Acids and Bases

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Acids and Bases
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Learning Objectives
Define an acid and a base, and give an example of
each.
Recognize the structure of the functional groups
carboxylic acids and amines, and show with a
chemical reaction why each is either an acid or
base.
Solve problems using the ion product of water
equation

HOH-
1 x 10-14 M2
Solve problems using the definition of pH
pH
-log H

Use the value of a pKa to predict ionizatiion
states at a given pH.
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Learning Objectives
Use the Henderson-Hasselbalch equation to
calculate the pH of a solution.
Describe the bicarbonate buffer system with
respect to the production of lactic acid and
ammonia, and how the rate of breathing changes in
response to changes in blood pH.
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Acids and bases were first recognized by simple
properties acids have a sour taste bases are
bitter acids and bases change the color of
certain dyes
called indicators
From the color changes of indicators, it was
observed that acids and bases can reverse the
effects of each other.
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The Swedish chemist Arrhenius proposed the first
successful concept of acids and bases. He
defined acids and bases in terms of the effect
these substances have on water.
Acids are substances that increase the
concentration of H ion in water. Bases are
substances that increase the concentration of OH-
ion in water.
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This explanation for acids and bases is
straightforward for the strong acids and bases.
When HCla strong aciddissolves in water, it
dissociates completely into hydrogen ions and
chloride ions
HCl
H
Cl-

This increases the concentration of H, and thus
HCl is an acid.
When NaOHa strong basedissolves in water, it
dissociates completely into Na ions and OH- ions
NaOH
Na

OH-
This increases the concentration of OH-, and thus
NaOH is a base.
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Weak acids and weak bases are very important from
a biochemical point of view.
Weak acids and weak bases require a slightly
different interpretation. However, weak
acidswhen dissolved in water.will increase the
concentration of H. And weak bases.when
dissolved in water.will increase the
concentration of OH ions.
To understand weak acids and weak bases, the
dissociation of water must be understood.
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Dissociation of Water Molecules
Water dissociates very slightly into OH ions and
protons, H.
H2O
OH-
H

H

H2O
H3O
Free protons do not exist in solution. Hydrogen
ions formed in water are immediately hydrated to
hydronium ions. Hydrogen bonding between water
molecules makes hydration of dissociated protons
virtually instantaneous.
H3O
Hydronium ion
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Formation of the hydronium ion
-1
d-
O
O
d
H
H
H
O
1
d
H
H
H
d-
O
d
d
The oxygen in water has two unshared pairs of
electrons. It can accept a H ionoxygen uses
one pair of unshared electrons to form an
additional single bond with a third hydrogen.
H
H
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The carboxylic acid, RCOOH
Carboxylic acids are weak acids they do not
completely dissociate in water like strong acids.
O
O


C
OH
R
C
O-
R
H

The dissociation of a carboxylic acid in water
yields a carboxylate ion and a proton, H. This
fits the definition of an acid since by
dissociating a proton, the concentration of H
ions increases.
The carboxylic acid also fits a better definition
of an acid an acid is a compound that donates a
proton.
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The Danish chemist Johannes Bronsted and,
independently, the British chemist Thomas Lowry
pointed out that acid-base reactions can be seen
as proton-transfer reactions. As such, acids and
bases can be defined in terms of proton-transfer.
An acid may be defined as a proton donor (a H
donor), and a base defined as a proton acceptor
(a H acceptor).
How does a carboxylic acid measure up to this
definition?
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O
O


C
OH
R
H2O
C
O-
R
H3O

O
O


C
O
R
C
O-
R
H
1
O
d
H
H
H
d-
O
The carboxylic acid donates a proton to a water
molecule upon dissociation in aqueous solution.
Thus, a carboxylic group is an acid.
d
d
H
H
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Amines are weak bases
The unshared electron pair on nitrogen in an
amine can accept a proton (H). As such, an
amine is a proton acceptor, and is classified as
a base.
R-NH2

H
R-NH3
Where does the proton come from? When an amine
is dissolved in water, the proton comes from
water. And leaves behind a OH- ion. Thus, an
amine dissolved in water increases the
concentration of OH- ions. This agrees with the
original definition of a base, and with the
concept that a base is a proton acceptor.
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Acceptance of a proton by an amine from a water
molecule
R-NH2 H2O
R-NH3 OH-
-1
d-
O
O
d
H
H
H
H
1
N
R
d
H
H
d
d-
N
H
R
A base is a proton acceptor.
d
H
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Water can act as either an acid or a base.
Sometimes water acts as an acid and donates a
proton
R-NH2 H2O
R-NH3 OH-
In this reaction, water donates a proton to the
amine. Thus, water isin this reaction.an acid.
The amine accepts the proton, and is the base.
In the reverse direction, the protonated amine
acts as an acid, and donates a proton to the OH
group which acts as a base a proton acceptor.
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Sometimes water acts as a base
O
O


C
OH
R
H2O
C
O-
R
H3O

In this reaction, water accepts a proton from the
carboxylate group. Thus, water is in this
reactiona base. The carboxylic acid donates the
proton, and is the acid.
In the reverse direction, the carboxylate ion
accepts a proton and is the base. The hydronium
ion donates a proton and is an acid.
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A carboxylic acid is a proton donor once the
proton has been dissociated, the carboxylate ion
becomes a proton acceptor.
O
O


C
OH
R
C
O-
R
carboxylate ion
carboxylic acid
A proton donor and its corresponding proton
acceptor make up a conjugate acid-base pair.
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The same situation occurs with amines an amine
is a proton acceptor but once it accepts a
proton, it becomes a proton donor.
R-NH2
R-NH3
proton acceptor a base
proton donor an acid
These two molecular species form a conjugate
acid-base pair.
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Dissociation of Water Molecules
Water dissociates into hydroxyl ions and protons
(hydronium ions). This is sometimes referred to
as self-ionization.
H2O
OH-
H

This is far from a complete dissociation. Water
only slightly dissociates.
The ionization of water can be expressed by an
equilibrium constant
HOH-
Keq

H2O
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The value for Keq is 1.8 x 10-16 M (determined by
electrical conductivity measurements).
HOH-

1.8 x 10-16 M
H2O
The concentration of pure water at 25 oC is 55.5
M.
H2O

55.5 M
HOH-

1.8 x 10-16 M
55.5 M

HOH-

55.5 M
X
1.8 x 10-16 M
1 x 10-14 M2
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HOH-
1 x 10-14 M2
The ion product of water, Kw, is defined as
HOH-
Kw


Kw
Therefore
1 x 10-14 M2
In pure water, H OH-, the solution is said
to be neutral, and the H is calculated to be 1
x 10-7 M.
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HOH-
1 x 10-14 M2
The ion product of water equation
If H OH-, then
H H 1 x 10-14 M2
H2 1 x 10-14 M2
H 1x 10-7 M
and OH- must therefore equal 1x 10-7 M.
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Problems
What is the concentration of H in a solution of
0.1 M NaOH?
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The pH scale designates the H and OH-
concentrations
The ion product of water is the basis for the pH
scale. It is a convenient means of designating
the concentration of H (and OH-) in any aqueous
solution in the range between 1.0 M H and 1.0 M
OH-.
The term pH is defined by the expression
pH
-log H

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Math Review
Exponents
The number 3 is called an exponent
10 x 10 x 10 103
10 is called the base
2 x 2 x 2 x 2 24
4 is the exponent, and the base is 2
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A logarithm is an exponent.
Here is a simple equation 102 100
The left side of this equation shows 10 as the
base and the exponent is 2. Obviously, 10 x 10
equals 100.
Here is this same relationship expressed in a
logarithmic equation
log10 100 2
The subscript 10 indicates the base.
This equation reads the logarithm (base 10) of
100 is 2
Remember logarithms are exponents
2 is the exponent
Of what? Of the base 10
102
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The logarithm (base 10) of some number x is
simply the exponent of 10 needed to equal x.
For the even powers of 10 (100, 1000, 10000, etc)
this is easy to see. However, logarithms exist
for all positive numbers, and these will be
decimals (2.35, 1.74, etc).
The term log always means to the base 10
these are called common logarithms.
The term ln always refers to the base e
these are called the natural logarithms.
Your calculator will have buttons labeled log
and ln for each type of logarithm.
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pH
-log H

This isagaina simple logarithmic equation.
The term log implies a base of 10.
pH is the exponent (of 10).
H is the hydrogen ion concentration expressed
as moles/liter or molar concentration (M).
If the H is 1 x 10-5 M, what is the pH?
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The pH of some aqueous fluids.
pH is always a positive number between 0 and 14
Blood pH is normally around pH 7.4.
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pH of body fluids
Fluid
pH
blood 7.4 interstitial fluid 7.4 cytoplasm
of cells 7.0 saliva 5.8-7.1 gastric
juice 1.6-1.8 pancreatic juice 7.5-8.8 intest
inal juice 6.3-8.0 urine 4.6-8.0 sweat
4.0-6.8
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Definition of pKa
The tendency of any acid (HA) to lose a proton
and form its conjugate base (A-) is defined by an
equilibrium constant for the reversible reaction
HA-
Keq

HA
H
A-

HA
Equilibrium constants for ionization reactions
are called ionization or dissociation constants,
and given the symbol Ka.
Analogous to pH,
HA-
Ka

HA
pKa

-log Ka
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The Henderson-Hasselbalch Equation
A-
pH
pKa
log


HA
A- is the molar concentration of the conjugate
base HA is the molar concentration of the
conjugate acid
CONJUGATE BASE
pH
pKa
log


CONJUGATE ACID
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O
O


C
OH
CH3
C
O-
H

CH3
acetic acid
acetate
pKa 4.76
conjugate base
conjugate acid
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Calculate the pH of a mixture of 0.1 M acetic
acid and 0.2 M sodium acetate. The pKa of acetic
acid is 4.76.
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When the pH equals the pKa, the concentration of
the conjugate acid equals the concentration of
the conjugate base.
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CH3COO-
8
7
CH3COOH
CH3COO-

6
5
4
pKa4.76
3
pH
2
1
CH3COOH
0
0
50
100
Percent titrated with base
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buffer a combination of substancesusually a
weak acid and its conjugate basethat act
together to prevent a drastic change in pH when
either another acid or base is added.
Three major buffer systems in vertebrates (1)
the bicarbonate buffer system (2) phosphate
system (3) proteins (the amino acid side chains
can act as proton donors or acceptors)
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pH optima of some enzymes
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Blood, lungs, and the bicarbonate buffer system