Chapter 19 Notes: Part II
Acid/Base Theories
 There are three ways to define
acids and bases.
 This reflects the fact that
science is always revising itself.
1) Arrhenius Acids/Bases
 An acid is a chemical that gives off
hydrogen ions in solution.
 A base is a chemical that gives off
hydroxide ions in solution.
Example: HCl + NaOH  NaCl + H2O
**BASICALLY…acids start with H+ and bases end with OH-!!!
Arrhenius Acids/Bases
There are three ways to describe an acid:
I. Monoprotic-gives off one H+ in sol’n
II. Diprotic-gives off two H+
III. Triprotic-gives off three H+
Bronsted-Lowry Acids/Bases
 The Arrhenius definition is a good
one, but does not encompass
everything that shows acidic/basic
qualities.
 To account for this, Johannes
Bronsted and Thomas Lowry
proposed a new idea.
2) Bronsted-Lowry Acids/Bases
 An acid is a hydrogen-ion
(proton) donor.
 A base is a hydrogen-ion
(proton) acceptor.
Conjugate Acids/Bases
 A conjugate acid is the particle
that is formed when a base
gains a hydrogen ion.
 A conjugate base is the particle
that is formed when an acid
donates a hydrogen ion.
Conjugate Acids/Bases
 A conjugate acid/base pair
consist of two substances
related by the loss of a single
hydrogen ion.
Conjugate Pairs
NH3 + H2O  NH4 +
+
OH
proton donator
 Which is the initial acid?
H2O
proton acceptor
 Which is the initial base?
formed after base gains H+
 What is the conjugate acid?
formed after acid donates H+
NH3
NH4+
 What is the conjugate base? OH-
HCl + H2O  H3
+
O
+
 Which is the initial acid?
 Which is the initial base?
 What is the conjugate acid?
 What is the conjugate base?
Cl
HCl
H2O
H3O+
Cl-
Amphoteric Substances
 Note that in one of the previous
examples H2O was acidic and basic
in the other.
 A substance that can act as an acid
or a base is called amphoteric.
3) Lewis Acids/Bases
 A Lewis acid is a substance that
can accept a pair of electrons to
form a covalent bond.
 A Lewis base is a substance that
can donate a pair of electrons to a
covalent bond.
**MUST draw Lewis dot structures to
determine whether a compound will
gain or lose e-!!!
7
5
1
 Ex… BF3 + NH3
x
x
xx
xx
Fx x xB F
xx
x
x
xFx
xx
x
x
x
x
ACID
(not happy)
accept e- pair
xH
HN
x
H
x
BASE
(happy)
donate e- pair
Hx N x H
x
H
3
F3BNH3
x
x
xx
xx
Fx x xB F
xx
x
x
xFx
xx
x
x
x
x
pH scale
 The pH scale is a way of
expressing the strength of
acids and bases.
 Instead of using very small
numbers based on the Molarity
of the H+ (or OH-) ion ,
 We use the NEGATIVE power of
10 on the Molarity of the H+ (or
OH-) ion. pH = - log [H+]
Under 7 = acid
7 = neutral
Over 7 = base
The Inverse
Relationship
between the pH
and pOH Scales
For any neutral
solution,
pH + pOH = 14.00
(at 25°C)
pH of Common Substances
pH
[H+]
[OH-]
pOH