7/19/2011
Models of Solution
Chemistry- III
Acids and Bases
Chapter 8
Ionic Atmosphere Model : Revisiting
Ionic Strength
Ionic strength - a measure of total concentration
of ions in the solution
μ=
1
2
∑ (ci zi )
2
ci = concentrat ion of the ith species
zi = charge on the ith species
Ionic strength
• Increases as the number of ions increased
• Increasing ionic strength reduces attraction between positive
and negative ions
• Strongly influences the equilibrium of any chemical reaction in
aqueous solution
Chapter 8
Ionic Atmosphere Model : Revisiting
Ionic Strength
Ionic strength calculation - one half the sum of the
concentration of each ion multiplied by the charge of the
ion squared
μ=
1
2
∑ (ci zi )
2
ci = concentration of the ith species; zi = charge on the ith species
What is the ionic strength of
• 0.10 M NaNO3 and 0.20 M Na2SO4 ?
• 0.50 M KCl and 0.80 M CaBr2 ?
1
7/19/2011
Chapter 8
Calculation Ionic Strength
Electrolytes
1 formula unit of NaCl
dissociates to produce exactly
1 Na+ ion and 1 Cl– ion
1:1 electrolyte
C 2 - 1:2
CaBr
1 2 electrolyte
l
l
Na2SO4 - 2:1 electrolyte
Na3PO4 - 3:1 electrolyte
Electrolyte
Molarity
1:1
M
Ionic Strength
M
1:2
M
3M
1:3
M
6M
2:2
M
4M
Chapter 8
Activity Coefficients
To account for the effect of ionic strength the
concentrations are replaced with activities
AX =[X] × γX
γX = Activity coefficient of X - a measure of the
deviation from the “ideal behavior”
- at low
l ionic
i i strengthh γX approaches
h to unity
i
aA+ bC
cC + dD
We will express the chemical equilibrium
K=
[ AC ]c [ AD ]d
[ AA ]a [ AB ]b
=
[C]c γ C [D]d γ D
[A]a γ A [B]b γ B
Chapter 8
Activity Coefficients
For example,
CaSO4 (s)
Ca2+ (aq) + SO42- (aq)
2−
K SP = [ ACa 2+ ][ ASO 2- ] = [Ca 2+ ](γ Ca 2+ )[SO 4 ](γ SO 2− )
4
4
The activity
acti it coefficients are obtained from the ionic
atmosphere model using extended Debye-Hückel equation
that relates charge (z), size of the ionic atmosphere (α), and
the ionic strength (μ)
log γ =
− 0.51z 2 μ
at 25 °C
⎛
μ ⎞⎟
⎜1 + α
⎜
305 ⎟⎠
⎝
Units : z no unit needed
α in pm
μ in M
2
7/19/2011
Chapter 6
Acids and Bases
Chapter 6
Arrhenius Definitions
Definition 1: Arrhenius definitions of acids and bases
‰ Acids – produce H+ ions (or hydronium ions
H3O+)
H3O+ (aq) + A- (aq)
HA (s) + H2O (l)
‰ Bases – produce OH– ions
BOH
B+ + OH–
Exceptions: Some bases don’t have hydroxide ions!
e.g. Ammonia NH3
Chapter 6
Arrhenius Definitions
Arrhenius acid is a substance that produces H+ (H3O+) in
water
Arrhenius base is a substance that produces OH– in water
3
7/19/2011
Chapter 6
Brønsted – Lowry Definitions
Definition 2: Brønsted – Lowry
‰ Acids – proton donor
‰ Bases – proton acceptor
base
acid
conjugate
base
conjugate
acid
Chapter 6
Brønsted – Lowry Definitions
Conjugate pairs
‰ Acids – proton donor
‰ Bases – proton acceptor
HCO3– (aq)
( ) + H2O (l)
acid
base
H3O+ (aq)
( ) + CO32– (aq)
( )
conjugate
acid
conjugate
base
Chapter 6
Find Out Conjugate Acid-Base Pairs
1. HCl(aq) , Cl–(aq) and H2O(l) , H3O+(aq)
2. H2O(l) , OH–(aq) and NH3(aq) , NH4+(aq)
3. HCl(aq) , Cl–(aq) and NH3(aq) , NH4+(aq)
4. H3PO4(aq) , H2PO4–(aq) and H2O(l) , H3O+(aq)
5. H2PO4– (aq) , HPO42–(aq) and H2O(l) , H3O+(aq)
4
7/19/2011
Chapter 6
Find Out Conjugate Acid-Base Pairs
Chapter 6
Lewis Acid and Lewis Base Definitions
Definition 3: Lewis acid and Lewis
base
Lewis acid : A chemical species
that accepts an electron pair
Lewis base : A chemical species
that donates an electron pair
Chapter 6
Lewis Acid and Lewis Base Definitions
5
7/19/2011
Lewis Acid and Lewis Base Definitions
Dioxygen
C
N
Fe
Fe center
The heme group in
hemoglobin can
interact with O2
and
d CO
CO.
„ The Fe ion in
hemoglobin is a
Lewis acid
„ O2 and CO can act
as Lewis bases
„
Chapter 6
pH Scale
„
The pH scale is to express the
strength of acids and bases with
easily handled numbers. Instead
of using very small numbers, we
just use the negative power of 10
of the Molarity of the H+ (or OH–)
ion.
Under 7 = acid
7 = neutral
Over 7 = base
Chapter 6
Strength of Acids and Bases
‰
‰
‰
‰
Strength: The tendency to donate or accept a
proton.
Weak acid has weak proton-donating tendency;
while a strong acid has a strong proton-donating
tendency. The definition is true for bases: a
strong base has strong proton accepting
tendency.
One can only define strength in relative sense.
Strength measured relative to some reference, in
the present case, the solvent water.
Strength is judged on both the acid and base
involved in an acid-base reaction.
6
7/19/2011
Chapter 6
Strength of Acids and Bases
Example of a strong acid such as HCl.
‰
H+
Cl─
H+
Cl─
H+
Cl─
H+
Cl─
H+
Cl─
Chapter 6
Strength of Acids and Bases
Example of an weak acid such as acetic acid.
‰
CH3COO─
CH3COO─
H+
H+
CH3COO─
H+
CH3COO─
H+
CH3COO─
H+
Chapter 6
Strength of Acids and Bases
Quantitative Determination of Strength by
Dissociation Constant
Consider the first step of the dissociation of any acid
Ka
H+ (aq) + A- (aq)
HA (aq)
Ka =
‰
+
[H ][A - ]
[HA]
The larger Ka, the stronger the acid; the
smaller Ka, the weaker the acid.
7
7/19/2011
Chapter 6
Strength of Acids and Bases
Acids
Strength: The tendency to donate or accept a proton.
‰ pKa = - log Ka
‰ Thus, the larger pKa, the weaker the acid; the smaller
pKa, the stronger the acid.
acid
‰
‰
‰
‰
‰
Similarly,
pH = - log [H+]
pOH = - log [OH─]
pX = - log X
Chapter 6
Strength of Acids and Bases
Bases
‰ Strength: The tendency to accept a proton.
‰
A─ (aq)+ H2O (l)
Kb
Kb =
HA (aq) + OH─ (aq)
[HA ][OH − ]
[A − ]
‰
pKb = - log Kb
‰
The larger pKb, the weaker the base; the smaller pKb,
the stronger the base.
Chapter 6
Strength of Acids and Bases
Self ionization of water
‰
Neutrality is defined by the condition: [H+] = [OH-].
‰
Water is
i amphoteric
h
i
Kw
H2O (l)
H+ (aq) + OH─ (aq)
‰
Kw = [H+ ][OH− ]
8
7/19/2011
Chapter 6
Strength of Acids and Bases
Self ionization of water
Show that at neutral condition, pH=7.00
‰
H2O (l)
Kw
H+ (aq) + OH─ (aq)
K w = [H + ][OH − ] = 10 −14
Neutrality is defined by the condition: [H+] = [OH-]
i.e. [H+]2 = 1014
or, [H+] = 107
or, pH = -log[H+] = 7
Chapter 6
Strength of Acids and Bases
Conjugate Acids and Bases
‰
H+ + A─
H2O
‰
A- + H2O
‰
HA
H+ + OH─
Kb =
1/Ka
Kw
HA + OH─ Kb
Kw
Ka
The stronger an acid, the weaker is the conjugate base,
and vice versa
Chapter 6
Strength of Acids and Bases
Free Energy Change in an Acid Dissociation
Ka
HA (aq)
‰
H+ (aq) + A- (aq)
pKa = - log Ka
Gibb' s free energy change due to the dissociation,
ΔG o = − RT ln K a = −2.303RT log K a
where R and T are universal gas constant
( 8.314472 J·K -1 ·mol-1and temperature, respectively.
9
7/19/2011
Chapter 6
Strength of Acids and Bases
Free Energy Change in an Acid Dissociation
Ka
HA (aq)
H+ (aq) + A- (aq)
Calculate the free energy change associated to the
dissociations of the following acids with the given
pKas
acetic (ethanoic)
acid
CH3COOH
4.75
citric acid
C6H8O7
3.13
4.76
6.40
carbonic acid
H2CO3
6.37
hydrochloric acid*
HCl
-4
hydrogen sulfide
H2S
7.04
10.25
pH Calculations
„
Strong acid and strong base
Situation 1: Equivalence point
H+ + OHEquation: H2O
x
x
⇒x2 = Kw = 10-14
⇒x=10
10-7
⇒pH=7.00
pH Calculations
„
Weak acid and strong base
Situation 2: Before adding base
Equation: HA
H+ + AF–x
2
x
= Ka
F−x
x
x
⇒x=?
⇒ pH =?
Let Ka = 10-6.27 and we started from 0.020 M HA
10