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Chapter 9
Charge-Transfer
Reactions:
Acids and Bases and
Oxidation-Reduction
9.1 Acids and Bases
• Acids: Taste sour, dissolve some metals, cause
plant dye to change color
• Bases: Taste bitter, are slippery, are corrosive.
• Two theories that help us to understand the
chemistry of acids and bases.
1. Arrhenius Theory
2. Brønsted-Lowry Theory
Arrhenius Theory of Acids and Bases
• Acid - a substance, when dissolved in water, dissociates to produce hydrogen ions
– Hydrogen ion: H+ also called “protons”
HCl is an acid:
HCl(aq)  H+(aq) + Cl-(aq)
• Base - a substance, when dissolved in water,
dissociates to produce hydroxide ions.
NaOH is a base
NaOH(aq)  Na+(aq) + OH-(aq)
• Where does NH3 fit? When it dissolves in water it
is basic but it does not have OH- ions in it.
Brønsted-Lowry Theory of Acids and Bases
• Acid - proton donor
• Base - proton acceptor
Notice that it is not defined using water.
• When writing the reactions, both accepting and
donation are evident.
HCl(aq) + H2O(l)  Cl-(aq) + H3O+(aq)
acid
base
• Now let us look at NH3 and see why it is a base.
NH3(aq) + H2O(l)
NH4+(aq) + OH-(aq)
base
acid
Conjugate Acids and Bases
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• The acid base reaction can be written in the general
form:
HA + B
A- + HB+
acid
base
base
acid
• Notice the reversible arrows.
• The products are also an acid and base called the
conjugate acid and base.
• Conjugate Acid - what the base becomes after it
accepts a proton. Conjugate Base - what the acid
becomes after it donates its proton.
• Conjugate Acid-Base Pair - The acid and base on
the opposite sides of the equation.
Strong and Weak Acids
• The reversible arrow isn’t always written. Some
acids or bases essentially dissociate 100% and a one
way arrow is used.
• Example: HCl + H2O  Cl- + H3O+
• HCl is called a strong acid-an acid that dissociates
100%
• Weak acid - one which does not dissociate 100%.
9.2 Solutions of Acids and Bases
Strength of Acids and Bases
• Acid and base strength - degree of dissociation
– Not a measure of concentration, different thing
• Strong acids and bases - reaction with water is
virtually 100% (Strong electrolytes)
• Strong Acids:
– HCl, HBr, HI
– HNO3
– H2SO4
Hydrochloric Acid, etc.
Nitric Acid
Sulfuric Acid
• Strong Bases:
– NaOH, KOH, Ba(OH)2 (all are metal hydroxides)
• Weak acids and bases - only a small percent
dissociates. (Weak electrolytes)
• Weak acid example:
– Acetic acid:
CH3COOH(aq) + H2O(l)
CH3COO-(aq) + H3O+(aq)
• Weak base example:
– Ammonia:
NH3(aq) + H2O(l)
NH4+(aq) + OH-(aq)
The Dissociation of Water
• Pure water is virtually 100% molecular.
• Very small number of molecules dissociate
– Dissociation of acids and bases is often called
ionization.
H2O(l)
H+(aq) + OH-(aq)
• Called autoionization. Very weak electrolyte.
• H+ is called the hydrogen ion. In pure water at
room temperature:
[H+] = 1 x 10-7 M
[OH-] = 1 x 10-7 M
• Therefore the equilibrium expression for:
H2O(l)
H+(aq) + OH-(aq)

Keq  [H ][OH ]
-
• Remember, liquids are not included.
• This constant is called the ion product for water and
has the symbol Kw
• Since [H+] = [OH-] = 1.0 x 10-7 M, what is the
value for Kw? 1.0 x 10-14.
– Remember, it is without units.
The pH Scale
• pH scale - a scale that indicates the acidity or
alkalinity of a solution.
– Ranges from 0 (very acidic) to 14 (very basic)
• As we do the problems, keep in mind that since
1 x 10-14 = [H+][OH-],
– if we know one concentration, can calculate the other,
– if add an acid, [H+]  and [OH-] 
– if add a base, [OH-]  and [H+] 
• The pH of a solution is defined as:
pH = -log[H+]
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9.3 Reactions between Acids and Bases
• Neutralization reaction - the reaction of an acid
with a base to produce a salt and water.
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HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
• An analytical technique to determine the
concentration of an acid or base is the titration.
• Titration involves the addition of measured amount
of a standard solution (solution of known
concentration) to neutralize the second, unknown
solution.
• The equivalence point is when the moles H+ and
OH- are equal.
Polyprotic Substances
• The previous examples have the acid and base at a
1:1 combining ratio.
– Not all acid-bases do this.
• Polyprotic substance - donates or accepts more
than one proton per formula unit.
H2SO4(aq) + 2NaOH(aq)  Na2SO4(aq) + 2H2O(l)
• Other polyprotics: Nitric Acid, Sulfuric Acid, and
Phosphoric Acid.
9.4 Acid-Base Buffers
• Buffer solution - solution which resists large
changes in pH when either acids or bases are
added.
• The Buffer Process
• Buffers consist of either
– a weak acid and its salt or
– a weak base and its salt
• Examples:
– Acetic acid (CH3COOH) with sodium acetate
(CH3COONa).
– An equilibrium is established in solution between the
acid and the salt anion.
CH3COOH(aq) + H2O(l)
CH3COO-(aq) + H+(aq)
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Addition of Base (OH-) to a Buffer Solution.
CH3COOH(aq) + H2O(l)
CH3COO-(aq) + H+(aq)
• The OH- will react with the H+, removing it from
the above equilibrium.
• Which way will the equilibrium shift? To the right.
Addition of Acid (H+) to a Buffer solution.
• The acid increases the concentration of H+.
• Which way will the equilibrium shift? To the left.
• Buffer Capacity - a measure of the ability of a
solution to resist large changes in pH when a
strong acid or strong base is added.
Preparation of a Buffer Solution
CH3COOH(aq) + H2O(l)

CH3COO-(aq) + H+(aq)
-
[H 3O ][CH3COO ]
Ka 
[CH3COOH]
9.5 Oxidation-Reduction Reactions
• Oxidation - defined by one of the following
– loss of electrons
– loss of hydrogen atoms
– gain of oxygen atoms
• Example: NaNa+ + e– Oxidation half of the reaction
• Reduction - defined by one of the following:
– gain of electrons
– gain of hydrogen
– loss of oxygen
• Example: Cl + e-  Cl– Reduction half of the reaction
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Na
 Na+ + eCl + e-  ClNa + Cl Na+ + ClReducing Agent
• Is oxidized
• Loses electrons
• Causes
reduction
Oxidizing Agent
• Is reduced
• Gains electrons
• Causes
oxidation
Applications of Oxidation and Reduction
• Corrosion - the deterioration of metals caused by
an oxidation-reduction process.
– Example: rust (oxidation of iron)
4Fe(s) + 3O2(g)  2Fe2O3(s)
• Combustion of Fossil Fuels
– Example: natural gas (methane) furnaces.
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
• Bleaching - Most bleaching agents are oxidizing
agents. The oxidation of the stains produces
compounds that do not have color.
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