William Brown
Thomas Poon
www.wiley.com/college/brown
Chapter Two
Acids and Bases
Arrhenius Acids and Bases
• In 1884, Svante Arrhenius proposed these
definitions.
– Acid: A substance that dissolves in water to produce
H3O+ ions.
– Base: A substance that dissolves in water to produce
OH- ions.
– This definition of an acid is a slight modification of the
original Arrhenius definition, which was that an acid
produces H+ in aqueous solution.
– Today we know that H+ reacts immediately with a
water molecule to give a hydronium ion H3O+.
Arrhenius Acids and Bases
– When HCl, for example, dissolves in water, it reacts
with water to give a hydronium ion and a chloride ion.
H2 O(l) + HCl(aq)
H3 O+ (aq) + Cl- (aq)
– We use curved arrows to show the change in position
of electron pairs during this reaction.
Arrhenius Acids and Bases
• With bases, the situation is slightly different.
– Many bases are metal hydroxides such as KOH, NaOH,
Mg(OH)2, and Ca(OH)2.
– These compounds are ionic solids and, when they
dissolve in water, their ions merely separate.
NaOH(s)
H2 O Na+ (aq)
+ OH (aq)
– Other bases are not hydroxides; these bases produce
OH- by reacting with water molecules.
NH3 ( aq) + H2 O(l)
NH4 + ( aq) + OH- (aq)
Arrhenius Acids and Bases
– We use curved arrows to show the transfer of a
proton from water to ammonia.
Brønsted-Lowry Acids & Bases
• Acid: A proton donor.
• Base: A proton acceptor.
Conjugate Acids & Bases
 Conjugate base: The species formed form on acid
when an acid donates a proton to a base.
– Conjugate acid: The species formed from a base when
the base accepts a proton from an acid.
– Acid-base reaction: A proton-transfer reaction.
– Conjugate acid-base pair: Any pair of molecules or
ions that can be interconverted by transfer of a
con jugate acid-bas e pair
proton.
con jugate acid-bas e pair
HCl( aq)
Hydrogen
chloride
(acid )
+ H2 O(l)
Water
(b ase)
-
Cl ( aq) +
Chlorid e
ion
(conjugate
bas e of HCl)
H3 O+ (aq)
Hydron ium
ion
(conjugate
acid of H 2 O)
Brønsted-Lowry Acids & Bases
– Brønsted-Lowry definitions do not require water as a
reactant.
con jugate acid-bas e pair
con jugate acid-bas e pair
CH3 COOH
Acetic acid
(acid)
+ NH3
Ammonia
(base)
-
+
CH3 COO +
NH4
Acetate
Ammonium
ion
ion
(con jugate base (conjugate acid
acetic acid)
of ammonia)
Brønsted-Lowry Acids & Bases
– We use curved arrows to show the flow of electrons
that occurs in the transfer of a proton from acetic acid
to ammonia.
Brønsted-Lowry Acids & Bases
• Note the following about the conjugate acidbase pairs in Table 2.1:
1. An acid can be positively charged, neutral, or
negatively charged; examples of each type are H3O+,
H2CO3, and H2PO4- .
2. A base can be negatively charged or neutral;
examples are OH-, Cl-, and NH3.
3. Acids are classified a monoprotic, diprotic, or
triprotic depending on the number of protons that
each may give up; examples are HCl, H2CO3, and
H3PO4.
Brønsted-Lowry Acids & Bases
– Carbonic acid, for example, can give up one proton to
become bicarbonate ion, and then the second proton
to become carbonate ion.
H2 CO3 + H2 O
Carbonic
acid
HCO3 - + H3 O+
Bicarbon ate
ion
-
CO3
+ H3 O
Carbonate
ion
HCO3 + H2 O
Bicarbonate
ion
2-
+
4. Several molecules and ions appear in both the acid
and conjugate base columns; that is, each can
function as both an acid and as a base.
Brønsted-Lowry Acids & Bases
5. There is an inverse relationship between the strength
of an acid and the strength of its conjugate base.
– The stronger the acid, the weaker its conjugate base.
– HI, for example, is the strongest acid in Table 2.1 and
its conjugate base, I-, is the weakest base in the table.
– CH3COOH (acetic acid) is a stronger acid that H2CO3
(carbonic acid); conversely, CH3COO- (acetate ion) is a
weaker base that HCO3- (bicarbonate ion).
Acid and Base Strength
• Strong acid: One that reacts completely or almost
completely with water to form H3O+ ions.
• Strong base: One that reacts completely or
almost completely with water to form OH- ions.
– Here are the six most common strong acids and the
four most common strong bases.
Formula
HCl
HBr
HI
HNO3
H2 SO4
HClO4
N ame
Hydrochloric acid
Hydrob romic acid
Hydroiodic acid
N itric acid
Su lfu ric acid
Perch loric acid
Formula
LiOH
NaOH
KOH
Ba( OH) 2
N ame
Lith iu m h yd roxide
Sodiu m hydroxide
Potass iu m h yd roxide
Barium hydroxide
Acid and Base Strength
• Weak acid: A substance that only partially
dissociates in water to produce H3O+ ions.
– Acetic acid, for example, is a weak acid; in water, only
4 out every 1000 molecules are converted to acetate
ions.
CH3 COOH(aq) + H2 O( l)
Acetic acid
CH3 COO-( aq) + H3O+ ( aq)
Acetate ion
• Weak base: A substance that only partially
dissociates in water to produce OH- ions.
– ammonia, for example, is a weak base
NH3 (aq) + H2 O( l)
NH4 + (aq) + OH-( aq)
Acid-Base Reactions
– Acetic acid is incompletely ionized in aqueous
solution.
O
O
H2 O
CH3 COH +
Acid
Base
(w eaker acid ) (w eaker base)
-
+
CH3 CO
Conju gate b ase
of CH 3CO 2H
(stron ger bas e)
+
H3 O
Conju gate acid
of H 2O
(s tronger acid)
– The equation for the ionization of a weak acid, HA, is
HA + H2 O
A
-
+ H3 O+
+
-
Ka = Keq[ H2 O] = [H3 O ] [A ]
[HA]
Acid-Base Equilibria
• To determine the position of equilibrium in an acidbase reaction:
– Identify the two acids in the equilibrium; one on the left
and one on the right.
– Use the information in Table 2.2 to determine which is the
stronger acid and which is the weaker acid.
– Remember that the stronger acid gives the weaker
conjugate base, and the weaker acid gives the stronger
conjugate base.
– The stronger acid reacts with the stronger base to give the
weaker acid and weaker base.
– Equilibrium lies on the side of the weaker acid and the
weaker base.
Acid-Base Equilibrium
• Equilibrium in the following acid-base reaction
lies to the right, on the side of the weaker acid
and the weaker base.
O
CH3 COH
O
+
NH3
Acetic acid
Ammonia
pK a 4.76
(stron ger bas e)
(s tronger acid)
CH3 CO-
+
NH4
+
A cetate ion Ammon ium ion
(w eak er b ase)
pK a 9.24
(w eaker acid )
Structure and Acidity
• The most important factor in determining the
relative acidity of an organic acid is the relative
stability of the anion, A-, formed when the acid,
HA, transfers a proton to a base.
• We consider these four factors:
1. The electronegativity of the atom bonded to H in HA.
2. Resonance stabilization of A-.
3. The inductive effect.
4. The size and delocalization of charge in A- .
Structure and Acidity
• Electronegativity of the atom bearing the
negative charge.
• Within a period
– The greater the electronegativity of the atom bearing the negative charge,
the more strongly its electrons are held.
– The more strongly the electrons are held, the more stable the anion A-.
– The more stable the anion A- the greater the acidity of the acid HA.
Structure and Acidity
• Resonance delocalization of the charge on A– Compare the acidity of a carboxylic acid and an alcohol, both of
which contain an -OH group.
– Carboxylic acids are weak acids. Values of pKa for most unsubstituted
carboxylic acids fall within the range of 4 to 5.
- +
+
+
pKa = 4.76
CH3 COOH H2 O
CH3 COO
H3 O
– Alcohols are very weak acids. Values of pKa for most alcohols fall
within the range of 15 to 18.
CH3 CH2 O-H + H2 O
An alcohol
CH3 CH2 O + H3 O+
An alkoxide
ion
-
pKa = 15.9
– How do we account for the fact that carboxylic acids are stronger
acids than alcohols?
Structure and Acidity
– The greater the resonance stabilization of the
anion, the more acidic the compound.
– There is no resonance stabilization in an alkoxide
anion.
– We can write two equivalent contributing
structures for the carboxylate anion; the negative
charge is spread evenly over the two oxygen
atoms.
O
O
CH3 C
+
O H
H2 O
CH3 C
O
CH3 C
O
+
+
H3 O
O
The carboxylate an ion is stabilized
by delocalization of th e negative ch arge
Structure and Acidity
• Inductive polarization of electron density
transmitted through covalent bonds by a nearby
atom of higher electronegativity
Structure and Acidity
• The more stable the anion A-, the greater the
acidity of the acid HA.
– The larger the volume over which the charge on an
anion (or cation) is delocalized, the greater the
stability of the anion (or cation).
– When considering the relative acidities of the
hydrogen halides, (HI > HBr > HCl > HF), we need to
consider the relative stabilities of the halide ions.
– Recall from general chemistry that atomic size is a
periodic property.
– For main group elements, atomic radii increase going
down a group and increase going across a period.
Structure and Acidity
– For the halogens, iodine has the largest atomic radii,
fluorine has the smallest I > Br > Cl > F.
Anions are always larger than the atoms from which they
are derived. For anions, nuclear charge is unchanged but
the added electron(s) introduce new repulsions and the
electron clouds swell.
Among the halide ions, I- has the largest atomic radius, and
F- has the smallest atomic radius.
Thus, HI is the strongest acid because the negative charge
on iodide ion is delocalized over a larger volume than the
negative charge on chloride, etc.
Note that the result of both resonance and the inductive
effect is due to the delocalization of charge.
Lewis Acids and Bases
• Lewis acid: Any molecule or ion that can form a
new covalent bond by accepting a pair of
electrons.
• Lewis Base: Any molecule or ion that can form a
new covalent bond by donating a pair of
electrons.
A
Lewis
acid
+
:B
Lewis
base
-
+
A B
new covalent bond
formed in this Lewis
acid-base reaction
Lewis Acids and Bases
When HCl dissolves in water, for example, the
strongest available Lewis base is an H2O
molecule, and the following proton-transfer
reaction takes place.
H O
+ H Cl
H
H O H +
Cl
H
Hyd roniu m ion
When HCl dissolves in methanol, the strongest
available Lewis base is a CH3OH molecule, and
the following proton-transfer reaction takes place.
CH3 O
H
Methan ol
+ H Cl
CH3 O H +
H
An oxoniu m
ion
Cl
Lewis Acids and Bases
Lewis Acids and Bases
• Another type of Lewis acid we will encounter in later chapters is an
organic cation in which a carbon is bonded to only three atoms and bears
a positive formal charge.
– such carbon cations are called carbocations
bromin e uses a lon e
pair of electrons to form
a new b on d to carbon
+
CH3 -CH-CH3 +
Br
A n organic cation Bromide ion
(a Lew is acid)
(a Lew is bas e)
Br
CH3 -CH-CH3
2-Bromobutan e
Acids and
Bases
End Chapter 2