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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

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Nitrogen compounds in bat urine can decompose and release ammonia into the air.

CHEMISTRY & YOU

Why are high levels of ammonia harmful to you?

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Arrhenius Acids and Bases

How did Arrhenius define an acid and a base?

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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.

Arrhenius Acids and Bases

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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.

Arrhenius Acids and Bases

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In 1887, the Swedish chemist Svante Arrhenius proposed a new way of defining and thinking about acids and bases.

According to Arrhenius, acids are hydrogen-containing compounds that ionize to yield hydrogen ions (H⁺) in solution. Bases are compounds that ionize to yield hydroxide ions (OH^–) in aqueous solution.

Arrhenius Acids and Bases

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Arrhenius Acids

Some Common Acids
Name	Formula
Hydrochloric acid	HCl
Nitric acid	HNO₃
Sulfuric acid	H₂SO₄
Phosphoric acid	H₃PO₄
Ethanoic acid	CH₃COOH
Carbonic acid	H₂CO₃

Arrhenius Acids and Bases

Acids vary in the number of hydrogens they contain that can form hydrogen ions.

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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.

Arrhenius Acids

Arrhenius Acids and Bases

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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.

Arrhenius Acids

Arrhenius Acids and Bases

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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.

Arrhenius Acids

Arrhenius Acids and Bases

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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.

Arrhenius Acids

Arrhenius Acids and Bases

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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.

Arrhenius Acids

Arrhenius Acids and Bases

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Ethanoic acid (CH₃COOH), which is commonly called acetic acid, is an example of a molecule that contains both hydrogens that do not ionize and a hydrogen that does ionize.

Although its molecules contain four hydrogens, ethanoic acid is a monoprotic acid.

Arrhenius Acids

Arrhenius Acids and Bases

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The three hydrogens attached to a carbon atom are in weakly polar bonds.

They do not ionize.

Only the hydrogen bonded to the highly electronegative oxygen can be ionized.

Arrhenius Acids

Arrhenius Acids and Bases

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Arrhenius Bases

Some Common Bases
Name	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

Arrhenius Acids and Bases

The table below lists four common bases.

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The base sodium hydroxide (NaOH) is known as lye.

Sodium hydroxide is an ionic solid.
It dissociates into sodium ions in aqueous solution.

Arrhenius Bases

Arrhenius Acids and Bases

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The base sodium hydroxide (NaOH) is known as lye.

Arrhenius Bases

Arrhenius Acids and Bases

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.

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Potassium hydroxide (KOH) is another ionic solid.

It dissociates to produce potassium ions and hydroxide ions in aqueous solution.

Arrhenius Bases

Arrhenius Acids and Bases

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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.

Arrhenius Bases

Arrhenius Acids and Bases

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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, slow-healing wounds if not immediately washed off.

Arrhenius Bases

Arrhenius Acids and Bases

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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?

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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.

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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.

Arrhenius Bases

Arrhenius Acids and Bases

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Can every hydrogen from every molecule form hydrogen ions, therefore acting as an Arrhenius acid?

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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.

Can every hydrogen from every molecule form hydrogen ions, therefore acting as an Arrhenius acid?

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Brønsted-Lowry Acids and Bases

What distinguishes an acid from a base in the Brønsted-Lowry theory?

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Sodium carbonate (Na₂CO₃) and ammonia (NH₃) act as bases when they form aqueous solutions.

Neither of these compounds is a hydroxide-containing compound, so neither would be classified as a base by the Arrhenius definition.

Brønsted-Lowry Acids and Bases

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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.

Brønsted-Lowry Acids and Bases

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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.

Brønsted-Lowry Acids and Bases

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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.

Brønsted-Lowry Acids and Bases

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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.

Brønsted-Lowry Acids and Bases

Ammonia is a Brønsted-Lowry base because it accepts hydrogen ions.
Water is a Brønsted-Lowry acid because it donates hydrogen ions.

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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.

Conjugate Acids and Bases

Brønsted-Lowry Acids and Bases

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In essence, the reversible reaction of ammonia and water has two acids and two bases.

Brønsted-Lowry Acids and Bases

Conjugate Acids and Bases

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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.

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Conjugate Acids and Bases

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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.

Brønsted-Lowry Acids and Bases

Conjugate Acids and Bases

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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.

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Conjugate Acids and Bases

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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.

Brønsted-Lowry Acids and Bases

Conjugate Acids and Bases

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Brønsted-Lowry Acids and Bases

Conjugate Acids and Bases

In this reaction, hydrogen chloride is the hydrogen-ion 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.

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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 acid-base pairs.

Brønsted-Lowry Acids and Bases

Conjugate Acids and Bases

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Interpret Data

Some Conjugate Acid-Base Pairs
Acid	Base
HCl	Cl^–
H₂SO₄	HSO₄^–
H₃O⁺	H₂O
HSO₄^–	SO₄²^–
CH₃COOH	CH₃COO^–
H₂CO₃	HCO₃⁻
HCO₃^–	CO₃²^–
NH₄⁺	NH₃
H₂O	OH^–

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Note that water appears in both the list of acids and the list of bases.

Sometimes water accepts a hydrogen ion.
At other times, it donates a hydrogen ion.
How water behaves depends on the other reactant.

Amphoteric Substances

Brønsted-Lowry Acids and Bases

Some Conjugate Acid-Base Pairs
Acid	Base
HCl	Cl^–
H₂SO₄	HSO₄^–
H₃O⁺	H₂O
HSO₄^–	SO₄²^–
CH₃COOH	CH₃COO^–
H₂CO₃	HCO₃⁻
HCO₃^–	CO₃²^–
NH₄⁺	NH₃
H₂O	OH^–

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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.

Brønsted-Lowry Acids and Bases

Amphoteric Substances

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How can one substance, such as water, be both an acid and a base, according to the Brønsted-Lowry definition?

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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.

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Lewis Acids and Bases

How did Lewis define an acid and a base?

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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.

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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.

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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.

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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.

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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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The Lewis definition is the broadest.
It extends to compounds that the Brønsted-Lowry theory does not classify as acids and bases.

Acid-Base Definitions
Type	Acid	Base
Arrhenius	H⁺ producer	OH^–producer
Brønsted-Lowry	H⁺ donor	H⁺ acceptor
Lewis	electron-pair acceptor	electron-pair donor

This table compares the definitions of acids and bases.

Interpret Data

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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

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When a Lewis acid reacts with a Lewis base, the base donates a pair of electrons and the acid accepts the donated pair.

Analyze Identify the relevant concepts.

1

Sample Problem 19.1

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Solve Apply concepts to this problem.

2

Identify the reactant with the unshared pair of electrons and the reactant that can accept the pair of electrons.

Ammonia has an unshared pair of electrons to donate.
The boron atom can accept the donated electrons.

Sample Problem 19.1

Draw electron dot structures to identify which reactant has an unshared pair of electrons.

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Solve Apply concepts to this problem.

2

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.

Sample Problem 19.1

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Are hydrogen-ion donors also electron-pair acceptors?

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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.

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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.

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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

Glossary Terms

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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 acid-base 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

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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.

BIG IDEA

Reactions

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