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Chapter 2: Aqueous Soln’
Properties of water (H2O)
very few inorganic compounds are liquids at earth’s temperature. The
physical and chemical characteristics of water unique to earth (70% of
the earth’s surface) which makes the existence of life possible.
H2O Structure
• O-H bond length: 0.958 Å
(1 Å=10-8 cm = 0.1 nm)
• Bond angle: 104.5̓
• Dipole moment: asymmetric of
charge across the molecule.
• Solvent ੝‫܄‬: very polar
• Effective VDW radius: 1.4-1.6 Å
• Dipole moment: 1.85 Debye unit
H-bondings in H2O
The hydrogen bonds generated by electrostatic nature of the molecule
hold molecules together
G-D-HG… GA
Unique properties of water because
of the H-bondings:
Density- 1.0
Latent heat of melting: 79.72 cal/g
Laten heat of evaporation: 540 cal/g
Specific heat: 1
most other substances-near 0.2
liquid NH3-1.23
liquid H2-3.4
Surface tension: highest except for
mercury
ణᗖ‫߄ޑ‬Ң‫ݤ‬:
D-H…AǴ‫ځ‬ύD-Hࣁ১ለ‫“ޑ܄‬donor group”Ǵ
‫ٯ‬ӵN-H‫܈‬O-H
Aࣁ১ᡵ‫“ޑ܄‬acceptor atom”Ǵ‫ڀ‬Ԗlone pair
electronsǴ‫ٯ‬ӵN‫܈‬O
H-bondૈໆ: ~20 KJ /mole
ణᗖऊౣ‫ڀ‬Ԗ10%‫ޑ‬Ӆሽᗖ੝‫܄‬.
H-bondߏࡋ: 1.4 Å
ణᗖ‫ޑ‬ຯᚆКӚձচηvan der Waalsъ৩ᕴ‫ک‬
อऊ0.5 Å.
H-bondings in Ice
every oxygen at the center of a
tetrahedron formed by 4 oxygen atoms;
as ice melts open lattice is disrupted.
Ice is much less dense than water. At 0
±C, bonds rupture and lattice
disorganization increase density
From 4 ±C on, increased energy leads to
increasing inter-atomic distances.
Ӣࣁ‫ڰ‬ᄊН(Ӈ)ࣁopen structure, Ѭ‫ޑ‬
ஏࡋ0.92 g/mlКНե(1.0 g/ml)
ӇૈӧН΢ੌ୏ǴΨӢԜѬૈፓ࿯ࢩࢬϷӦౚ‫ࡋྕޑ‬Ǵ೭ঁ౜ຝӧ
ғᄊ΢‫ڀ‬ख़ε‫ޑ‬ཀကǶ
H-bondings also persist in the liquid state
The heat of sublimation of ice at 0 ºC is 46.9 KJ/mole
kinetic energy of gaseous water: ~ 6 KJ/mole
41 KJ/mole required to disrupt the H-bonding force in ice.
Heat of fusion of ice: 6 KJ/mole
Liquid water is ~15% less hydrogen bonded than ice at 0 ºC .
ᡉҢНଯϣᆫΚ‫ޑ‬੝‫ٰ߯܄‬ԾణᗖǶ
Ice: 4 H-bonds per water molecule, water at 0 ºC: 4.4 H-bonds, Water at 10 ºC:
2.3 H-bonds per water molecule
This indicates tetrahedral like H-bonding environment but becomes less ordered
in liquid water.
Нϩηӧόӕ࣬ᄊਔ‫ޑ‬ᡂ୏ೲ౗
Ice: H-bond lifetime - about 10 microsec
Water: H-bond lifetime - about 10 psec
ӧНύ‫ޑ‬ణᗖᆛࢂ‫ס‬Ԕ‫ޑ‬Ǵҗ3-6ঁНऊ‫׎‬ԋᕉ‫ރ‬ణᗖᕉნǴԶЪᡂ୏ೲ౗ཱུଯǴऊ20 psecǶ
Liquid water is a rapidly fluctuating space-filling network of hydrogen bonded,
over a short distances, resembles that of ice.
Solvent Property of water
‫ނ‬፦ྋှ‫ࡓۓ‬: like dissolves like
Hydrophilic compound: ionic or polar in nature
Hydrophobic compound: nonpolar (eg: oil, CH4 etc)
Н‫܄ཱུޑ‬ଯǴ٬‫ځ‬ԋࣁཱུ‫܈܄‬ᚆη੝‫ނ܄‬፦‫๊ޑ‬٫ྋᏊǶ
Dielectric constantΔD:
The force to separate two opposite charged to infinite
distance. Thus, the dielectric constant of a solvent is a
measure of the ability to dissolve ionic compounds. The
D value of H2O is 78.5.
F
k
kq1q2
Dr 2
8.99 x109 J ˜ m ˜ C 2
The ions are solvated or hydrated in solvent.
Solvent properties of water
For ionic compound dissolved in water, the Ions are always
hydrated in water and carry around a "hydration shell“
Water forms H-bonds with polar solutes
A nonpolar solute "organizes"
water
The H-bond network of water
reorganizes to accommodate
the nonpolar solute
This is an increase in "order"
of water. ”a decrease in
ENTROPY”
Amphiphilic molecules
ӭኧғϯϩηӕਔ‫ڀ‬Ԗཱུ‫܄‬Ϸߚཱུ‫୔܄‬ǴӢԜΨӕਔ‫ڀ‬ԖᒃϷ౧Н‫ޑ܄‬ТࢤǶ
Also called "amphipathic"
Refers to molecules that
contain both polar and
nonpolar groups.‫ٯ‬ӵ: fatty
acidsǶ
Equivalently refer to
molecules that are attracted to
both polar (hydrophilic) and
nonpolar (hydrophobic)
environments
Micelle and bilayer
Water tends to hydrate the
hydrophilic portion of the
amphiphile, but it tends to exclude
its hydrophobic portion. Therefore,
Micelle are globular structure with
thousand amphiphiles aggregate in
ordered fashion as shown in the
figure.
The interactions stabilized a micelle
or bilayer are collectively described
hydrophobic forces or
hydrophobic interactions.
ϩη‫ޑ‬౧Н੝‫܄‬Ԗշ‫ܭ‬،‫ۓ‬၀ϩη‫่ޑ‬ᄬǴаϷ
suprastructure‫ޑ‬ಔӝǴ‫ٯ‬ӵmembraneǶ
Basics about Acids, Bases, and Buffer
Definition of acid or base
Arrhenius: substance donates protons and hydroxides ions
Bronsted & Lowry:
Acid: substance donates protons
Base: substance accepts protons
CH3COOH + H2O Æ CH3COO- + H3O+
NH3 + H2O Æ NH4+ + OH-
Lewis: (aqueous & non-aqueous soln’)
Acid: substance accept electron pair
Base: substance donate electron pair
The Lewis definition encompasses the Bronsted-Lowry definition: In the reaction of H+ and OH-,
H+ is a Lewis acid because it accepts an electron pair from the OH-. Since the OH- donates an
electron pair we call it a Lewis base.
As an example not described by the Bronsted-Lowry definition, Al3+ in water is a Lewis acid. It
reacts with water to form an aqua complex: the Al3+ accepts the electron-pair from water molecules.
In this example the water acts as a Lewis base.
Ka (dissociation constant)
In biologically relevant compounds various weak acids and bases are encountered, e.g. the acidic and
basic amino acids, nucleotides, phospholipids etc.
Weak acids and bases in solution do not fully dissociate and,therefore, there is an equilibrium
between the acid and its conjugate base. This equilibrium can be calculated and is termed the
equilibrium constant = Ka. This is also sometimes referred to as the dissociation constant as it pertains to
the dissociation of protons from acids and bases.
For weak acid dissociates in water
HA + H2O ÅÆ A- + H+
the equlibrium constant can be calculated
Ka
[H ][A - ]
[HA]
‫ۓ‬ကpKa = -logKa
log Ka
pKa
[H ][A - ]
log(
)
[HA]
Ka (dissociation constant)
log Ka
pKa
[H ][A-]
log(
)
[HA]
[A-]
log[H ] - log(
)
[HA]
‫ۓ‬ကpH = -log[H+]ǴӢԜ
pKa
[A-]
pH log(
)
[HA]
The smaller the pKa value the stronger is the acid. This is due to the
fact that the stronger an acid the more readily it will give up H+ and,
therefore, the value of [HA] in the above equation will be relatively
small.
The Henderson-Hasselbalch Equation
ຓჴthe dissociation constant of weak acid or base can be
determined experimentally.
pKa
[A-]
pH log(
)
[HA]
ÆÆ
pH
[A-]
pKa log(
)
[HA]
At the point of the dissociation where the concentration of the
conjugate base [A-] = to that of the acid [HA]
pH = pKa + log[1]
= pKa
җჴᡍෳ‫ۓ‬solute‫ޑ‬Kaॶ
pH
[A-]
pKa log(
)
[HA]
At pH below the pK, [HA] > [A-]
When [HA] = [A-]
At pH = pK, In other words, the
term pKa is that pH at which an
equivalent distribution of acid and
conjugate base (or base and
conjugate acid) exists in solution.
At pH above the pK, [HA] < [A-]
Buffer Solution )጗ፂྋన*
ϙሶࢂ጗ፂྋన(buffer solution)?
Consider a reaction HA + H2O ÅÆ A- + H3O+
‫ځ‬ύHA‫ک‬A- ᆀࣁconjugate acid‫ک‬base (Ӆ೫ለᡵჹ)
(ёаᇥ:ӵ݀΋ঁྋన֖ԖӅ೫ለᡵჹǴջԖёૈࢂ጗ፂྋన)
጗ፂྋన‫ޑ‬੝‫@܄‬
• Resist to change in pH of the solution. This phenomenon is
known as buffering.
• Solution must contain conjugated weak acid-base pair.
• Buffer Capacity: amount of acid or base added in order to
change pH of the solution by 1.
Titration* curve of buffer solution
CH3COOH + H2O Æ CH3COO- + H3O+
pH
[A-]
pKa log(
)
[HA]
When [A-]/[HA]>10, pH changes
rapidly.
The effective pH buffering zone of weak acid or base can
be calculated from the dissociation constant
pH = pKa ̈́ 1
Importance of biological pH buffering
The pH of blood is maintained in a narrow range around 7.4 (diagram). The primary
buffers in blood are hemoglobin in erythrocytes and bicarbonate ion (HCO3-) in the
plasma. The role played by the bicarbonate is as the followings
CO2 + H2O ÅÆ H2CO3
H2CO3 ÅÆ H+ + HCO3Henderson-Hasselbalch equation
pH = 6.1 + log [HCO3-/(0.03)(PCO2)]
Since CO2 is a dissolved gas.This factor has been shown to be approximately 0.03 times
the partial pressure of CO2 (PCO2).
• In most biochemical studies it is important to perform
experiments, that will consume H+ or OH- equivalents, in a solution
of a buffering agent that has a pKa near the pH optimum for the
experiment.
Transport of CO2 and the Bohr effect
Representation of the transport of CO2 from the tissues to the blood with delivery of O2 to the tissues. The opposite process
occurs when O2 is taken up from the alveoli of the lungs and the CO2 is expelled. All of the processes of the transport of
CO2 and O2 are not shown such as the formation and ionization of carbonic acid in the plasma. The latter is a major
mechanism for the transport of CO2 to the lungs, i.e. in the plasma as HCO3-. The H+ produced in the plasma by the
ionization of carbonic acid is buffered by phosphate (HPO42-) and by proteins. Additionally, some 15% of the CO2 is
transported from the tissues to the lungs as hemoglobin carbamate.
buffers mimic biological environment
CH2 OH
HOH2 C
C
NH 2
Tris
CH2 OH
(CH 2 )2 OH
N
CH 2 OH
C
(CH 2 )2 OH
CH 2 OH
Bis-Tris
CH 2 OH
N
N
SO 3 H
SO 3 H
PIPES
N
N
SO 3H
OH
HEPES
Titration of polyprotic substance
• Polyprotic:
Substances contain more than one acid-base group. eg. H3PO4 or H2CO3
are called polyprotic acid.
H3PO4 ÅÆ H2PO4- + H+
K1=7.08 X 10-3 pK1=2.15
H2PO4- ÅÆ HPO42- + H+
K2=1.51 X 10-7 pK2=6.82
HPO42- ÅÆ PO43- + H+
K3=4.17 X 10-13 pK3=12.38
Dissociation of more H+ becomes difficult as
pK values go up.
The difference between successive pK value
is > 4.0, it can be assumed that, at a given pH,
the members of the conjugate acid-base pair
with nearest pK value predominate.
Polyprotic acids with closely spaced pK’s
For substance where the pK’s of a polyprotic acid differ by less than ~2 pH, the
resulting ionization is the average ionization constant of the groups involved
(molecular ionization constants).
K1
[ H ]([ AH ] [ HA ])
[ HAH ]
KA KB
KA >> KB, then K1~KA
[ H ][ A2 ]
K2
[ AH ] [ HA ]
KC K D
( KC K D )
If KD >> KC, then K2~KC
Ampholytes, Polyampholytes, pI and Zwitterion
Ampholytes:
substances contain both acidic and basic groups(one acidic and one basic
group)
Polyampholytes:
If substances contain many acidic and basic groups.
Isoelectric point, pI:
Proteins contains many different amino acids some of which contain
ionizable side groups, both acidic and basic. Therefore, the pI is
described as the pH at which the effective net charge on a molecule is
zero.
(A molecule with a low pI would contain a predominance of acidic
groups, whereas a high pI indicates predominance of basic groups.)
Titration of Simple Amino Acids
Amino acids are more complicated than simple weak acids
since amino acids have at least 2 ionizing groups.
Glycin, for example, has both a carboxylic acid and an amino
group that can ionize
Zwitterion:
Substances bear charged groups
of opposite polarity.
Gly is still neutral because the +
charge is netualized by the charge.
pI of Glycin amino acid (titration diagram)
ÅÆ
ÅÆ
pK1=pH 2.35
pI
pI
pK2=pH 9.78
( pK i pK j )
2
( 2.35 9.78 )
2
6.01
Glycine is neutral at pH 6; it has no net charge here.
Complex titration curves of proteins
5 Complex Amino Acids are: glutamic
acid, aspartic acid, lysine, arginine and
histidine. Each of these 5 amino acids
has 3 ionizable groups and therefore, 3
pKs.