ACIDS AND BASES
UNIT 9
PRACTICE
• Consider two acid solutions, 0.01 M
hydrochloric acid (HCl) and 0.01 M acetic
acid (CH3COOH). How will the pH of the
two solutions compare? Explain. Calculate
the pH of each solution.
WHAT ARE ACIDS AND BASES?
BRØNSTED-LOWRY MODEL
• An acid is a substance that produces H+
ions in solution:
HCl(g)  H+(aq) + Cl-(aq)
• An acid is a proton (H+) donor:
BRØNSTED-LOWRY MODEL
• An acid is a substance that produces H+
ions in solution:
HCl(g)  H+(aq) + Cl-(aq)
• An acid is a proton (H+) donor:
BRØNSTED-LOWRY MODEL
• A base is a substance that produces OHions in solution:
NaOH(g)  Na+(aq) + OH-(aq)
• A base is a proton (H+) acceptor:
HYDROGEN IONS
+
(H )
• A proton is just a hydrogen ion (H+)
• When a hydrogen atom (H) loses its only
electron only a proton is left
Protons:
Electrons:
Hydrogen Atom (H)
1
1
Hydrogen Ion (H+)
1
0
Neutrons:
0
0
HYDRONIUM ION
• A hydronium ion, H3O+(aq), is a hydrated proton
• When an acid donates a proton (H+) in an aqueous
solution, it is picked up by a water molecule and a
hydronium ion is formed:
H+ + H2O  H3O+
• H3O+ and H+ are used interchangeably
– They both represent the ion responsible for the acidic
properties of an aqueous solution
pH
• The acidity of a solution is expressed either as the
molar hydrogen ion concentration [H+] or the pH
of a the solution
• pH and hydrogen ion concentration are related in
the following way:
pH = -log[H+]
• In aqueous solutions the pH ranges from 0 to 14
pOH SCALE
Analogous to the pH
scale:
pOH = -log[OH-]
pH + pOH = 14
CALCULATING [H+] FROM pH
• If we are given the pH of a solution and
asked to calculate [H+]:
[H3O+] = 10-pH or [H+] = 10-pH
PRACTICE
• Calculate the pH of
(a) a 1.0 x 10-3 M HCl solution.
(b) a 0.020 M Ba(OH)2 solution.
Note: in (a) and (b) we neglected the
contribution of the autoionization of water to
[H+] and [OH-] because 1.0 x 10-7 M is so small
compared to 1.0 x 10-3 M and 0.020 M
THE ACID-BASE PROPERTIES OF
WATER
• Water is an example of an amphoteric substance,
which is a substance that can act either as an acid
or a base
• In reactions with acids it functions as a base, and
in reactions with bases it acts like an acid
THE AUTOIONIZATION OF WATER
• Water ionizes to a small extent:
H2O(l) ⇌ H+ (aq) + OH- (aq)
• A water molecule transfers a proton to another
water molecule
• This reaction is called the autoionization of water
• Water is neutral because it produces an equal
number of H+ and OH-
THE AUTOIONIZATION OF
WATER
THE ION PRODUCT CONSTANT
OF WATER (Kw)
• The ion product constant (Kw) for water is
the equilibrium constant for the
autoionization of water:
H2O(l) ⇌ H+ (aq) + OH- (aq)
Kc = Kw = [H3O+] [OH-]
or
Kw = [H+] [OH-]
THE ION PRODUCT CONSTANT
OF WATER (Kw)
• In pure water at 25oC the concentrations of
H+ and OH- are equal and found to be 1.0 x
10-7M, therefore:
Kw = [H+][OH-]
Kw =[1.0 x 10-7M][1.0 x 10-7M]= 1.0 x 10-14
Kw = 1.0 x 10-14 at 25oC
THE ION PRODUCT CONSTANT
OF WATER (Kw)
• Whether we have pure water or an aqueous
solution, the following relationship always
holds at 25oC:
Kw = [OH-][H+] = 1.0 x 10-14
Acidic solution [H+] > [OH-]
Neutral solution [H+] = [OH-]
Basic solution [H+] < [OH-]
Concentrations and Kw
Solution
[H3O+]
[OH-]
Kw = [H3O+] [OH-]
Pure water
1.0 x 10-7 M
1.0 x 10-7 M
1.0 x 10-14
1.0 x 10-1 M 1.0 x 10-13 M
1.0 x 10-14
0.010 M strong acid 1.0 x 10-2 M 1.0 x 10-12 M
1.0 x 10-14
0.10 M strong base 1.0 x 10-12 M 1.0 x 10-2 M
1.0 x 10-14
0.010 M strong base 1.0 x 10-13 M 1.0 x 10-1 M
1.0 x 10-14
0.10 M strong acid
CONJUGATE ACID-BASE PAIRS
• An extension of the Brönsted-Lowry definition of
acids and bases is the concept of the conjugate
acid-base pair
– A conjugate acid-base pair is an acid and its
conjugate base or a base and its conjugate acid
• Conjugate acid-base pairs are two substances in
aqueous solution whose formulas differ by an H+
• The acid is the more positive species having the
extra H
CONJUGATE ACID-BASE PAIRS
• Conjugate base: The substance remaining
after an acid donates its H+ to a base
• Conjugate acid: The substance that results
after the base accepts a H+ from an acid
CONJUGATE ACID-BASE PAIRS
STRONG ACIDS AND BASES
• Strong acids and bases ionize completely
when dissolved in water (100%
disassociation)
• Strong acids and strong bases are strong
electrolytes
• Notice the single arrow to the right-the
reaction goes to completion
HCl  H+ + Cl-
STRONG ACIDS AND BASES
HCL is a Strong Acid
NaOH is a Strong Base
THE SIX MOST IMPORTANT STRONG ACIDS
Strong acid
Ionization Reaction
Hydrochloric (HCl)
HCl  H+ + Cl-
Hydrobromic (HBr)
HBr  H+ + Br-
Hydroiodic (HI)
HI H+ + I-
Perchloric acid (HClO4)
HClO4  H+ + ClO4-
Nitric acid (HNO3)
HNO3  H+ + NO3-
Sulfuric acid (H2SO4)
Only the first H+ is strong; the
second only ionizes slightly
H2SO4  H+ + HSO4-
STRONG ACIDS
STRONG BASES
• Like strong acids,
strong bases are
strong electrolytes
that ionize completely
in water
Strong Base
Solubility
NaOH
soluble
KOH
soluble
Ca(OH)2
Slightly soluble
Ba(OH)2
soluble
STRONG ACIDS/BASES
• Generally if an acid is strong, its conjugate base
has no measurable strength
• Likewise, if a base is strong, its conjugate acid is
very weak
• Generally the aqueous anions of strong acids and
the aqueous cations of strong bases are neutral
ions
WEAK ACIDS
• Weak acids are acidic substances that only
partially ionize in aqueous solution
– An equilibrium is established between the acid
and its conjugate base:
HF(aq) + H2O(l) ⇌ F-(aq) + H3O+(aq)
WEAK ACID
WEAK ACIDS
COMMON WEAK ACIDS
IONIZATION REACTION
Acetic acid, CH3COOH
CH3COOH ⇌ H+ + CH3COO-
Hydrofluoric acid, HF
HF ⇌ H+ + F-
Carbonic acid, H2CO3
H2CO3 ⇌ H+ + HCO3-
Phosphoric acid, H3PO4
H3PO4 ⇌ H+ + H2PO4-
Hypochlorous acid, HClO
HClO ⇌ H+ + ClO-
Chlorous acid, HClO2
HClO2 ⇌ H+ + HClO-
Chloric acid, HClO3
HClO3 ⇌ H+ + HClO2-
HCl, Strong Acid
HF, Weak Acid
WEAK ACIS AND ACID
IONIZATION CONSTANTS
• Consider a generic weak monoprotic acid,
HA
• Its ionization in water is represented by:
HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)
or
HA(aq) ⇌ H+(aq) + A-(aq)
ACID IONIZATION CONSTANT (Ka)
• Because an equilibrium is established
between a weak acid and its conjugate
base, an equilibrium expression can be
written:
HA(aq) ⇌ H+(aq) + A-(aq)
Ka = [H3O+] [A-]
[HA]
or
Ka = [H+] [A-]
[HA]
ACID IONIZATION CONSTANT (Ka)
• The acid equilibrium constant, Ka, is the
equilibrium constant for the ionization of a
weak acid
HA(aq) ⇌ H+(aq) + A-(aq)
Ka = [H3O+] [A-]
[HA]
or
Ka = [H+] [A-]
[HA]
ACID IONIZATION CONSTANT (Ka)
• At a given temperature, Ka is a measure of
the strength of an acid
• The larger the Ka greater the H+
concentration and the stronger the acid
• Keep in mind only weak acids have Ka
values
WEAK BASES
• Like weak acids weak bases only partially ionize in
solution
• Ammonia is a weak base it ionizes to a very
limited extent in water:
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
CONJUGATE ACID-BASE PAIRS
• Conjugate acid-base pairs are two
substances in aqueous solution whose
formulas differ by an H+
• The acid is the more positive species having
the extra H
Acid
Conjugate base
Acetic acid, CH3COOH
Acetate ion, CH3COO-
CONJUGATE ACID-BASE PAIRS
• One of the rules for chemical equilibria:
When two reactions are added to give a
third reaction, the equilibrium constant for
the third reaction is the product of the two:
CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
Ka = 1.8 x 10-5
CH3COO-(aq) + H2O(l) ⇌ CH3COOH(aq) + OH(aq)
Kb = 5.6 x 10-10
H2O(l) ⇌ H+(aq) + OH-(aq)
Kw = 1.0 x 10-14
THE RELATIONSHIP BETWEEN Ka
AND Kb
• For any conjugate acid-base pair:
KaKb = Kw
Acid
Conjugate base
Acetic acid, CH3COOH
Ka = 1.8 x 10-5
Acetate ion, CH3COOKb = 5.6 x 10-10
CALCULATING [H+] OR pH OF AN
WEAK ACID OR BASE AT
EQUILIBRIUM
• Calculate the pH of a 0.50 M HF solution at
25oC. The Ka for HF is 7.1 x 10-4