Acids and Bases
Autoionization of Water and the pH Scale
+
H2O(l)
H2O(l)
+
H3O+(aq)
OH-(aq)
The nature of the hydrated proton.
Brønsted-Lowry Acid-Base Definition
An acid is a proton donor, any species which donates a H+.
A base is a proton acceptor, any species which accepts a H+.
+
H O H +
-
O
H
H
H O
+ H O
H
H
H
+
H O
H
H +
N
H
H
H
H O
H
+
+
H N H
H
The extent of dissociation for strong and weak acids
Strong acid: HA(g or l) + H2O(l)
H2O+(aq) + A-(aq)
The extent of dissociation for strong and weak acids
Weak acid: HA(aq) + H2O(l)
H2O+(aq) + A-(aq)
Strong acids dissociate completely into ions in water.
HA(g or l) + H2O(l)
H3O+(aq) + A-(aq)
Kc >> 1
Weak acids dissociate very slightly into ions in water.
HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
Kc << 1
The Acid-Dissociation Constant
[H3O+][A-]
Kc =
[H2O][HA]
Kc[H2O] = Ka =
stronger acid
higher [H3O+]
larger Ka
[H3O+][A-]
[HA]
smaller Ka
lower [H3O+]
weaker acid
ACID STRENGTH
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H2O(l) + H2O(l)
Kc =
H3O+(aq) + OH-(aq)
[H3O+][OH-]
[H2O]2
The Ion-Product Constant for Water
Kc[H2O]2 = Kw = [H3O+][OH-] = 1.0 x 10-14 at 250C
A change in [H3O+] causes an inverse change in [OH-].
In an acidic solution, [H3O+] > [OH-]
In a basic solution, [H3O+] < [OH-]
In a neutral solution, [H3O+] = [OH-]
The relationship between [H3O+] and [OH-] and the relative acidity of solutions.
[H3O+]
Divide into Kw
[OH-]
[H3O+] > [OH-]
[H3O+] = [OH-]
[H3O+] < [OH-]
ACIDIC
SOLUTION
NEUTRAL
SOLUTION
BASIC
SOLUTION
The pH values of
some familiar
aqueous solutions
pH = -log [H3O+]
The relations among [H3O+], pH, [OH-], and pOH
Acid/Base Activity
PROBLEM:
Calculating [H3O+], pH, [OH-], and pOH
In a restoration project, a conservator prepares copper-plate
etching solutions by diluting concentrated HNO3 to 2.0M, 0.30M,
and 0.0063M HNO3. Calculate [H3O+], pH, [OH-], and pOH of
the three solutions at 250C.
Important pKa values to know
Compound
Structure
pKa
H
Alkanes (sp3)
51
H C H
H
Alkenes
Alkynes
H
H
H
H
H
44
25
H
33-35
H
Amines
N H
15-19
R
Alcohols
R
O
H
15.7
H
3-5
Water
H
Carboxylic acids
Special Carboxylic
acids
O
R
O
H
-0.2
H
-9
-7
-5.2 (first ionization)
O
F
Strong Acids
O
HBr O
F HCl
F
H2SO4
Brønsted-Lowry Acid-Base Definition
An acid is a proton donor, any species which donates a H+.
A base is a proton acceptor, any species which accepts a H+.
An acid-base reaction can now be viewed from the standpoint
of the reactants AND the products.
An acid reactant will produce a base product and the two will
constitute an acid-base conjugate pair.
Proton transfer as the essential feature of a Brønsted-Lowry acid-base reaction
Lone pair
binds H+
+
+
HCl
H 2O
(acid, H+ donor)
Cl-
H 3 O+
(base, H+ acceptor)
Lone pair
binds H+
+
+
NH3
(base, H+ acceptor)
H2O
(acid, H+ donor)
NH4+
OH-
The Conjugate Pairs in Some Acid-Base Reactions
Conjugate Pair
Acid
+
Base
Base
+
Acid
Conjugate Pair
Reaction 1
HF
+
H2O
F-
+
H3O+
Reaction 2
HCOOH +
CN-
HCOO-
+
HCN
Reaction 3
NH4+
+
CO32-
NH3
+
HCO3-
Reaction 4
H2PO4-
+
OH-
HPO42-
+
H2O
Reaction 5
H2SO4
+
N2H5+
HSO4-
+
N2H62+
Reaction 6
HPO42-
+
SO32-
PO43-
+
HSO3-
Acid/Base Activity
PROBLEM:
Identifying Conjugate Acid-Base Pairs
The following reactions are important environmental processes.
Identify the conjugate acid-base pairs.
(a) H2PO4-(aq) + CO32-(aq)
(b) H2O(l) + SO32-(aq)
HPO42-(aq) + HCO3-(aq)
OH-(aq) + HSO3-(aq)
Strengths of
conjugate acidbase pairs
Acid
Weaker Ethan e
acid
Ethylene
Ammonia
Hyd rogen
Acetylene
Ethan ol
Water
pK a
Formula
CH3 CH3
CH2 =CH2
NH3
H2
HC CH
CH3 CH2 OH
H2 O
Methylammon ium ion CH3 NH3
+
-
Ammoniu m ion
Hyd rogen su lfid e
HCO3
C6 H5 OH
+
NH4
H2 S
Carbonic acid
Acetic acid
Benzoic acid
Phosp horic acid
H2 CO3
CH3 COOH
C6 H5 COOH
H3 PO4
Bicarbonate ion
Phen ol
Hyd roniu m ion
Sulfuric acid
Hyd rogen ch loride
Stron ger Hyd rogen bromide
acid
Hyd rogen iod ide
+
H3 O
H2 SO4
HCl
HBr
HI
51
44
38
35
25
15.9
15.7
10.64
10.33
9.95
Conju gate Bas e
CH3 CH 2
S tronger
- conju gate
CH2 =CH
base
NH2
H
HC C
CH3 CH 2 O
-
HO
CH3 NH2
2-
9.24
7.04
6.36
4.76
4.19
CO3
C6 H5 O
NH3
HS
HCO3
CH3 COO
C6 H5 COO
2.1
-1.74
-5.2
-7
-8
-9
H2 PO4
H2 O
HSO4
Cl
Br
I
-
Weaker
conju gate
base
3.18 HF
Equilibrium favors reaction of the stronger acid and stronger
base to give the weaker acid and weaker base
CH3
O
H
+ N H
C
H
O H
pKa 4.76
H
O
H N H
+
CH3 C
H
O
acetate ion ammonium
pKa 9.24
acetic acid + sodium bicarbonate (omit Na+ )
CH3
O
O
O-H
C
+
C
O
O H
pKa= 4.76
CH3
O
C
O
O
O-H
C
+
O
H
6.36
CO2
+
H2O
Acid/Base Activity
PROBLEM:
Predicting the K Acid-Base Reaction
“Will this deprotonate that”
Predict the net direction and whether Ka is greater or less than 1
for each of the following reactions (assume equal initial
concentrations of all species):
(a) H2PO4-(aq) + NH3(aq)
(b) H2O(l) + HS-(aq)
HPO42-(aq) + NH4+(aq)
OH-(aq) + H2S(aq)
In Class
Use balanced equations showing the net direction
of the reaction to describe each of the following
observations.
a) You smell ammonia when NH3 dissolves in water
b) The odor goes away when you add an excess of HCl to the solution in part a)
c) The odor returns when you add an excess of NaOH to the solution in part b)
Acid/Base Activity
PROBLEM:
Finding the Ka of a Weak Acid from the pH of
Its Solution
Phenylacetic acid (C6H5CH2COOH, simplified here as HPAc)
builds up in the blood of persons with phenylketonuria, an
inherited disorder that, if untreated, causes mental retardation
and death. A study of the acid shows that the pH of 0.12M
HPAc is 2.60. What is the Ka of phenylacetic acid?
Acid/Base Activity
PROBLEM:
Determining Concentrations from Ka and
Initial [HA]
Propanoic acid (CH3CH2COOH, which we simplify and HPr) is
an organic acid whose salts are used to retard mold growth in
foods. What is the [H3O+] of 0.10M HPr (Ka = 1.3x10-5)?
[HA]dissociated
Percent HA dissociation =
x 100
[HA]initial
Polyprotic acids
acids with more than more ionizable proton
H3PO4(aq) + H2O(l)
H2PO4
-(aq)
+ H3
O+(aq)
Ka1 =
[H3O+][H2PO4-]
[H3PO4]
= 7.2x10-3
H2PO4
-(aq)
+ H2O(l)
HPO4
2-(aq)
+ H3
O+(aq)
Ka2 =
[H3O+][HPO42-]
[H2PO4-]
= 6.3x10-8
HPO4
2-(aq)
+ H2O(l)
PO4
3-(aq)
+ H3
O+(aq)
Ka1 > Ka2 > Ka3
Ka3 =
[H3O+][PO43-]
[HPO42-]
= 4.2x10-13
ACID STRENGTH
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Acid/Base Activity
PROBLEM:
Calculating Equilibrium Concentrations for a
Polyprotic Acid
Ascorbic acid (H2C6H6O6; H2Asc for this problem), known as
vitamin C, is a diprotic acid (Ka1 = 1.0x10-5 and Ka2 = 5x10-12)
found in citrus fruit. Calculate [H2Asc], [HAsc-], [Asc2-], and the
pH of 0.050M H2Asc.
Abstraction of a proton from water by methylamine
Lone pair
binds H+
+
CH3NH2
H 2O
methylamine
+
CH3NH3+
methylammonium ion
OH-
BASE STRENGTH
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Acid/Base Activity
PROBLEM:
Determining pH from Kb and Initial [B]
Dimethylamine, (CH3)2NH, a key intermediate in detergent
manufacture, has a Kb of 5.9x10-4. What is the pH of 1.5M
(CH3)2NH?
Acid/Base Activity
PROBLEM:
Determining the pH of a Solution of A-
Sodium acetate (CH3COONa, or NaAc for this problem) has
applications in photographic development and textile dyeing.
What is the pH of 0.25M NaAc? Ka of acetic acid (HAc) is
1.8x10-5.
Bond strength decreases,
acidity increases
The effect of atomic and molecular properties on
nonmetal hydride acidity.
6A(16)
7A(17)
H 2O
HF
H 2S
HCl
H2Se
HBr
H2Te
HI
Electronegativity increases,
acidity increases
The relative strengths of oxoacids
H
O


I
< H
O


Br
<
H
O


O
H
O


Cl
<<
H

O

Cl
O
O
Cl
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Ka Values of Some Hydrated Metal Ions at 250C
Hydrated Ion
Ka
Fe3+
Fe(H2O)63+(aq)
6 x 10-3
Sn2+
Sn(H2O)62+(aq)
4 x 10-4
Cr3+
Cr(H2O)63+(aq)
1 x 10-4
Al3+
Al(H2O)63+(aq)
1 x 10-5
Be2+
Be(H2O)42+(aq)
4 x 10-6
Cu2+
Cu(H2O)62+(aq)
3 x 10-8
Pb2+
Pb(H2O)62+(aq)
3 x 10-8
Zn2+
Zn(H2O)62+(aq)
1 x 10-9
Co2+
Co(H2O)62+(aq)
2 x 10-10
Ni2+
Ni(H2O)62+(aq)
1 x 10-10
ACID STRENGTH
Free Ion
The acidic behavior of the hydrated Al3+ ion
Electron density
drawn toward Al3+
Nearby H2O acts
as base
H 3 O+
H 2O
Al(H2O)63+
Al(H2O)5OH2+
Acid/Base Activity
Predicting Relative Acidity of Salt Solutions
PROBLEM:
Predict whether aqueous solutions of the following are acidic,
basic, or neutral, and write an equation for the reaction of any
ion with water:
(a) Potassium perchlorate, KClO4
(b) Sodium benzoate, C6H5COONa
(c) Chromium trichloride, CrCl3
(d) Sodium hydrogen sulfate, NaHSO4
Acid/Base Activity
PROBLEM:
Predicting the Relative Acidity of Salt
Solutions from Ka and Kb of the Ions
Determine whether an aqueous solution of zinc formate,
Zn(HCOO)2, is acidic, basic, or neutral.
Molecular Structure and Acidity
relative acidities the more A:(-) stable, greater the acidity of H-A
CH3
O
H
+ N H
C
H
O H
A-H
:B
O
CH3 C
O
A:-
+
Ways to stabilize A:(-) the negative charge
ON a more electronegative atom
ON larger atom
RESONANCE delocalized
STABILIZED by inductive effect
IN an orbital with more s character
H
H N H
H
H-B+
Molecular Structure and Acidity
within a period - the greater the electronegativity of A:(-)
the more A:(-) is stablized
 the stronger the acid
Conjugate base
Acid
pKa 16
H3 C O H
CH3 O
pKa 38
H3 C N H
H
CH3 N
pKa 51
H
H3 C C H
H
H
C
H
H
CH3
Molecular Structure and Acidity
Size of A:(-) Within a column the larger the atom bearing the (-), the greater its stability
H3 C S H
pK a 7.0
+ CH3 O
CH3 S
+
H3 C O H
pK a 16
Molecular Structure and Acidity
Resonance delocalized of charge in ACompare alcohol and carboxylic acid acidity
H
H
H
C
H C O + O
HH H
H
CH3
O
C
O
H
+
H
O
H
H
H
C
H C O
HH
+ H O
H
H
pK a = 15.9
alkoxide ion
CH3
CH3
O
C
O
+ H O
H
H
O
C
O
resonance stabilization
pK a = 4.76
Molecular Structure and Acidity
curved arrows show the flow of electrons in an acid-base
reaction
CH3
O
H
+ N H
C
H
O H
H
O
H N H
+
CH3 C
H
O
acetate ion ammonium
Molecular Structure and Acidity
molecules may have 2 or more sites that can accept a H+
e.g. carboxylic acids, esters, and amides
protonation favored where the charge is more delocalized
which oxygen is protonated?
O
O
H3C
+ H OSO3H
C
H3C
H
C
OR
O
O
H
O
H3C
C
O
H
H
H
+ HSO4 -
Molecular Structure and Acidity
resonance
octets greater
contribution even
with plus charge on O
O
H3C
H
C
O
H
O
H3C
H
C
O
H
O
O
H3C
+ H2 SO4
C
H3C
H
C
OR
O
O
H
O
H3C
C
O
H
H
H
+ HSO4 -
Molecular Structure and Acidity
O
H3 C
H
H+ on the hydroxyl
C
O
 H+ onO the
H
O
H3C
H
H3 C C
carbonyl
C
O
O
H
H
O
O
H3 C
+ H2 SO4
C
H3 C
H
C
OR
O
O
H
H
O
H3C
C
O
H
H
+ HSO4 -
H
can “write” contributing structures
create & separate charge
Molecular Structure and Acidity
inductive effect, electron-withdrawing
covalent bonds transmit electronegativity (polarizing) effects
push or pull shared e(-)s of adjacent atoms
pK a: 15.9
H H
H C C O
H
H H
:B
H H
H C C O
H H
12.4
14.6
F H
F C C O
H
F H
F H H
F C C C O
H
F H H
:B
F H
F C C O
F H
15.4
F H H H
F C C C C O
H
F H H H
decreases w/ distance
Molecular Structure and Acidity
inductive effect: butanoic and chlorobutanoic acids
O
O
OH
pK a 4.82
Cl
Cl
OH
pK a 4.52
O
O
OH
pK a 3.98
OH
Cl
pK a 2.83
Molecular Structure and Acidity
Hybridization
greater the % s character with (-) the more stable the anion
Weak
Acid
alkyne
Conjugate
pK a
Base
H C C H
H
alkene
alkane
w ater
H
C C
H
H
H
H
H C C H
H
H
HO-H
H C C
25
H
C C
H
H
H
H C C H
H
H
44
HO–
15.7
51
Molecules as Lewis Acids
An acid is an electron-pair acceptor.
A base is an electron-pair donor.
F
B
F
F
acid
F
H
+
H
N
HH
base
B
F
F
N
HH
adduct
M(H2O)42+(aq)
M2+
H2O(l)
adduct