Name
CHAPTER 9
Class
Date
Acids, Bases, and Salts
SECTION
21 Reactions of Acids with Bases
KEY IDEAS
As you read this section, keep these questions in mind:
• What happens when you combine an acid with a base?
• In what ways can you use salts?
What Is an Acid-Base Reaction?
Have you ever used an antacid to feel better when you
had an upset stomach? You can get an upset stomach if
the acid in your stomach irritates your esophagus. An
antacid contains a base. Taking an antacid reduces the
amount of acid in your stomach.
The reaction between an acid and a base is called a
neutralization reaction. Neutralization reactions have
many uses.
REACTIONS BETWEEN IONS
READING TOOLBOX
Summarize Before you read,
copy the three equations on
this page onto a large sheet
of paper. As you read, label
the hydronium ions, the
hydroxide ions, and spectator
ions. Indicate which parts of
the third equation are from a
base and which are from an
acid.
Recall that a strong acid, such as hydrochloric acid,
ionizes completely when it is dissolved in water, as
shown below:
HCl + H2O H3O+ + Cl−
A strong base, such as sodium hydroxide, also ionizes
completely in water, as shown here:
NaOH Na+ + OH−
EHHDBG@<EHL>K
If the acid and base are mixed together, a neutralization reaction occurs, as shown below:
1. Identify What uncharged
product results from a
neutralization reaction?
H3O+ + Cl− + Na+ + OH− Na+ + Cl− + 2H2O
A chemical reaction occurs between the hydronium
ions and hydroxide ions to form water molecules. In this
case, the Na+ and Cl– are spectator ions. Spectator ions
do not change during the reaction between H3O+ and
OH− ions. If you combine equal amounts and concentrations of a strong acid and a strong base, the H3O+ and
OH− ions react to form H2O. Thus, the solution that
results is neutral.
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Acids, Bases, and Salts
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SECTION 2
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Date
Reactions of Acids with Bases continued
FORMATION OF SALTS
If you remove the spectator ions from the equation for
a neutralization reaction, you will have the following:
H3O+ + OH− 2 H2O
READING CHECK
2. Explain What happens
to the spectator ions in a
neutralization reaction?
The spectator ions do not chemically react or change
their identity. However, they do combine to form a salt.
A salt is an ionic compound made of a positive ion from
the base (Na+) and the negative ion from the acid (Cl−), as
shown below:
Na+ + Cl− NaCl
Examine the figure below. The middle beaker looks
like the others, even though the neutralization reaction
has produced new compounds. This is because the salt
stays dissolved in the water. However, when the water
evaporates, the salt will remain in the beaker as a solid.
Chloride
ion, Cl–
Hydronium
ion, H3O+
Sodium
ion, Na+
Water molecule,
H2O
Chloride
ion, Cl–
Hydroxide
ion, OH¯
Sodium
ion, Na+
Water
molecule,
H2O
EHHDBG@<EHL>K
3. Explain The center beaker
contains the products of a
neutralization reaction. Why
do you not see the salt that
has formed?
What Are Some Common Salts?
When you hear the word salt, you probably think of
the salt you sprinkle on food. However, to a scientist, a
salt can be almost any combination of a cation and an
anion. The table on the next page gives some examples
of salts.
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Reactions of Acids with Bases continued
Some Common Salts
Salt
Formula
Uses
Aluminum sulfate
Al2(SO4)3
purifying water; used in
antiperspirants
Ammonium sulfate
(NH4)2SO4
flameproofing fabrics;
used as fertilizer
Barium sulfate
BaSO4
used to diagnose some
medical conditions
Calcium chloride
CaCl2
used to de-ice streets
and highways; used in
some kinds of concrete
Potassium chloride
KCl
treating potassium
deficiency; used as
substitute for table salt
Sodium carbonate
Na2CO3
making glass; added to
wash to soften water
Sodium chloride
NaCl
flavoring food
Sodium hydrogen
carbonate
NaHCO3
treating upset stomach;
ingredient in baking
powder; used in fire
extinguishers
Sodium stearate
NaOOCC17H34
used as soap; used in
deodorants
Sodium lauryl sulfonate
NaSO3C12H25
used as detergent
EHHDBG@<EHL>K
4. Identify Give three common salts that you may find
in foods.
All of these products
contain salts.
Like all animals, we need elements such as potassium,
sodium, calcium, magnesium, iron, phosphorus, and
iodine to stay healthy. However, many of these nutrients
are harmful in their elemental forms. You get the ions of
these elements from minerals. Minerals contain the salts
that you need.
Salts that contain calcium are important for your
teeth and bones. Your nerves and muscles need calcium,
potassium, and sodium ions in order to work properly.
Sodium and potassium salts help balance the amount of
water that goes in and out of your cells.
READING CHECK
5. Explain Why do we get
the many of the nutrients we
need from salts rather than
from elemental forms?
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Reactions of Acids with Bases continued
Why Don’t Some Neutralization Reactions
Produce Neutral Solutions?
READING CHECK
6. Identify What two factors
determine the pH of solution
formed in an acid-base
reaction?
Reactions between acids and bases do not always
produce neutral solutions. The pH of the final solution
depends on how much acid and base you combine.
The pH also depends on the strength of the acid and
base you use.
Recall that weak acids and bases do not ionize completely and that strong acids and bases do ionize completely. If a strong acid reacts with an equal amount of
a weak base, the solution will be acidic. If a weak acid
reacts with an equal amount of a strong base, the solution
will be basic.
How Can You Use Acid-Base Reactions?
READING CHECK
7. Define What is a titration?
Suppose you were given a solution of sodium
hydroxide, but you didn’t know its concentration. How
could you find out? One way would be to carry out a
titration. A titration is a process in which you add
carefully measured amounts of one solution to another
solution. In an acid-base titration, the concentration of
one solution is generally known. The concentration of
the other solution is unknown.
Think about how pH changes as an acid and a base
mix. For example, the graph below shows how the pH of
a solution of HCl changed as potassium hydroxide (KOH)
was added to it. Hydrochloric acid, HCl, was titrated with
potassium hydroxide, KOH.
Moles of KOH Added vs. pH
12
10
Graphing Skills
8
pH
8. Identify What is the initial
pH of the solution before
KOH is added?
6
4
9. Analyze Does the
solution’s acidity increase or
decrease as KOH is added?
2
0
0
0.2
0.4
0.6
KOH added (mol)
0.8
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Reactions of Acids with Bases continued
INTERPRETING A TITRATION GRAPH
As KOH was added to the acid solution, the pH
increased slowly. Then, when 0.4 mol of KOH had been
added, the pH increased very quickly. The pH at that
point is equal to the pH at the center of the vertical line:
about 7. When more KOH was added, the pH rose above 7.
Recall that a pH below 7 indicates an acidic solution.
Therefore, when fewer than 0.4 mol of KOH had been
added to the solution, the solution was acidic. This shows
that not all of the HCl reacted with the KOH. When more
than 0.4 mol of KOH was added, the pH was above 7. This
shows that the solution was basic. In other words, there
was no HCl left in the solution to react with the KOH.
Remember that when a strong acid reacts with an
equal amount of a strong base the resulting solution has
a pH of 7. It took 0.4 mol of KOH to produce a solution
with a neutral pH. Therefore, there must have been 0.4
mol of HCl in the original acid solution.
The point at which there is no extra acid or base in the
mixture is called the equivalence point of the titration. In
the example on the previous page, the equivalence point
occurred when 0.4 mol of KOH were added to the HCl. At
that point, all of the HCl had reacted with the KOH, but
there was no extra KOH in the solution. How many moles
of KOH were added to reach the equivalence point? Use
the steps below to figure it out:
Step 1: Locate the equivalence point
on the graph.
A strong acid was titrated with a
strong base. The y-axis indicated
pH, so the equivalence point in the
titration curve has a y-value of 7.
Step 2: Read the moles of KOH from
the graph.
The x-axis indicates how many moles
of KOH were added. At pH = 7, 0.4
mol of KOH was added.
8g^i^XVa I]^c`^c\
10. Apply Concepts When
enough KOH had been
added to the HCl solution
to create a neutral solution,
what were the components
of the solution?
READING CHECK
11. Define What is the
equivalence point of a
titration?
The pH of the mixture at the equivalence point
depends on which acids and bases are reacting. If a
strong acid reacts with a strong base, the pH at the
equivalence point will be 7. If a strong acid reacts with a
weak base, the pH at the equivalence point will be less
than 7. If a weak acid reacts with a strong base, the pH at
the equivalence point will be greater than 7.
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Section 2 Review
SECTION VOCABULARY
neutralization reaction the reaction of the
ions that characterize acids and the ions that
characterize bases to form water molecules
and a salt
salt an ionic compound that forms when a metal
atom or a positive radical replaces the hydrogen of an acid
1. Compare How does the everyday definition of salt differ from the one used
by scientists?
2. Identify What two types of compounds are produced by a neutralization reaction?
3. Identify In the reaction described by the equation below, what are the
spectator ions?
H3O+ + Br− + Li+ + OH− Li+ + Br− + 2H2O
4. Predict Suppose you combine a weak acid with an equal amount of a strong base.
Will the final solution be acidic or basic?
5. Write Equations Write two equations for the neutralization of nitric acid, HNO3,
with magnesium hydroxide, Mg(OH)2. In the first equation, include the spectator
ions. In the second, do not include the spectator ions.
6. Predict If you react equal amounts of sulfuric acid, H2SO4, and ammonia, NH3, will
the solution that results be acidic, basic, or neutral? Explain your answer.
7. Identify What are two ways calcium salts are important for your health?
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