Chapter 18
Acids and
Bases
Just as some animals
can be classified as
dogs or cats; some
compounds can be
classified as acids or
bases.
Properties of Acids
and Bases
Common to both:
• Electrolytes – aqueous
solutions conduct
electric current;
ions free to move
Common to acids and
bases
• Change color in
presence of indicators
Cabbage juice:
acid – pink
base - green
Litmus:
acids are red
bases are blue
Properties of acids
• taste sour
• react with many metals
to form hydrogen gas
Single replacement rxn;
metal replaces hydrogen
in acid
Ex: zinc reacts with
hydrochloric acid
Zn(s) + 2HCl(aq) 
ZnCl2(aq) + H2(g)
Properties of acids
•react with carbonates
to form carbon dioxide
gas and water
Double replacement rxn;
carbonic acid (a product)
decomposes to carbon
dioxide and water.
Baking soda (sodium
hydrogen carbonate)
reacts with vinegar
(acetic acid).
Chemical weathering
Philadelphia
Merchant’s
Exchange
(1832)
Limestone (CaCO3)
reacts with acid rain.
Properties of bases
• taste bitter
• dilute solutions
feel slippery
Definition of acidic, basic
and neutral solutions.
Self ionization of water
H2O(l) + H2O(l)
+
H3O (aq) + OH (aq)
Occurs to a small degree;
most particles remain as
molecules.
Definition of acidic, basic
and neutral solutions.
Pure water and all dilute
aqueous solutions
contain:
• water molecules
+
• hydronium ions (H3O )
• hydroxide ions
(OH )
Definition of acidic, basic
and neutral solutions.
Neutral solution:
+
[H3O ] = [OH ]
Acidic solution:
+
[H3O ] > [OH ]
Basic solution:
+
[H3O ] < [OH ]
Naming acids
Binary acids
acid that contains only
two different elements:
hydrogen and a nonmetal
hydro root of 2nd element ic
Ex: HCl hydrochloric acid
Oxyacid
acid that contains
hydrogen, oxygen and a
3rd element (nonmetal)
hydrogen + polyatomic ion
Name of oxyacid based
on name of polyatomic ion:
ion
acid
-ite  -ous
-ate  -ic
Ex: HNO2 nitrous acid
HNO3 nitric acid
Binary acid
hydro _____ ic acid
Oxyacid
______ ic acid
Arrhenius model
Acid - a substance that
contains hydrogen and
ionizes to produce
+
hydrogen ions (H ) in
aqueous solution
HCl(aq) + H2O(l) 
+
H3O (aq) + Cl (aq)
Arrhenius model
Base – substance that
contains hydroxide
group and dissociates
to produce hydroxide
ions (OH ) in aqueous
solution
NaOH(s)
+
Na (aq)
+
OH (aq)
Bronsted-Lowry Model
focuses on hydrogen
+
ion, (H ) or proton
Review
Hydrogen atom:
1 p+
0n
1e
Hydrogen ion:
1 p+
0n
+
H
0e
same as a proton
Bronsted-Lowry Model
Acid – a hydrogen ion
(proton) donor
Base – a hydrogen ion
(proton) acceptor
Bronsted-Lowry Model
Acid as proton donor
HF(aq) + H2O(l)
+
H3O (aq) + F (aq)
acid
All Arrhenius acids are
also Bronsted-Lowry
acids.
Bronsted-Lowry Model
Base as a proton acceptor
HF(aq) + H2O(l)
+
H3O (aq) + F (aq)
base
Bronsted-Lowry Model
• ammonia (NH3) acts as
a base
NH3(aq) + H2O(l)
+
NH4 (aq) + OH (aq)
• amines(- NH2) also bases
Bronsted-Lowry Model
Conjugate acid – species
that results when a base
accepts a proton
“after picture”
Example:
NH3(aq) + H2O(l)
+
NH4 (aq) + OH (aq)
NH3 base;
+
NH4 conjugate acid
Bronsted-Lowry Model
Conjugate base – species
that is formed when an
acid donates a proton
“after picture”
Example
HF(aq) + H2O(l)
+
H3O (aq) + F (aq)
HF acid;
F conjugate base
Bronsted-Lowry Model
Conjugate acid-base
pair
two substances that
differ only in a hydrogen
ion (proton)
Bronsted-Lowry Model
H2O(l) + H2O(l)
+
H3O (aq) + OH (aq)
base
acid
Bronsted-Lowry Model
Amphoteric – substances
that can act as both
acids and bases
Acid – Base Strength
Strength of an acid
or base depends on the
degree to which it forms
ions (by ionization or
dissociation.)
Strong acids
essentially all molecules
ionize
strong electrolytes
HCl(aq) + H2O(l) 
+
H3O (aq) + Cl (aq)
Ex: HCl, HNO3, H2SO4
Weak acids
most molecules do
not ionize
weak electrolytes
HF(aq) + H2O(l)
+
H3O (aq) + F (aq)
Ex: CH3COOH, H3PO4,
H2CO3, HF
Weak acids
HF(aq) + H2O(l)
+
H3O (aq) + F (aq)
Write equilibrium constant
expressions for the above
reaction.
Acid ionization
constant Ka
value of the equilibrium
constant expression for
a weak acid
small Ka reactants are
favored; equilibrium
lies to the left
Strong bases
completely dissociate in
water; strong electrolytes
hydroxides of Groups 1
and 2 metals
NaOH(s)
+
Na (aq)
+
OH (aq)
Weak bases
Ammonia and amines
are weak bases; most
molecules do not ionize.
Weak bases
NH3(aq) + H2O(l)
+
NH4 (aq) + OH (aq)
Write equilibrium constant
expression for the above
reaction.
Base ionization constant
Kb
value of the equilibrium
constant expression for a
weak base
small Kb reactants are
favored; equilibrium lies
to the left
Weak bases
hydroxides of metals
other than Groups 1
and 2 are not soluble
do not produce many
hydroxide ions
considered weak bases
Ka and Kb
only written for weak acid
or weak base
must be determined
experimentally
Relative strengths of
acid-base conjugate
pairs
The stronger an acid,
the weaker its
conjugate base.
HCl(aq) + H2O(l) 
+
H3O (aq) + Cl (aq)
HCl strong acid;
Cl weak base
HF(aq) + H2O(l)
+
H3O (aq) + F (aq)
HF weak acid;
F strong base
Relative strengths of
acid-base conjugate
pairs
The stronger the base,
the weaker the
conjugate acid.
NH3(aq) + H2O(l)
+
NH4 (aq) + OH (aq)
NH3 weak base;
OH strong acid
Monoprotic and polyprotic
acids
Monoprotic acid
acid that donates only
one proton (H+)
Ex: HF, CH3COOH
only ionizable hydrogen
atoms can be donated
Polyprotic acid
acids that donate more
than one proton
happens in a series of
steps, one proton at a
time
Ex: H2SO4 diprotic
H3PO4 triprotic
Lewis acid base model
focuses on electron
pair being donated or
accepted
Lewis acid base model
Lewis acid
atom, ion, or molecule
that accepts an electron
pair to form a covalent
bond
Lewis base
donates an electron pair
Ammonia can donate an
electron pair; Lewis base
Boron trifluoride can
accept an electron pair;
Lewis acid
Self-ionization of water
Water
amphoteric – can act
as Bronsted-Lowry acid
(donate proton) and as
Bronsted-Lowry base
(accept proton)
base
acid
Self-ionization of water
a proton is transferred
from one water molecule
to another producing a
hydronium ion and a
hydroxide ion
Concentration (mol/L) of
hydronium ions in pure
o
water at 25 C
[H3O+] = 1.0 x 10-7 M
In pure water:
[H3O+] = [OH-] = 1.0 x 10-7 M
Ionization constant of water
Kw = [H3
=
+
O ]
[OH ]
-7
(1.0x10 )
-7
(1.0x10 )
= 1.0 x 10–14 (at 25oC)
Applies to all dilute
aqueous solutions at a
given temperature.
Solutions can be neutral,
acidic, or basic.
For any neutral solution:
[H3
+
O ]
=
[OH ]
Adding an acid to water
increases the hydronium
ion concentration.
+
O ]
[OH ]
If [H3
>
then
solution is acidic.
Adding a base to water
increases the hydroxide
ion concentration.
If [H3O+] < [OH-] then
solution is basic.
Kw = [H3
+
O ]
[OH ]
=
–14
1.0x10
inversely proportional
Acidic solution
[H3O+] > 1.0 x 10-7 M
Basic solution
[OH-] > 1.0 x 10-7 M
Is a solution with hydronium
ion concentration of
-4
1.0 x 10 acidic, basic,
or neutral?
+
H3O
The
concentration in a
-5
cup of coffee is 1.0 x 10 M.
OH
What is the
ion
concentration in the coffee?
Kw = [H3O+] [OH-] = 1.0x10–14
Is the coffee acidic, basic,
or neutral?
pH scale
Sorensen
(Danish biochemist)
• devised a system for
expressing acidity of
a solution
• less cumbersome than
writing out hydronium
ion concentration
pH scale
pH
negative of the common
logarithm of the hydronium
ion concentration
pH = -log [H3O+]
Calculate the pH of a
solution with hydronium
ion concentration of
1 x 10-5 M
+
O ]
pH = -log [H3
= -log (1 x 10-5)
= 5.0 (watch sig. fig.)
power of Hydrogen
For a neutral solution
[H3
+
O ]
=
[OH ]
= 1.0 x
pH = 7.00
-7
10
M
Acidic solution
[H3O+] > 1.0 x 10-7 M
pH < 7.00
Basic solution
[H3O+] < 1.0 x 10-7 M
pH > 7.00
Calculating pH
+
O ]
pH = -log [H3
What is the pH of a
vinegar solution that
has a hydronium ion
concentration of
-4
4.0 x 10 M?
Is solution acidic, basic,
neutral?
Calculate
+
[H3O ]
pH = -log [H3
from pH
+
O ]
The pH of a soft drink
is 3.08. What is the
hydronium ion
concentration?
Calculating pOH
pOH = -log
[OH ]
An antacid solution has a
hydroxide-ion concentration
-5
of 3.2 x 10 M. Calculate
the pOH of the solution.
Relating pH and pOH
For all dilute aqueous
solutions:
Kw = [H3O+] [OH-] = 1.0 x 10–14
pH + pOH = 14.00
In a strong acid, essentially
all molecules ionize.
Determine the hydronium
and hydroxide ion
concentrations in a solution
that is 1x10-4 M HCl.
Kw = [H3O+] [OH-] = 1.0x10–14
Determine the hydronium
and hydroxide ion
concentrations in a
solution that is 3.0x10-2 M
NaOH.
Determine the hydronium
and hydroxide ion
concentrations in a solution
that is 1.0 x 10-4 M Ca(OH)2.