 + pKa2)/2 in cysteine
Cysteine has three acidic protons

What’s the pI of aspartic acid and lysine?


11
12

A diprotic acid has two acid dissociation constants,
designated Ka1 and Ka2 (where Ka1 ＞ Ka2)：

The two base association constants are
designated Kb1 and Kb2 (Kb1 ＞ Kb2)：
13
Relation Between Ka and Kb

Ka1 (or K1) refers to the acidic species with the
most protons, and Kb1 refers to the basic species
with no acidic protons.
14
Box 11-1
Carbon dioxide in the air and ocean

Carbon dioxide is the principal greenhouse gas in the
atmosphere, with a significant role in regulating the
temperature of earth’s surface.

Earth absorbs sunlight and then radiates energy away
by discharging infrared radiation to space.

A greenhouse gas is so named because it absorbs
infrared radiation emitted from the ground and
reradiates back to the ground.
15
Carbon dioxide in the air and ocean
(cont.)

The balance between sunlight absorbed and radiation
to space determines the surface temperature.

The ocean is a major reservoir for CO2. When the
concentration of dissolved CO2 goes up, the pH of the
ocean goes down.

CO2 could be trapped in ice cores in Antarctica and
the measurement could track the CO2 concentration
in history.
16
17
11-2 Finding the pH in Diprotic Systems
A molecule that can both donate and accept a
proton is said to be amphiprotic..
.
.
.
18

The pH of a solution of the intermediate form of a
diprotic acid is approximately midway between pK1 and
pK2, almost independent of concentration. (p. 243-248).
Example: pH of Intermediate Form of a Diprotic Acid
 Potassium hydrogen phthalate, KHP, is a salt of the
intermediate form of phthalic acid. Calculate the pH of
0.10 M KHP and of 0.010 M KHP.
SOLUTION:
the pH of potassium hydrogen phthalate is estimated
as 1/2(pK1+pK2)=4.18, regardless of concentration.
19
11-3 Which Is the Principal Species?


What is the principal form of benzoic acid at pH 8? (A-)
What is the principal form of benzoic acid at pH 3? (HA)
20
Which is the principal species?
pH < pKa

major species: HA
pH > pKa
major species: AIn monoprotic acid, the major species can be
defined by pKa.
21
Which is the principal species in
diprotic acid?
Slide 22 of 119
Which is the principal species in
triprotic acid?

If the charge of H3A is +1 (or +2), what is the pI?
23
Figure 11-1
Figure 11-1 Fractional composition diagram
for fumaric acid (trans-butenedioic acid).
24
Example:Principal Species-Which One and How Much?

What is the predominated form of ammonia in a solution
at pH 7.0? Approximately what fraction is in this form?
SOLUTION:

At pH=9.24, [NH4＋] = [NH3]. Below pH 9.24, NH4＋ will be
the predominant form. Because pH = 7.0 is about 2 pH
units below pKa, the quotient [NH3]/[NH4＋] will be about
1:100. Approximately 99% is in the form NH4＋.
25
26
Example :Principal Species in a Polyprotic system
27
The ammonium group next to the carboxyl group is
more acidic than the substituent ammonium group at
the right. What is the principal form of arginine at pH
10.0? Approximately what fraction is in this form?
What is the second most abundant form at this pH?
SOLUTION :


The predominant species between pK2 = 8.99 and
pK3=12.1 is HA. At pK2, [H2A＋] = [HA]. At pK3, [HA] =
[A－]. Because pH 10.0 is about one pH unit higher
than pK2, we can say that [HA]/[H2A＋]～10：1. About
90% of arginine is in the form HA. The second most
abundant species is H2A＋ at ~10%.
28
11-4 Titrations in Polyprotic Systems

All Henderson-Hasselbalch equation are always
true for a solution at equilibrium.

The pK1 and pK2 play important roles to show the
character of this diprotic acid.
29
Figure 11-2
Figure 11-2 shows
calculated curves for 50.0
mL each of three different
0.020 0 M diprotic acids,
H2A, titrated with 0.100 M
OH－.
Figure 11-2 Calculated titration curves
for three different diprotic acids, H2A.
30
Proteins are polymers made of amino
acids
Therefore, proteins are polyprotic acids and bases.
32
The End
33
Exploring Chemical Analysis
Daniel C. Harris (Fourth Edition)
12
A Deeper Look at Chemical
Equilibrium
國防醫學院
生化學科
王明芳老師
2012-3-29
34
Outline
 The effect of ionic strength
on solubility of salts
 Activity coefficients
35
12-1 Chemical equilibrium in the
environment
 Because of the acid drainage from abandoned coal
mines, part of the north branch of the Potomac River
is lifeless.
 As the river passes a paper mill and wastewater
treatment increase the pH from 4.5 to 7.2.
CaCO3(s) + CO2(aq) + H2O(l)
Calcium carbonate
Ca2+(aq) + 2HCO3-(aq)
Dissolved calcium bicarbonate
HCO3-(aq) + H+(aq) neutralization CO2(g) + H2O(l)
Acid from river
36
The effect of ionic strength on
solubility of salts
37

When slightly soluble lead (II) iodide dissolves in
pure water, many species are formed.
Effect of KNO3 on the solubility of
PbI2
 When KNO3 is added to the saturated PbI2 solution,
the total concentration of dissolved iodine increases.
 The dissolved iodine includes free iodide and the
iodide attached to lead.
 Why does solubility increase when salts are added
to the solution?
39
The effect of ionic strength on
solubility of salts
 The I- ion is surrounded by cations (K+, Pb2+) and
anions (NO3-, I-) in the solution.
 The typical anion (I-) has more cations than anions
near it.
 We call this region the ionic atmosphere.
 The net charge in the atmosphere is less than the
charge of the anion at the center.
 The ionic atmosphere attenuates (decrease) the
attraction between ions in solutions.
41
The effect of ionic strength on
solubility of salts (cont.)
 The higher the concentration of ions in a solution, the
higher the charge in the ionic atmosphere.
 Each ion-plus-atmosphere contains less net charge
and there is less attraction between any particular
cation and anion.
 The greater the ionic strength of the solution, the
greater the charge in each ionic atmosphere.
43
The effect of ionic strength on
solubility of salts (cont.)
 What is ionic strength? Ionic strength, m, is a measure
of the total concentration of ions in solution. The more
highly charged an ion, the more it is counted.
m = (c1z12 + c2z22 + c3z32 + …)/2 = 1/2 S cizi2
Where ci is the concentration of the ith
species and zi is its charge.
44
The effect of ionic strength on
solubility of salts (cont.)

Each of the following reactions is driven to the
right if KNO3 is added.
Fe(SCN) 2+
Fe 3+ + SCN-
45
Example: Calculation of Ion Strength
Find the ionic strength of (a) 0.10 M NaNO3; (b) 0.010 M
Na2SO4; and (c) 0.020 M KBr plus 0.010 M Na2SO4.
SOLUTION:
46
Activity coefficients of ions
47
12.2 Activity coefficients
aA + bB
cC + dD
K=
[C]c[D]d
[A]a[B]b
Activity of C: Ac = [C] gC,
gC is the activity coefficient of C
Therefore, we can rewrite the constant:
[C]c gCc [D]d gDd
ACc ADd
K= a b =
[A]a gAa [B]b gBb
AA AB
Ksp = (A Pb2+) (A I-)2 = [Pb2+]gPb2+ • [I-]2gI-2
48
Extended Debye-Hückel equation
2 m
-0.51
z
Log g =
1+ (a m / 305
)
 g is the activity coefficient of an ion of charge z
and size a (picometers, pm) in an aqueous solution
of ionic strength m.
 The equation works fairly well for m ≤ 0.1 M
49
Extended Debye-Hückel equation
(cont.)
 The size a is the effective hydrated
radius of the ion and its tightly
bound sheath of water molecules.
50
Cations attract the negatively charged oxygen
atom of H2O and anions attract the positively
charged H atoms.
51
Extended Debye-Hückel
equation (cont.)
 Ionic and hydrated radii of fluoride
and iodide.
 The smaller F- ion binds water
molecules more tightly than does I-,
so F- has the larger hydrated radius.
52
Extended Debye-Hückel
equation (cont.)
The smaller Li+ ion binds water
molecules more tightly than does
the larger K+ ion, so Li+ has the
larger hydrated diameter
53
Effect of ionic strength, ion charge, and ion
size on the activity coefficient
 As ionic strength increases, the activity coefficient
decreases.
 As the charge of the ion increases, the departure of its
activity from unity increases.
 The smaller the hydrated radius of the ion, the more
important activity effects become.
54
Activity coefficients for differently charged ions with a constant
Hydrated radius of 500 pm. At zero ionic strength, g = 1
One easy way to obtain activity
coefficients
 We may obtain the activity coefficients by using the
Extended Deby-Hückel equation. However, this
method could be very complicated and time
consuming.
 We may utilize Table 12-1 (p. 269) to obtain the
activity coefficients. This method is very easy.
 If the activity coefficient does not exist on the table,
what would you do?
56
57
Welcome to the real world!
 Activity coefficients of nonionic compound: Neutral
molecules (benzene, etc.) have no ionic atmosphere
because they don’t have charge. Therefore, the
activity of a neutral molecule will be assumed to be
equal to its concentration.
 The real definition of pH:
pH = - log AH+ = - log [H+] gH+
58
High ionic strength
 In fact, above an ionic strength of approximately 1 M,
activity coefficients of most ions increases.
 In concentrated solutions, the ‘solvent’ is no longer
just H2O.
59
Example: Using Table 12-1

Find the activity coefficient of Mg2＋ in a solution of
3.3 mM Mg(NO3)2.
SOLUTION:
.
In table 12-1, Mg2+ has a size of 800 pm . When m = 0.010
M, g = 0.69
61
Example: Interpolating Activity Coefficients

Calculate the activity coefficient of H＋ when μ =
0.025 M.
SOLUTION:
Another Solution：
62
Example: A Better Estimate of the Solubility of PbI2

From the solubility product alone, you estimated in Ask
Yourself 12-A that the concentration of dissolved iodine in a
saturated solution of PbI2 is 2.5 mM.

The observed concentration of dissolved iodine in the absence
of KNO3 in Figure 12-1 in 3.8 mM, which is 50% higher than
the predicted concentration of I－ of 2.5 mM. The Pb2＋ and I－
ions increase the ionic strength of the solution and therefore
increase the solubility of PbI2. Use activity coefficients to
estimate the increased solubility.
63
SOLUTION:
 .
64
The End
65