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Chapter 16
Overview:
•Definitions
Arrhenius
Bronsted -- Conjugate Pairs
Hydronium Ion
•Relative Strengths
Strong/Weak acids and reactions
Strong/Weak bases and reactions
Ka’s and Kb’s
•pH and H2O Ionization
•Calculating Equilibrium Concentrations
•Hydrolysis
Acidic and Basic Salts
•Polyprotic Acids
•Molecular Structure/Bonding
•Lewis Acids and Bases
Acids
•sour
•corrosive
•reddens blue vegetable colors
•react with bases
Bases
•bitter
•soapy
•restores vegetable colors reddened by acids
•react with acids
Arrhenius Acid:
substance that releases or produces H+
HCl(aq) H+(aq)
+
Cl -(aq)
Arrhenius Base:
substance that releases or produces OH NaOH(aq) Na+(aq)
+ OH -(aq)
Bronsted Acid:
substance that donates a H+ to another
HNO3(aq) + H2O(l)
H3O+(aq)
+ NO3-(aq)
Bronsted Base:
substance that accepts a H+ from another
CO32-(aq) + H2O(l)
HCO3-(aq)
+ OH -(aq)
Water Dissociation:
2H2O
H3O+ +
OH -
Hydronium Ion -- because
bare protons are unlikely
d+
O H
H
d-
-
O H
O +H O H
H
H
H+ transfer
H
+
pH and Water Ionization:
2H2O(l)
H3O+(aq) +
OH -(aq)
K = [H3O+][OH -] = K [H2O]2 = [H3O+][OH -]
[H2O] 2
Kw = [H3O+][OH -] = 1.0 x 10 -14
(at 25C)
Reactant Favored
ion-product constant for water
pH = - log [H+] = - log [H3O+]
neutral solution:
2H2O
[H3O+] = [OH -]
H3O+
+
OH -
1.0 x 10 -14 = [H3O+][OH -] = [H3O+]2
[H3O+] = 1.0 x 10 -7 M
pH = - log (1.0 x 10 -7) = 7.0
non-neutral solutions:
acidic solutions:
[H+] > [OH-]
pH < 7.0
basic solutions:
[H+] < [OH-]
pH >
7.0
pH of Strong Acid and Base Solutions:
Calculate the pH of a solution containing 0.00100 M of
a strong acid such as HCl.
HCl(aq)
0.00100
H+(aq) + OH-(aq)
0.00100
0.00100
pH = - log (0.00100) = 3.00
but. . . .
What about H3O+ from water?
2 H2O
initial
change
equil.
H3O+
+
0.00100
OH 0
+x
+x
0.00100 + x
x
1 x 10 -14 = (0.00100 + x)(x)
x = H3O+ and OH- from disso. of water
Kw = [H3O+] [OH -]
0
x is very small
compared to
0.00100 & can be
neglected
1 x 10 -14 = (0.00100 + x)(x)
x = 1.00 x 10 -11 M = [OH -]
pOH = 11.0
[H+] [H3O+]
[H3O+] (1.0 x 10-11) = 1.0 x 10 -14
[H3O+] = 1.0 x 10 -3 M
the conc. of H+ in a
solution of a strong
acid is the conc. of
the strong acid
pH = 3.0
Note: pH + pOH = 14.0
Bottom Line:
[H+] = [H3O+] = [H5O2+] = [H9O4+] hydrated
hydrogen ions
neutral solution [H3O+] = [OH-] pH = 7
acidic solution [H3O+] > [OH-] pH < 7
basic solution [H3O+] < [OH-] pH > 7
the concentration of [H3O+] in a strong acid
is the concentration of the acid
Calculate the pH of a solution containing
0.010 M KOH.
KOH
0.010 M
K+(aq)
+
0.010 M
OH-(aq)
0.010 M
strong base, complete rxn, stoichiometric
1.0 x 10 -14 = [H+] (0.010 M)
[H+] = 1.0 x 10 -12 M
pH = 12.0
contribution of [H+] from
dissociation of H2O is
negligible
Calculate the [H+] and [OH -] in a
solution that has a pH = 8.60
- log [H+] = pH
- log [H+] = 8.60
log [H+] = -8.60
[H+] = anti log (-8.60)
[H+] = 1.8 x 10 -4 M
[OH -] = 5.6 x 10 -11 M
pH
14
[H3O+]
10 -14
[OH-]
1
pOH
0
Basic
ammonia
human blood
milk
7.0
10 -7
10 -7
7.0
vinegar, cola
0
1
10 -14
14
Neutral
Acidic
Measuring pH:
pH meter -- electrodes measure pH
– most precise method
acid-base indicators
– less precise but good when a pH meter is not
available
– substances which are differently colored at
different pH values
– litmus, phenolphthalein, thymol blue
Arrhenius Acid:
substance that releases or produces H+
HCl(aq) H+(aq)
+
Cl -(aq)
Arrhenius Base:
substance that releases or produces OH NaOH(aq) Na+(aq)
+ OH -(aq)
Bronsted Acid:
substance that donates a H+ to another
HNO3(aq) + H2O(l)
H3O+(aq)
+ NO3-(aq)
Bronsted Base:
substance that accepts a H+ from another
CO32-(aq) + H2O(l)
HCO3-(aq)
+ OH -(aq)
Examples:
H+
NH4+(aq) + H2O(l) H3O+(aq) + NH3(aq)
acid
base
H+
conjugate
acid
conjugate
base
conjugate pair
PO43-(aq) + H2O(l) HPO42-(aq) + OH -(aq)
base
acid
conjugate
acid
conjugate
base
conjugate pair
Some species can act as an acid or base:
H+
HCO3-(aq) + H2O(l)
acid
H3O+(aq) + CO32-(aq)
conjugate
acid
base
conjugate
base
H+
HCO3-(aq) + H2O(l)
base
acid
H2CO3(aq) + OH -(aq)
conjugate
acid
conjugate
base
HCO3- is an amphiprotic substance
You Must Know:
what an acid and a base is and how to
identify both
know definitions and properties
the reaction of an acid and a base with
water
how to identify acid, base, conjugate
acid and conjugate base
what the hydronium ion is and the
ionization reaction of water
Problems:
HX + H2O H3O+ + X A
B
CA
•What is the conjugate base of:
H2S
NH4+
NH3
H2O
OH -
HS NH3
NH2OH O2-
•What is the conjugate acid of:
NO3HNO3
HPO42H2PO4H3SO4+
H2SO4
CB
Relative Strengths:
HCl +
stronger A
H2O
stronger B
H3O+ + Cl -
weaker A
weaker B
equilibrium is a competition between the bases
H2O and Cl - -- the equilibrium will lie toward
the direction of the weaker acid and base
in this case, H2O is a stronger base than Cl - as
it competes much more effectively for the H+
HCN +
weaker A
H2O
weaker B
H3O+ + CN stronger A
H2O < CN -
stronger B
Which is the weaker acid:
H2S + CN -
HCN + HS -
HCN
HCO3- + SO42-
HSO4- + CO32-
HCO3-
HClO4 + H2O
H3O+ + ClO4-
H3O+
NH4+ +
H3O+ + NH3
NH4+
H 2O
The stronger the acid the weaker its conjugate base:
strongest acid
weakest acid
weakest
conjugate base
strongest
conjugate base
Given the following, which is the weaker
conjugate:
acids
bases
HCl > CH3CO2H
Cl -
HCN < H3PO4
CN -
H2SO4 > H2SO3
HSO4
NH3 > H2O
NH4
H
-
>
> H2PO4 -
2-
NH3
< HSO3<
+
HSO4 < CO3
-
< CH3CO2-
H3O+
H2SO4 > HCO3 H2
<
NH4+
Predicting Direction of Acid/Base Rxns.
CH3CO2H
stronger A
stronger B
+ CN -
weaker B
HCN + CH3CO2 -
weaker A
stronger A
HSO4-
+ NH3
stronger B
weaker A
NH4+
+ SO42-
weaker B
Strong Acids and Bases
•Strong Acids
HCl
HNO3
HClO4
HClO3
H2SO4
HBr
HI
HX + H2O
H3 O+ + X -
•Strong Bases
Grp I hydroxides
Grp II hydroxides
(except Be)
MOH
essentially complete rxns.
M+ + OH -
Weak Acids and Bases:
HX + H2O
H3O+ + X -
Ka = [H3O+][X -]
[HX]
B + H2O
< 1
BH+ + OH -
Kb = [BH+][OH -]
[B]
< 1
Weak Acids Can Be:
•cations
NH4+ or [Cu(H2O)6]2+
•anions
H2PO4- or HCO3-
•neutral
CH3CO2H or HCO2H
Weak Bases Can Be:
•anions
CO32- or
•neutral
NH3
or
CN (CH3)3N
You Must:
Be able to determine direction of rxn
based on acid or base relative strengths
Know the strong acids and strong bases
Be able to recognize weak acids and bases
Know water ionization rxn, constant and
expression
Know what pH is and how to calculate it
Know how to calculate equil. conc., pH,
pOH for weak acids and bases
Problem 1: A 0.015 M solution of an
unknown base has a pH of 10.09. Is it a strong
or weak base? What is the Kb, if it is weak?
Problem 2: What are the equil. conc. of
H3O+, acetate ion, acetic acid in a 0.20 M
aqueous solution of acetic acid, CH3CO2H?
Use the approximation whenever possible: When the initial
weak acid/base conc. > 100 *Ka/b neglect x when it is added
to or subtracted from the initial conc.
Answer 1: If this were a strong base then
[OH-] = 0.015 M and pOH = 1.8 and pH = 12.2.
B + H2O
HB+ + OH-
pH = 10.09 and pOH = 3. 91 [OH-] = 1.2 x 10-4
Kb = [HB+][OH-] = (1.2 x 10 -4)2
[B]
(0.015)
= 9.6 x 10 -7
Answer 2:
CH3CO2H + H2O
initial 0.20
change -x
equil. 0.20 -x
H3O+ + CH3CO20
+x
x
0
+x
x
Ka = 1.8 x 10 -5 = [H3O+][CH3CO2-]
[CH3CO2H]
Ka = 1.8 x 10 -5 = (x)(x)
(0.20 -x)
x2 = 3.6 x 10 -6
x = 1.9 x 10 -3
[CH3CO2-]=[H3O+] = 1.9 x 10 -3 M
[CH3CO2H] = 0.20 M
pH = 2.7
What is the percent ionization?
% ion = [H3O+] x 100
[CH3CO2H]
= 0.95 %
Polyprotic Acids and Bases:
H3PO4 + H2O
H3O+ + H2PO4- Ka1
H2PO4- + H2O
H3O+ + HPO42- Ka2
HPO42- + H2O
H3O+ + PO43-
H3PO4 + 3H2O
Ka3
3H3O+ + PO43--
Ka tot = Ka1 Ka2 Ka3
(7.5 x 10 -3)(6.2 x 10 -8)(3.6 x 10 -13) = 1.7 x 10 -22
Weak Bases:
Can be anions such as:
– CN-, HSO3-, SO32-, HCO3-, CO32-, etc.
Can be N-containing compounds, such as:
– NH3, (CH3)NH2, (CH3)2NH, (CH3)3N, etc.
React with water
– B- + H2O
HB
+
OH-
– B + H2O
HB+ +
OH-
or
Have base dissociation constants, Kb
Example:
NH3 + H2O
initial 0.20 M
change -x
equil. 0.20 -x
NH4+ + OH 0
+x
x
0
+x
x
Kb = 1.8 x 10 -5 = [NH4+][OH -]
[NH3]
Kb = 1.8 x 10 -5 = (x)(x)
(0.20 -x)
x2 = 3.6 x 10 -6
x = 1.9 x 10 -3
[NH4+]=[OH-] = 1.9 x 10 -3 M
[NH3] = 0.20 M
pOH = 2.7
pH = 12.3
CO3 -2 + H2O
OH - + HCO3--
Kb1
HCO3 -- + H2O
OH - + H2CO3
Kb2
CO3 + 2H2O
2OH -- + H2CO32--
Kb tot = Kb1 Kb2
(2.1 x 10 -4)(2.4 x 10 -8) = 5.0 x 10 -12
Relationship of an acid Ka and the Kb
of its conjugate base:
conjugate pair rxns with water
NH3 + H2O
NH4+ +
NH4+ + H2O
2H2O
NH3
H 3O+
+
+
OH H3 O +
OH -
Kb = 1.8 x 10 -5
Ka = 5.6 x 10 -10
Kw = 1 x 10 -14
For Conjugate Acid/Base Pairs:
Kb Ka = K w
or
pKb + pKa = pKw
Find Ka for the conjugate acids of
the following bases: Ka = Kw/Kb
Base
Kb
Conj. Acid
NH3
1.8 x 10 -5
NH4+
C5H5N 1.7 x 10 -9
HS CO3
2-
Ka
5.6 x 10 -10
C5H5NH + 5.9 x 10 -6
1.8 x 10 -7
H2S
5.6 x 10 -8
1.8 x 10
HCO3 -
5.6 x 10 -11
-4
Acid/Base Hydrolysis
Many salts produce solutions that are acidic or basic
Why?
because either (or both) the cation or anion of the salt
acted as a weak acid or a weak base
HX + H2O
H3O+ + X -
weak acid
or
B + H2O
weak base
BH+ + OH -
Which cations will not hydrolyze?
Cations of strong bases -- Grp I and II (except
Be)
Li+ Na+ K+ Rb+ Cs+ Mg2+ Ca2+ Sr2+ Ba2+
Anions of strong acids -ClO4- ClO3- SO42-
NO3-
Cl -
Br -
I-
All other cations and anions will hydrolyze
Which cations will hydrolyze?
Conjugate acids of weak bases:
NH4+ Al3+ Cu2+ etc.
Conjugate bases of weak acids:
CO32 - CH3CO2- NH3 F - etc.
What is the pH of the following salt solutions?
•NaCl
Na+ -- neutral
Cl - -- neutral
N
•NaF
Na+ -- neutral
F - -- basic
B
•NH4Cl
NH4+ -- acidic
Cl - -- neutral
A
•Na2CO3
Na+ -- neutral
CO32- -- basic
B
•Cr(NO3)3
Cr3+ -- acidic
NO3- -- neutral
A
•KNO3
K+ -- neutral
NO3+ -- neutral
N
•NaC3H2O4
Na+ -- neutral
C3H2O4- -- basic
B
•NH4CH3CO2
NH4+ -- acidic
CH3CO2- -- basic ?
Is NH4CH3CO2 acidic or basic?
NH4+ +
H 2O
H3O+
+
NH3
Ka = 5.6 x 10 -10
CH3CO2- +
H2O
CH3CO2H + OH -
Kb = 5.6 x 10 -10
weak acid & weak base strength equal -- neutral soln.
You Must:
Know what hydrolysis is
Know which cations hydrolyze or produce acidic
solutions
Know which anions hydrolyze or produce basic
solutions
Be able to estimate and calculate the pH of a
salt solution
Know how to determine Ka’s & Kb’s of conjugate
pairs
Strength of Acids:
Can be increased by anything that facilitates
the loss of a Proton (H+)
•Bond Strength
•Polarity or Delectronegativity
•Central Atom Charge or Oxidation #
IVA
VA
VIA
VIIA
row 2
CH4
NH3
H2O
HF
row 3
SiH4
PH3
H2S
HCl
Increasing acid strength
Increasing acid strength
Examples:
HClO4
HClO3
HClO2
HClO
Cl ox. #
Increasing acid strength
Acid
Ka
+7
strong
+5
strong
+3
1.1 x 10 -2
+1
3.0 x 10 -8
HClO
HBrO
HIO
Ka
Increasing acid strength
Acid
3 x 10 -8
2.5 x 10 -9
2.3 x 10 -11
Organic Acids:
H
H
O
C
C
H
O-
H
Acetate ion
resonance
H
O-
C
C
H
O
H
H
O
C
C
OH
H
Acetic Acid
Ka = 1.8 x 10 -5
F
F
O
C
C
OH
F
Trifluoroacetic Acid
Ka = 5.0 x 10 -1
Lewis Acids and Bases:
Lewis Acid -- accepts a pair of electrons
Lewis Base -- donates a pair of electrons
formation of a coordinate covalent bond
H
H
B
N
H
F
F
F
NH3
H
H
Lewis Base -donates a pair of
electrons
+
H+
H+
N
H
NH4
+
Lewis Acid -accepts a pair of
electrons