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Ionic Equilibria
Compiled by
Dr. L. Coo
Acids and Bases
1. Arrhenius Theory
(Svante Arrhenius,1884)
Acid = a substance that contains hydrogenand produces H+ in aqueous soln.
Base = a substance that contains the OH(hydroxyl) group and produces hydroxideions (OH-) in aq. soln.
(explains rxns bet protic acids and metal hydroxides)
Neutralization rxn:
H+(aq) + OH-(aq) H2O(l)
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2. Bronsted-Lowry Theory
(J.N. Bronsted and T.M. Lowry, 1923)
Acid = a substance that donates a proton, H+
(proton donor)
Base = a substance that accepts a proton
(proton acceptor)
An acid-base rxn is the transfer of a proton
from an acid to a base.
p. 350
B-L acid-base rxns can be described in terms of
conjugate acid-base pairs
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1) H2O + HF H3O+ + F-
base1 acid 2 acid1 base2
2) NH3 + H2O NH4+ + OH-
base1 acid 2 acid1 base2
Acid Conj. BaseHCl __________
H2CO3 __________
HPO42- __________
HSO3- __________
NH4+ __________
CH3COOH __________
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Base Conj. Acid
NH3 __________SO4
2- __________
SO2- __________
PH3 __________
I- __________
3. Lewis Theory
(Gilbert N. Lewis, 1923)
Acid = any species that can accept a share
in an electron pair.
Base = any species that can make available,
or donate, a share in an electron pair
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Autoionization of water
H2O() H+(aq) + OH-(aq)
ion product constant:
*Kw = [H+][OH-]
= 1.0 x 10-14
at 25o
C
*valid for aq solns and pure water
[H+] = [OH-] soln is neutral
[H+] = [OH-] = Kw = 1.0 x 10-7 M
[H+] > [OH-] soln is acidic
[H+] < [OH-] soln is basic
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A. Indicate whether soln is acidic, basic or
neutral
1. [H+] = 2.4 x 10-5 M
2. [OH-] = 3.0 x 10-9 M
B. Calculate the [OH-] when
[H+] = 2.45 x 10-6 M
H+ = a simple proton with no surroundingvalence electron
= small positively charged particle
interacts strongly with the lone pairelectrons of water molecules to formhydrated ion
H
H+ + :O-H H3O+
H+
and H3O+
: interchangeably used torepresent the hydrated proton
2H2O() H3O+
(aq) + OH-(aq)
Kw = [H3O+][OH-]
= 1.0 x 10-14 at 25oC
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For convenience in representing small
concentrations of H+ or OH-, the
p function was introduced:
pH = -log [H+]
pOH = -log [OH-]
pCa = -log [ __ ]
pF = -log [ __ ]
pKw = 14 = pH + pOH
pH value
Acidic < 7.00
Basic > 7.00
Neutral = 7.00
Strong Acid-strong electrolyte, completely ionizes insoln.
-readily gives up proton
binary ternary
HCl HNO3HBr H2SO4HI HClO4
HClO3 (uncommon)
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Strong Base-strong electrolyte, completely ionizes in soln.
-readily accepts proton
1. Ionic hydroxides of alkali metals:
Most common : NaOH, KOH
Uncommon: LiOH, RbOH, CsOH
2. Alkaline earth metals : except for Be
3. Oxide ion, O2-: NaO, CaO
4. Ionic hydrides and nitrides: N3-, H-
O2-(aq) + H2O() 2OH-(aq)
H-(aq) + H2O() H2(g) + OH-(aq)
N3-(aq) + 3H2O() NH3(aq) + 3OH-(aq)
Sr(OH)2 is a strong base, so it is 100%dissociated.
Sr(OH)2(aq) Sr2+
(aq) + 2OH
(aq)
0.0105 M 0.0105 M 2(0.0105) M
So [Sr2+] = 0.0105 Mand [OH-] =0.0210 M
(the strong base is the only source of [OH-])
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HClO3 is a strong acid, so it is 100% ionized.
HClO3(aq) + H2O(l) H3O+(aq) + ClO3(aq)
0.0105 M 0.0105 M 0.0105 M
So [H3O+] = 0.0105 Mand
[ClO3] = 0.0105 M
(the strong acid is the only source of [H+])
Strong acid Ka
HCl 103.9
HBr 105.8
HI 1010.4
HNO3 101.4
1. What is the pH of a 0.040 M HClO4?
2. pH of HNO3 is 3.45, what is [H+]?
3. What is pH of 0.011 M Ca(OH)2?
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Weak Acid/Base
= partially ionizes in soln
The extent to which the weak acid ionizes is
expressed through the equilibrium
constant for the ionization reaction.
( the strength of the acid)
Ka, acid dissociation constant
HA(aq) H+
(aq) + A-(aq)
Ka = [H+][A-]
[HA]
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Weak Bases
Two categories of weak bases
1. Neutral substance: contains an atom with a
lone pair of electrons that can serve as a
proton acceptor.
Ex: Nitrogen-containing cpds
2. Anions of weak acids
Ex: ClO- (source is a salt, NaClO)
ClO-(aq) + H2O HClO(aq) + OH-(aq) Kb = 3.3x10
-7
Kb, base dissociation constant
B(aq) + H2O() BH+
(aq) + OH-(aq)
Kb = [BH+][OH-]
[B]
(Kb always refers to the equilibrium in which abase reacts with H2O to form the conj acid andOH-)
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Monofunctional (HCl, HOAc) : Ka
(NaOH, NH3) : Kb
Polyfunctional (H2SO4, H3PO4): Ka1, Ka2
(Kb1, Kb2
Extent of dissociation is measured thru themagnitude of its Ka (dissociation constant ofthe acid) or its Kb (dissociation constant ofthe base).
What is the pH of a 0.25 M HF?
HF H+(aq) + F-(aq)
I 0.25M 0 0
C - x x x
E 0.25-x x x
Ka = [H+][F-]
HF
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Ka = 7.2 x 10-4
7.2 x 10-4 = X2
0.25 x
x2 = 1.8 x 10-4 7.2 x 10-4x
x2 + 7.2 x 10-4x 1.8 x 10-4 = 0
Solving quadratic equations:
For a general quadratic equation
y = x2 + bx + c, x2 + bx + c = 0
the roots of the equation (i.e., the value of x when
the equation is zero, y = 0), the real value (+) =0.013 M
[H+] = [F-] = 0.013 M
% ionization[H+] x 100
[HF]i
0.013 M x 100 = 5.2 %
0.25 M
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What is the pH of a 0.10 M HOAc?
HOAc H+(aq) + OAc-(aq)
I 0.10M 0 0
C - x x x
E 0.10-x x x
Ka = [H+][OAc-]
[HOAc]
Ka = 1.8 x 10-5
1.8 x 10-5 = x2
0.100 x
Simplify eqn, the small value of Ka indicatesthat the acid is only slightly ionized, suchthat x
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NH3(aq) + H2O(l) NH4+
(aq) + OH
(aq)
0.25 x M x M x M
Kb = [NH4+][OH- ] (x)2
[NH3] = (0.25 -x) = 1.8 x 105
assumex
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Appln: Calcn of Ka from % ionization
In 0.0100 M soln HOAc is 4.2% ionized.Calc its Ka.
Polyprotic Acid
= acid that contains more than one ionizable
hydogen atom per formula unit
H2SO4
H3
PO4
H2C2O4
Stepwise ionization of
polyprotic acid
H2C2O4 H+ + HC2O4
- Ka1 = 5.9 x 10-2
HC2O4- H+ + C2O4
-2 Ka2 = 6.4 x 10-5
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Calculate all species present in
0.100 M H2C2O4
H2C2O4 H+ + HC2O4- Ka1 = 5.9 x 10-2
I 0.100M 0 0
C -x +x +x
E 0.100-x x x
5.9 x 10-2 = x2
0.100-x
Solve by quadratic eqn, Ka1 is too large to
neglect x relative to 0.100 M
x2 + (5.9 x 10-2)x - 5.9 x 10-3 = 0
x = [H+]1st = [HC2O4-] = 0.053 M
HC2O4- H+ + C2O4
-2 Ka2 = 6.4 x 10-5
0.053-y 0.053 +y y
Ka2 = (0.053 + y)(y)
0.053 -y
6.4 x 10
-5
= (0.053 + y)(y)0.053 y
y > [H+]2nd = 6.4 x 10
-5 M
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Relationship of Ka and Kb
HOAc H+(aq) + OAc-(aq) Ka = 1.8 x 10-5
OAc- + H2O HOAc + OH- Kb = ?
H2O H+ + OH- Kw = Ka x Kb
Therefore KbOAc- = Kw
KaHOAc
Pyridine, C5H5N has a Kb value of 1.5 x 10-9.
What is the Ka of C5H5NH+ ?
If Ka for HNO2 is 5.1 x 10-4, what is Kb for
NO2-?
Pyridine : used as pesticide, toxic by ingestion andinhalation (tolerance is 5 ppm in air), explosivein air at 1.8 2.4%
Acid-Base Properties of Salt Solns
Salt : ionic cpd with a cation other than H+
and an anion other than OH- or O2-
Dilute solns of soluble salts are assumed to
completely ionize in water
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The acid/base properties of salt solns
are due to the behavior of their cations
and anionsAqueous solns produced from salts of:
1. a strong acid and a strong base is neutral
2. a strong acid and a weak base is acidic
3. a weak acid and a strong base is basic
4. a weak acid and a weak base will
depend on the extent of hydrolysis of
each ion.
1.Salts of strong acids and strong bases.
Ex:
KNO3salt of nitric acid and potassium hydroxide
CaSO4salt of sulfuric acid and calcium hydroxide
2. Salts of weak acids and strong bases.Ex:
NaF
salt of hydrofluoric acid and sodium hydroxide
Ba(NO2)2
salt of nitrous acid and barium hydroxide
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3. Salts of strong acids and weak bases.
Ex:(NH4)2SO4
salt of sulfuric acid and aqueous ammonia
C5H5NHCl pyridiniumCl
salt of hydrochloric acid and pyridine
Al(NO3)3 (Al3+ + H2O Al(OH)
2+ + H+)
4. Salts of weak acids and weak bases.
Ex:
(NH4)2CO3salt of carbonic acid and aqueous ammonia
CH3NH3F methylammoniumF
salt of hydrofluoric acid and methylamine
pH depends on the extent of hydrolysis of each ion
1. Which is more acidic?
a) NaNO3, Fe(NO3)3
b) KBr, KBrO
c) CH3NH3Cl, BaCl
d) NH4NO2, NH4NO3
2. List the ff solns in order of increasing pH
0.10M Co(ClO4)2, RbCN, Sr(NO3)2, KOAc
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Predict whether the salt of Na2HPO4 will
form an acidic or basic soln on dissolutionin water.
Common Ion Effect
HOAc H+ + OAc-
What happens if a salt, NaOAc is added to
the HOAc soln?
In general, the dissociation of a weakelectrolyte is decreased by adding to the
soln a strong electrolyte that has an ion in
common with the weak electrolyte.
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1. What is the pH of a soln made by adding
0.30 mol of HOAc and 0.30 mole of
NaOAc to enough water to make 1.0 L ofsoln?
2. Calculate the [F-] and pH of a soln
containing 0.10 mol HCl and 0.20 mol HF
in 1.0 L.
Buffer
A soln that resists change in pH when a
smallamount of an acid or base is added
or when the soln is diluted.
Useful in maintaining the pH for a reaction
at an optimum value
Consists of a mixture of a weak acid andits conj base or a weak base and its conj
acid at predetermined concs or ratios.
Which of the following solns are buffer
systems?
a) KH2PO4/H3PO4
b) NaClO4/HClO4
c) C5H5N/C5H5NHCl
(pyridine, Kb= 1.7x10-9)
d) KF/HF
e) KBr/HBr
f) Na2CO3/NaHCO3
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Henderson-Hasselbalch Eqn
pH = pKa + log [conj base]
[acid]
pOH = pKb + log [conj acid]
[base]
Buffer capacity,
Measure of how well a buffered soln resists
changes in pH when strong acid or base is
added
= no of moles of acid(base)/pH unitchange
highest when ?
Buffer Range= 2 pH range
= the maximum range to which a buffer soln
should be exposed
effective buffer range
HOAc/NaOAc 3.7 5.7
NH3/NH4Cl 8.3 10.3
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A weak acid with Ka = 1.0 x 10-5 was usedto prepare a buffer.
What is the pH of a buffer that is 0.12 M in
benzoic acid and 0.20 M in sodium
benzoate? Ka = 6.5 x 10-5
1. Calculate the pH of a buffer soln preparedby dissolving 12.51 g NaB (121.135) and4.55 g HB (157.596) in 1.00 L water.
Ka,HB = 8.5 x 10-9
2. If we add 12.0 mL of 1.00 M HCl to the
above soln, what will be its new pH?
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Describe how you would prepare a
phosphate buffer with a pH of about 7.40
Given:H3PO4 H2PO4
- Ka1= 7.5 x 10-3
H2PO4-HPO4
2- Ka2 = 6.2 x 10-8
HPO42-PO4
3- Ka3 = 4.8 x10-13
Other buffer soln preparation
Buffers can also be prepared by adding
carefully measured quantity of strong base
to a soln of a weak acid (or strong acid to
a weak base)
AS LONG AS THE WEAK ACID OR WEAK
BASE REMAINS IN EXCESS.
Examples:
1. Na+OH- + HOAc OAc- + H2O
I 0.1 0.2 0
C -0.1 -0.1 + 0.1
E 0 0.1 0.1