19.1 Acid-Base Theories>
Chapter 19
Acids, Bases, and Salts
19.1 Acid-Base Theories
19.2 Hydrogen Ions and Acidity
19.3 Strengths of Acids and Bases
19.4 Neutralization Reactions
19.5 Salts in Solution
1
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19.1 Acid-Base Theories>
CHEMISTRY
& YOU
Why are high levels of ammonia harmful
to you?
Nitrogen compounds
in bat urine can
decompose and
release ammonia into
the air.
2
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Acids and Bases
How did Arrhenius define an acid and
a base?
3
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Acids and bases have distinct properties.
• Acids give foods a tart or sour taste.
• Aqueous solutions of acids are strong or
weak electrolytes.
• Acids cause certain dyes, called indicators,
to change color.
• Many metals, such as zinc and magnesium,
react with aqueous solutions of acids to
produce hydrogen gas.
4
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Acids and bases have distinct properties.
• Soap is a familiar material that has the
properties of a base.
– The bitter taste is a general property of bases.
– The slippery feel of soap is another property of
bases.
• Bases will cause an indicator to change color.
• Bases also form aqueous solutions that are
strong or weak electrolytes.
5
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
In 1887, the Swedish chemist Svante
Arrhenius proposed a new way of defining
and thinking about acids and bases.
According to Arrhenius, acids are hydrogencontaining compounds that ionize to yield
hydrogen ions (H+) in solution. Bases are
compounds that ionize to yield hydroxide
ions (OH–) in aqueous solution.
6
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Acids
Acids vary in the number of hydrogens they
contain that can form hydrogen ions.
Some Common Acids
Name
Formula
Hydrochloric acid
Nitric acid
Sulfuric acid
Phosphoric acid
Ethanoic acid
Carbonic acid
7
HCl
HNO3
H2SO4
H3PO4
CH3COOH
H2CO3
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Acids
A hydrogen atom that can form a hydrogen
ion is described as ionizable.
• Nitric acid (HNO3) has one ionizable hydrogen.
Nitric acid is classified as a monoprotic acid.
– The prefix mono- means “one,” and the stem protic
reflects the fact that a hydrogen ion is a proton.
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Acids
A hydrogen atom that can form a hydrogen
ion is described as ionizable.
• Nitric acid (HNO3) has one ionizable hydrogen.
Nitric acid is classified as a monoprotic acid.
• Acids that contain two ionizable hydrogens,
such as sulfuric acid (H2SO4), are called
diprotic acids.
• Acids that contain three ionizable hydrogens,
such as phosphoric acid (H3PO4), are called
triprotic acids.
9
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Acids
Not all compounds that contain hydrogen are
acids.
• Only a hydrogen that is bonded to a very
electronegative element can be released as
an ion. Such bonds are highly polar.
• When a compound that contains such bonds
dissolves in water, it releases hydrogen ions.
10
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Acids
In an aqueous solution, hydrogen ions are
not present. Instead, the hydrogen ions are
joined to water molecules as hydronium ions.
• A hydronium ion (H3O+) is the ion that forms
when a water molecule gains a hydrogen ion.
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Acids
Methane (CH4) is an example of a
hydrogen-containing compound that is not
an acid.
• The four hydrogen atoms in methane are
attached to the central carbon atom by
weakly polar C—H bonds.
• Methane has no ionizable hydrogens and is
not an acid.
12
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Bases
The table below lists four common bases.
Name
13
Some Common Bases
Formula
Solubility in Water
Sodium hydroxide
NaOH
High
Potassium hydroxide
KOH
High
Calcium hydroxide
Ca(OH)2
Very low
Magnesium hydroxide
Mg(OH)2
Very low
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Bases
The base sodium hydroxide
(NaOH) is known as lye.
• Sodium hydroxide is an ionic solid.
• It dissociates into sodium ions in aqueous
solution.
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Bases
The base sodium hydroxide
(NaOH) is known as lye.
• Sodium hydroxide is extremely
caustic.
– A caustic substance can burn or eat away materials
with which it comes in contact.
– This property is the reason that sodium hydroxide is a
major component of products that are used to clean
clogged drains.
15
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Bases
Potassium hydroxide (KOH) is another ionic
solid.
• It dissociates to produce potassium ions and
hydroxide ions in aqueous solution.
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Bases
Sodium and potassium are Group 1A
elements. Elements in Group 1A, the alkali
metals, react violently with water.
• The products of these reactions are aqueous
solutions of a hydroxide and a hydrogen gas.
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Bases
Sodium hydroxide and potassium hydroxide
are very soluble in water.
• The solutions would typically have the bitter
taste and slippery feel of a base, but you
would not want to test these properties.
• The solutions are extremely caustic to the
skin. They can cause deep, painful, slowhealing wounds if not immediately washed
off.
18
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19.1 Acid-Base Theories>
CHEMISTRY
& YOU
Visitors to Bracken Cave wear protective
gear to keep ammonia gas out of their
eyes and respiratory tracts. Think about
the properties of bases. Why are high
levels of ammonia harmful?
19
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19.1 Acid-Base Theories>
CHEMISTRY
& YOU
Visitors to Bracken Cave wear protective
gear to keep ammonia gas out of their
eyes and respiratory tracts. Think about
the properties of bases. Why are high
levels of ammonia harmful?
Ammonia is a base,
and bases are
caustic in high
concentrations.
20
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19.1 Acid-Base Theories> Arrhenius Acids and Bases
Arrhenius Bases
Calcium hydroxide, Ca(OH)2, and
magnesium hydroxide, Mg(OH)2, are
compounds of Group 2A metals.
• These compounds are not very
soluble in water. Their solutions are always
very dilute, even when saturated.
– The low solubility of magnesium
hydroxide makes the suspension safe
to consume.
– Some people use this suspension as an antacid.
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19.1 Acid-Base Theories>
Can every hydrogen from every
molecule form hydrogen ions,
therefore acting as an Arrhenius
acid?
22
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19.1 Acid-Base Theories>
Can every hydrogen from every
molecule form hydrogen ions,
therefore acting as an Arrhenius
acid?
No. Only hydrogens that are bonded to a
very electronegative element can be
released as ions. That means that only
molecules containing hydrogens bonded to
very electronegative elements are
Arrhenius acids.
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Brønsted-Lowry Acids and Bases
What distinguishes an acid from a
base in the Brønsted-Lowry theory?
24
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Sodium carbonate (Na2CO3) and ammonia
(NH3) act as bases when they form
aqueous solutions.
• Neither of these compounds is a hydroxidecontaining compound, so neither would be
classified as a base by the Arrhenius
definition.
25
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
In 1923, the Danish chemist Johannes
Brønsted and the English chemist Thomas
Lowry were working independently.
• Each chemist proposed the same definition
of acids and bases.
26
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
According to the Brønsted-Lowry
theory, an acid is a hydrogen-ion donor
and a base is a hydrogen-ion acceptor.
• This theory includes all the acids and
bases that Arrhenius defined.
• It also includes some compounds that
Arrhenius did not classify as bases.
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
You can use the Brønsted-Lowry theory to
understand why ammonia is a base.
• When ammonia dissolves in water, hydrogen
ions are transferred from water to ammonia
to form ammonium ions and hydroxide ions.
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
You can use the Brønsted-Lowry theory to
understand why ammonia is a base.
• When ammonia dissolves in water, hydrogen
ions are transferred from water to ammonia
to form ammonium ions and hydroxide ions.
– Ammonia is a Brønsted-Lowry base because it
accepts hydrogen ions.
29
– Water is a Brønsted-Lowry acid because it
donates hydrogen ions.
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Conjugate Acids and Bases
When the temperature of an aqueous
solution of ammonia is increased, ammonia
gas is released.
• HNH4+ reacts with OH– to form more NH3 and H2O.
• In the reverse reaction, ammonium ions donate
hydrogen ions to hydroxide ions.
• NH4+ (the donor) acts as a Brønsted-Lowry acid, and
OH− (the acceptor) acts as a Brønsted-Lowry base.
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Conjugate Acids and Bases
In essence, the reversible reaction of
ammonia and water has two acids and two
bases.
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Conjugate Acids and Bases
A conjugate acid is the ion or molecule
formed when a base gains a hydrogen ion.
• NH4+ is the conjugate acid of the base NH3.
32
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Conjugate Acids and Bases
A conjugate base is the ion or molecule
that remains after an acid loses a hydrogen
ion.
• OH– is the conjugate base of the acid H2O.
33
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Conjugate Acids and Bases
Conjugate acids are always paired with a
base, and conjugate bases are always
paired with an acid.
• A conjugate acid-base pair consists of two
ions or molecules related by the loss or gain
of one hydrogen ion.
34
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Conjugate Acids and Bases
• The ammonia molecule and the
ammonium ion are a conjugate acid-base
pair.
• The water molecule and the hydroxide ion
are also a conjugate acid-base pair.
35
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Conjugate Acids and Bases
In this reaction, hydrogen chloride is the hydrogenion donor and is by definition a Brønsted-Lowry
acid. Water is the hydrogen-ion acceptor and a
Brønsted-Lowry base.
• The chloride ion is the conjugate base of the acid HCl.
• The hydronium ion is the conjugate acid of the water
base.
36
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Conjugate Acids and Bases
The figure below shows the reaction that takes
place when sulfuric acid dissolves in water.
• The products are hydronium ions and hydrogen
sulfate ions.
• Use the figure to identify the two conjugate acidbase pairs.
37
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19.1 Acid-Base Theories>
Interpret Data
Some Conjugate
Acid-Base Pairs
Acid
Base
HCl
H2SO4
H3O+
HSO4–
CH3COOH
H2CO3
HCO3–
NH4+
H2O
38
Cl–
HSO4–
H2O
SO42–
CH3COO–
HCO3−
CO32–
NH3
OH–
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Amphoteric Substances
Note that water appears
in both the list of acids
and the list of bases.
Some Conjugate
Acid-Base Pairs
Acid
Base
HCl
H2SO4
H3O+
• Sometimes water
accepts a hydrogen ion. HSO4–
CH3COOH
• At other times, it
H2CO3
donates a hydrogen ion.
• How water behaves
depends on the other
reactant.
39
HCO3–
NH4+
H2O
Cl–
HSO4–
H2O
SO42–
CH3COO–
HCO3−
CO32–
NH3
OH–
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19.1 Acid-Base Theories> Brønsted-Lowry Acids and
Bases
Amphoteric Substances
A substance that can act as either an acid
or a base is said to be amphoteric.
• Water is amphoteric.
– In the reaction with hydrochloric acid,
water accepts a proton and is therefore a
base.
– In the reaction with ammonia, water
donates a proton and is therefore an acid.
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19.1 Acid-Base Theories>
How can one substance, such as
water, be both an acid and a base,
according to the Brønsted-Lowry
definition?
41
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19.1 Acid-Base Theories>
How can one substance, such as
water, be both an acid and a base,
according to the Brønsted-Lowry
definition?
Because water can act as both a
hydrogen-ion donator and a hydrogen-ion
acceptor, it can act as both an acid and a
base according to the Brønsted-Lowry
definition.
42
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19.1 Acid-Base Theories> Lewis Acids and Bases
Lewis Acids and Bases
How did Lewis define an acid and a
base?
43
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19.1 Acid-Base Theories> Lewis Acids and Bases
According to Gilbert Lewis, an acid
accepts a pair of electrons and a base
donates a pair of electrons during a
reaction.
• This definition is more general than
those offered by Arrhenius or by
Brønsted and Lowry.
44
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19.1 Acid-Base Theories> Lewis Acids and 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 form a
covalent bond.
– The Lewis definitions include all the
Brønsted-Lowry acids and bases.
45
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19.1 Acid-Base Theories> Lewis Acids and Bases
Consider the reaction of H+ and OH–.
• The hydrogen ion donates itself to the
hydroxide ion.
– H+ is a Brønsted-Lowry acid, and OH− is a
Brønsted-Lowry base.
46
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19.1 Acid-Base Theories> Lewis Acids and Bases
Consider the reaction of H+ and OH–.
• The hydroxide ion can bond to the hydrogen
ion because it has an unshared pair of
electrons.
– OH− is also a Lewis base, and H+, which
accepts the pair of electrons, is a Lewis
acid.
47
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19.1 Acid-Base Theories> Lewis Acids and Bases
A second example of a reaction between a
Lewis acid and a Lewis base is what
happens when ammonia dissolves in water.
• Hydrogen ions from the dissociation of water
are the electron-pair acceptor and the Lewis
acid.
• Ammonia is the electron-pair donor and the
Lewis base.
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19.1 Acid-Base Theories>
Interpret Data
This table compares the definitions of acids
and bases.
Type
Arrhenius
Acid-Base Definitions
Acid
Base
H+ producer
OH– producer
Brønsted-Lowry H+ donor
H+ acceptor
Lewis
electron-pair acceptor electron-pair donor
• The Lewis definition is the broadest.
• It extends to compounds that the Brønsted-Lowry
theory does not classify as acids and bases.
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19.1 Acid-Base Theories>
Sample Problem 19.1
Identifying Lewis Acids and Bases
Identify the Lewis acid and
the Lewis base in this reaction
between ammonia and boron
trifluoride.
NH3 + BF3 → NH3BF3
50
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19.1 Acid-Base Theories>
Sample Problem 19.1
1 Analyze Identify the relevant concepts.
When a Lewis acid reacts with a
Lewis base, the base donates a pair
of electrons and the acid accepts the
donated pair.
51
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19.1 Acid-Base Theories>
Sample Problem 19.1
2 Solve Apply concepts to this problem.
Identify the reactant with the unshared
pair of electrons and the reactant that
can accept the pair of electrons.
Draw electron dot
structures to identify which
reactant has an unshared
pair of electrons.
52
• Ammonia has an unshared pair of
electrons to donate.
• The boron atom can accept the donated
electrons.
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19.1 Acid-Base Theories>
Sample Problem 19.1
2 Solve Apply concepts to this problem.
Classify the reactants based on their
behavior.
• Lewis bases donate a pair of electrons, so
ammonia is the Lewis base.
• Lewis acids accept a pair of electrons, so
boron trifluoride is the Lewis acid.
53
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19.1 Acid-Base Theories>
Are hydrogen-ion donors also
electron-pair acceptors?
54
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19.1 Acid-Base Theories>
Are hydrogen-ion donors also
electron-pair acceptors?
Yes. All substances defined as acids by the
Brønsted-Lowry definition (an acid is a
hydrogen-ion donor) are also defined as
acids by the Lewis definition (an acid is an
electron-pair acceptor). That means that
these substances are both hydrogen-ion
donors and electron-pair acceptors.
55
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19.1 Acid-Base Theories> Key Concepts
According to Arrhenius, acids are
hydrogen-containing compounds that
ionize to yield hydrogen ions in aqueous
solution. Bases are compounds that ionize
to yield hydroxide ions in aqueous solution.
According to Brønsted-Lowry theory, an
acid is a hydrogen-ion donor and a base is
a hydrogen-ion acceptor.
According to Lewis, an acid accepts a pair
of electrons and a base donates a pair of
electrons.
56
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19.1 Acid-Base Theories> Glossary Terms
• hydronium ion (H3O+): the positive ion
formed when a water molecule gains a
hydrogen ion
• conjugate acid: the particle formed when a
base gains a hydrogen ion; NH4+ is the
conjugate acid of the base NH3
• conjugate base: the particle that remains
when an acid has donated a hydrogen ion;
OH– is the conjugate base of the acid water
57
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19.1 Acid-Base Theories> Glossary Terms
• conjugate acid-base pair: two substances
that are related by the loss or gain of a single
hydrogen ion; ammonia (NH3) and the
ammonium ion (NH4+) are a conjugate acidbase pair
• amphoteric: a substance that can act as both
an acid and a base
• Lewis acid: any substance that can accept a
pair of electrons to form a covalent bond
• Lewis base: any substance that can donate a
pair of electrons to form a covalent bond
58
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19.1 Acid-Base Theories>
BIG IDEA
Reactions
• Chemists define acids and bases
according to the ions they yield in
aqueous solution.
• Chemists also define acids and bases
based on whether they accept or
donate hydrogen ions, and whether
they are electron-pair donors or
acceptors.
59
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19.1 Acid-Base Theories>
END OF 19.1
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