Chemical Reactions:
An Introduction
Chapter 4
Ions in Aqueous Solution
Ionic Theory of Solutions
Many ionic compounds dissociate into
independent ions when dissolved in water
H 2O


NaCl(s )  Na (aq)  Cl (aq)
These compounds that “freely” dissociate into
independent ions in aqueous solution are called
electrolytes.
Their aqueous solutions are capable of conducting
an electric current. Figure 4.2 illustrates this.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
2
Ions in Aqueous Solution
Ionic Theory of Solutions
Not all electrolytes are ionic compounds.
Some molecular compounds dissociate into
ions.


HCl(aq)  H (aq)  Cl (aq)
The resulting solution is electrically
conducting, and so we say that the molecular
substance is an electrolyte.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
3
Ions in Aqueous Solution
Ionic Theory of Solutions
Some molecular compounds dissolve but do
not dissociate into ions.
C6 H12O6 (s) (glucose)  C6 H12O6 (aq)
H 2O
These compounds are referred to as
nonelectrolytes. They dissolve in water to give
a nonconducting solution.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
4
Ions in Aqueous Solution
Ionic Theory of Solutions
Electrolytes are substances that dissolve in
water to give an electrically conducting
solution.
Thus, in general, ionic solids that dissolve in
water are electrolytes.
Some molecular compounds, such as acids,
also dissociate in aqueous solution and are
considered electrolytes..
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
5
Ions in Aqueous Solution
Ionic Theory of Solutions
Observing the electrical conductance of a
solution.
Figure 4.3 shows a simple apparatus that allows
you to observe the conductivity of a solution.
If the solution is conducting, the circuit is
complete and the bulb lights.
If the solution is nonconducting, the circuit is
incomplete and the bulb does not light.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
6
Ions in Aqueous Solution
Ionic Theory of Solutions
Strong and weak electrolytes.
A strong electrolyte is an electrolyte that exists
in solution almost entirely as ions.


NaCl(s)  Na (aq)  Cl (aq)
H 2O
Most ionic solids that dissolve in water do so
almost completely as ions, so they are strong
electrolytes.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
7
Ions in Aqueous Solution
Ionic Theory of Solutions
Strong and weak electrolytes.
A weak electrolyte is an electrolyte that
dissolves in water to give a relatively small
percentage of ions.



NH 4OH(aq)  NH4 (aq)  OH (aq)
Most soluble molecular compounds are either
nonelectrolytes or weak electrolytes.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
8
Ions in Aqueous Solution
Ionic Theory of Solutions
Strong and weak electrolytes.
Figure 4.4 illustrates the conductivity of weak
versus strong electrolytes.
Solutions of weak electrolytes contain only a
small percentage of ions. We denote this
situation by writing the equation with a double
arrow.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
9
Ions in Aqueous Solution
Ionic Theory of Solutions: Summary
In summary, substances that dissolve in water
are either electrolytes or nonelectrolytes.
Nonelectrolytes form nonconducting solutions
because they dissolve as molecules.
Electrolytes form conducting solutions because
they dissolve as ions.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
10
Ions in Aqueous Solution
Ionic Theory of Solutions: Summary
Electrolytes can be strong or weak.
Almost all ionic substances that dissolve are
strong electrolytes.
Molecular substances that dissolve are either
nonelectrolytes or weak electrolytes.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
11
Ions in Aqueous Solution
Molecular and Ionic Equations
A molecular equation is one in which the
reactants and products are written as if they
were molecules, even though they may
actually exist in solution as ions.
Ca(OH )2 (aq)  Na2CO3 (aq)  CaCO3 (s)  2NaOH(aq)
Note that Ca(OH)2, Na2CO3, and NaOH are all
soluble compounds but CaCO3 is not.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
12
Ions in Aqueous Solution
Molecular and Ionic Equations
An ionic equation, however, represents
strong electrolytes as separate independent
ions. This is a more accurate representation
of the way electrolytes behave in solution.
2

2

Ca (aq )  2OH (aq )  2Na (aq )  CO 3 (aq ) 


CaCO 3 (s )  2Na (aq )  2OH (aq )
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
13
Ions in Aqueous Solution
Molecular and Ionic Equations
Complete and net ionic equations
A complete ionic equation is a chemical
equation in which strong electrolytes (such as
soluble ionic compounds) are written as
separate ions in solution.
Ca( NO3 )2 (aq)  K 2CO3 (aq)  CaCO3 (s )  2KNO3 (aq)
(strong)
(strong)

(insoluble)
(strong)
2
Ca 2 (aq )  2NO 3 (aq )  2K  (aq )  CO 3 (aq ) 

CaCO 3 (s )  2K  (aq )  2NO 3 (aq )
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
14
Ions in Aqueous Solution
Molecular and Ionic Equations
Complete and net ionic equations.
A net ionic equation is a chemical equation
from which the spectator ions have been
removed.
A spectator ion is an ion in an ionic equation
that does not take part in the reaction.

2
Ca 2 (aq )  2NO 3 (aq )  2K  (aq )  CO 3 (aq ) 

CaCO 3 (s )  2K  (aq )  2NO 3 (aq )
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
15
Ions in Aqueous Solution
Molecular and Ionic Equations
Complete and net ionic equations
Let’s try an example. First, we start with a
molecular equation.
2HNO3 (aq)  Mg(OH )2 (s)  2H 2O(l )  Mg( NO3 )2 (aq)
Nitric acid, HNO3, and magnesium nitrate,
Mg(NO3)2, are both strong electrolytes.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
16
Ions in Aqueous Solution
Molecular and Ionic Equations
Complete and net ionic equations
Separating the strong electrolytes into separate
ions, we obtain the complete ionic equation.


2H (aq )  2NO 3 (aq )  Mg(OH )2 (s ) 
2

2H 2O(l )  Mg (aq )  2NO 3 (aq )
Note that the nitrate ions did not participate in
the reaction. These are spectator ions.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
17
Ions in Aqueous Solution
Molecular and Ionic Equations
Complete and net ionic equations
Eliminating the spectator ions results in the net
ionic equation.


2H (aq )  2NO 3 (aq )  Mg(OH )2 (s ) 

2
2H 2O(l )  Mg (aq )  2NO 3 (aq )

2
2H (aq)  Mg(OH )2 (s)  2H 2O(l )  Mg (aq)
This equation represents the “essential” reaction.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
18
Types of Chemical Reactions
Most of the reactions we will study fall into
one of the following categories
Precipitation Reactions
Acid-Base Reactions
Oxidation-Reduction Reactions
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
19
Types of Chemical Reactions
Precipitation Reactions
A precipitation reaction occurs in aqueous
solution because one product is insoluble.
A precipitate is an insoluble solid compound
formed during a chemical reaction in solution.
For example, the reaction of sodium chloride
with silver nitrate forms AgCl(s), an insoluble
precipitate.
NaCl(aq )  AgNO3 (aq )  AgCl(s)   NaNO3 (aq )
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
20
Types of Chemical Reactions
Precipitation Reactions
Solubility rules
Substances vary widely in their solubility, or
ability to dissolve, in water.
For example, NaCl is very soluble in water
whereas calcium carbonate, CaCO3, is insoluble
in water.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
21
Figure 4.5: Limestone Formations.Photo ©Corbis.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
22
Types of Chemical Reactions
Precipitation Reactions
Predicting Precipitation Reactions.
To predict whether a precipitate will form, we
need to look at potential insoluble products.
Table 4.1 lists eight solubility rules for ionic
compounds. These rules apply to the most
common ionic compounds.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
23
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
24
Types of Chemical Reactions
Precipitation Reactions
Predicting Precipitation Reactions.
Suppose you mix together solutions of
nickel(II) chloride, NiCl2, and sodium
phosphate, Na3PO4.
NiCl 2  Na3 PO 4 
How can you tell if a reaction will occur, and if
it does, what products to expect?
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
25
Types of Chemical Reactions
Precipitation Reactions
Predicting Precipitation Reactions.
Precipitation reactions have the form of an
“exchange reaction.”
NiCl 2  Na3 PO 4  Ni 3 ( PO 4 )2  NaCl
An exchange (or metathesis) reaction is a
reaction between compounds that, when written
as a molecular equation, appears to involve an
exchange of cations and anions.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
26
Types of Chemical Reactions
Precipitation Reactions
Predicting Precipitation Reactions.
Now that we have predicted potential products,
we must balance the equation.
3 NiCl 2 2 Na3 PO 4  Ni 3 ( PO 4 )2  6 NaCl
We must verify that NiCl2 and Na3PO4 are
soluble and then check the solubilities of the
products.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
27
Types of Chemical Reactions
Precipitation Reactions
Predicting Precipitation Reactions.
Table 4.1 indicates that our reactants, nickel(II)
chloride and sodium phosphate are both
soluble.
3NiCl 2(aq)  2Na3 PO 4(aq) 
Ni 3 ( PO 4 )2 (s)  6NaCl (aq)
Looking at the potential products we find that
nickel(II) phosphate is not soluble although
sodium chloride is.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
28
Types of Chemical Reactions
Precipitation Reactions
Predicting Precipitation Reactions.
We predict that a reaction occurs because
nickel(II) phosphate is insoluble and precipitates
from the reaction mixture.
To see the reaction that occurs on the ionic
level, we must rewrite the molecular equation
as an ionic equation.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
29
Figure 4.6:
Reaction of magnesium
chloride and silver nitrate.
Photo courtesy of American Color.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
30
Types of Chemical Reactions
Precipitation Reactions
Predicting Precipitation Reactions.
First write strong electrolytes (the soluble ionic
compounds) in the form of ions to obtain the
complete ionic equation
2


3
3Ni (aq)  6Cl (aq)  6Na (aq)  2PO 4 (aq) 
Ni 3 ( PO 4 )2 (s )  6Na  (aq )  6Cl  (aq )
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
31
Types of Chemical Reactions
Precipitation Reactions
Predicting Precipitation Reactions.
After canceling the spectator ions, you
obtain the net ionic equation.
2

3

3Ni (aq)  6Cl (aq)  6Na (aq)  2PO 4 (aq) 


Ni 3 ( PO 4 )2 (s )  6Na (aq )  6Cl (aq )
2
3
3Ni (aq )  2PO 4 (aq )  Ni 3 ( PO 4 )2 (s )
This equation represents the “essential” reaction.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
32
Types of Chemical Reactions
Acid-Base Reactions
Acids and bases are some of the most
important electrolytes.
They can cause color changes in certain dyes
called acid-base indicators.
Household acids and bases.
Red cabbage juice as an acid-base indicator.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
33
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
34
Figure 4.7: Household acids and bases. Photo courtesy of American Color.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
35
Figure 4.8: Preparation
of red cabbage juice as
an acid-base
indicator.Photo courtesy of James
Scherer.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
36
Types of Chemical Reactions
Acid-Base Reactions
The Arrhenius Concept
The Arrhenius concept defines acids as
substances that produce hydrogen ions, H+,
when dissolved in water.
An example is nitric acid, HNO3, a molecular
substance that dissolves in water to give H+ and
NO3-.


HNO 3 (aq ) 
 H (aq )  NO 3 (aq )
H 2O
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
37
Types of Chemical Reactions
Acid-Base Reactions
The Arrhenius Concept
The Arrhenius concept defines bases as
substances that produce hydroxide ions, OH-,
when dissolved in water.
An example is sodium hydroxide, NaOH, an
ionic substance that dissolves in water to give
sodium ions and hydroxide ions.


NaOH(s) 
 Na (aq)  OH (aq)
H 2O
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
38
Types of Chemical Reactions
Acid-Base Reactions
The Arrhenius Concept
The molecular substance ammonia, NH3, is a
base in the Arrhenius view,



NH 3 (aq )  H 2O(l )  NH 4 (aq )  OH  (aq )
because it yields hydroxide ions when it reacts
with water.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
39
Types of Chemical Reactions
Acid-Base Reactions
The Brønsted-Lowry Concept
The Brønsted-Lowry concept of acids and
bases involves the transfer of a proton (H+)
from the acid to the base.
In this view, acid-base reactions are protontransfer reactions.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
40
Types of Chemical Reactions
Acid-Base Reactions
The Brønsted-Lowry Concept
The Brønsted-Lowry concept defines an acid
as the species (molecule or ion) that donates a
proton (H+) to another species in a protontransfer reaction.
A base is defined as the species (molecule or
ion) that accepts the proton (H+) in a protontransfer reaction.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
41
Types of Chemical Reactions
Acid-Base Reactions
The Brønsted-Lowry Concept
In the reaction of ammonia with water,


NH 3 (aq )  H 2O(l )  NH 4 (aq )  OH  (aq )
H+
the H2O molecule is the acid because it donates a
proton. The NH3 molecule is a base, because it accepts
a proton.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
42
Types of Chemical Reactions
Acid-Base Reactions
The Brønsted-Lowry Concept
The H+(aq) ion associates itself with water to form
H3O+(aq).


H (aq )  H 2O(l )  H 3O (aq )
This “mode of transportation” for the H+ ion is
called the hydronium ion.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
43
Types of Chemical Reactions
Acid-Base Reactions
The Brønsted-Lowry Concept
The dissolution of nitric acid, HNO3, in water is
therefore a proton-transfer reaction

HNO 3 (aq )  H 2O(l )  NO 3 (aq )  H 3O  (aq )
H+
where HNO3 is an acid (proton donor) and H2O is a
base (proton acceptor).
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
44
Types of Chemical Reactions
Acid-Base Reactions
In summary, the Arrhenius concept and the
Brønsted-Lowry concept are essentially the
same in aqueous solution.
– The Arrhenius concept
acid: proton (H+) donor
base: hydroxide ion (OH-) donor
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
45
Types of Chemical Reactions
Acid-Base Reactions
In summary, the Arrhenius concept and the
Brønsted-Lowry concept are essentially the
same in aqueous solution.
– The Brønsted-Lowry concept
acid: proton (H+) donor
base: proton (H+) acceptor
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
46
Types of Chemical Reactions
Acid-Base Reactions
Strong and Weak Acids and Bases
A strong acid is an acid that ionizes completely
in water; it is a strong electrolyte.


HNO 3 (aq )  H 2O(l )  NO 3 (aq )  H 3O (aq )


HCl (aq )  H 2O(l )  Cl (aq )  H 3O (aq )
Table 4.3 lists the common strong acids.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
47
Types of Chemical Reactions
Acid-Base Reactions
Strong and Weak Acids and Bases
A weak acid is an acid that only partially
ionizes in water; it is a weak electrolyte.
The hydrogen cyanide molecule, HCN, reacts
with water to produce a small percentage of
ions in solution.



HCN(aq)  H 2O(l )  CN (aq)  H 3O (aq)
Table 4.2 lists some common weak acids.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
48
Types of Chemical Reactions
Acid-Base Reactions
Strong and Weak Acids and Bases
A strong base is a base that is present entirely
as ions, one of which is OH-; it is a strong
electrolyte.
H O
2


NaOH(s)  Na (aq)  OH (aq)
The hydroxides of Group IA and IIA
elements, except for beryllium hydroxide, are
strong bases. (see Table 4.3)
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
49
Types of Chemical Reactions
Acid-Base Reactions
Strong and Weak Acids and Bases
A weak base is a base that is only partially
ionized in water; it is a weak electrolyte.
Ammonia, NH3, is an example.



NH 3 (aq )  H 2O(l )  NH 4 (aq )  OH (aq )
Table 4.2 lists some common weak acids.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
50
Types of Chemical Reactions
Acid-Base Reactions
Strong and Weak Acids and Bases
You will find it important to be able to identify
an acid or base as strong or weak.
When you write an ionic equation, strong acids
and bases are represented as separate ions.
Weak acids and bases are represented as
undissociated “molecules” in ionic equations.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
51
Types of Chemical Reactions
Acid-Base Reactions
Neutralization Reactions
One of the chemical properties of acids and bases
is that they neutralize one another.
A neutralization reaction is a reaction of an acid
and a base that results in an ionic compound and
water.
The ionic compound that is the product of a
neutralization reaction is called a salt.
HCN(aq)  KOH(aq)  KCN(aq)  H 2O(l )
acid
Chapter
base
salt
Copyright © by Houghton Mifflin Company. All rights reserved.
52
Types of Chemical Reactions
Acid-Base Reactions
Neutralization Reactions
The net ionic equation for each acid-base
neutralization reaction involves a transfer of a
proton.
Consider the reaction of the strong acid ,
HCl(aq) and a strong base, LiOH(aq).
HCl(aq)  KOH(aq)  KCl(aq)  H 2O(l )
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
53
Types of Chemical Reactions
Acid-Base Reactions
Neutralization Reactions
Writing the strong electrolytes in the form of
ions (refer to Table 4.1 and 4.3) gives the
following complete ionic equation.




H (aq)  Cl (aq)  K (aq)  OH (aq) 


K (aq)  Cl (aq)  H 2O(l )
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
54
Types of Chemical Reactions
Acid-Base Reactions
Neutralization Reactions
Canceling the spectator ions results in the net
ionic equation. Note the proton transfer.




H (aq)  Cl (aq)  K (aq)  OH (aq) 
K  (aq)  Cl  (aq)  H 2O(l )
H  (aq)  OH  (aq)  H 2O(l )
H+
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
55
Types of Chemical Reactions
Acid-Base Reactions
Neutralization Reactions
In a reaction involving HCN(aq), a weak acid,
and KOH(aq), a strong base, the product is
KCN, a strong electrolyte
Referring to Tables 4.1, 4.2 and 4.3, we obtain
this net ionic equation:


HCN(aq)  OH (aq)  CN (aq)  H 2O(l )
H+
Chapter
Note the proton transfer.
Copyright © by Houghton Mifflin Company. All rights reserved.
56
Types of Chemical Reactions
Acid-Base Reactions
Acid-Base Reactions with Gas Formation
Carbonates react with acids to form CO2,
carbon dioxide gas.
Na2CO3  2HCl  2NaCl  H 2O  CO2 
Sulfites react with acids to form SO2, sulfur
dioxide gas.
Na2SO 3  2HCl  2NaCl  H 2O  SO 2 
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
57
Types of Chemical Reactions
Acid-Base Reactions
Acid-Base Reactions with Gas Formation
Sulfides react with acids to form H2S, hydrogen
sulfide gas.
Na 2S  2HCl  2NaCl  H 2S 
These reactions are summarized in Table 4.4.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
58
Types of Chemical Reactions
Oxidation-Reduction Reactions
Oxidation-reduction reactions involve the
transfer of electrons from one species to
another.
Oxidation is defined as the loss of electrons.
Reduction is defined as the gain of electrons.
Oxidation and reduction always occur
simultaneously.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
59
Types of Chemical Reactions
Oxidation-Reduction Reactions
The reaction of an iron nail with a solution of
copper(II) sulfate, CuSO4, is an oxidationreduction reaction.
The molecular equation for this reaction is:
Fe(s)  CuSO4 (aq)  FeSO4 (aq)  Cu(s)
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
60
Figure 4.11: The burning of
calcium metal in oxygen. Photo
courtesy of James Scherer.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
61
Types of Chemical Reactions
Oxidation-Reduction Reactions
The net ionic equation shows the reaction of
iron metal with Cu2+(aq) to produce iron(II) ion
and copper metal.
Loss of 2 e-1 oxidation
2
2
Fe(s)  Cu (aq)  Fe (aq)  Cu(s)
Gain of 2 e-1 reduction
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
62
Types of Chemical Reactions
Oxidation-Reduction Reactions
Oxidation Numbers
The concept of oxidation numbers is a simple
way of keeping track of electrons in a reaction.
The oxidation number (or oxidation state) of
an atom in a substance is the actual charge of
the atom if it exists as a monatomic ion.
Alternatively, it is hypothetical charge assigned
to the atom in the substance by simple rules.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
63
Types of Chemical Reactions
Oxidation-Reduction Reactions
Oxidation Number Rules
Rule Applies to
Statement
1
Elements
The oxidation number of an atom in an
element is zero.
2
Monatomic
ions
The oxidation number of an atom in a
monatomic ion equals the charge of the ion.
3
Oxygen
The oxidation number of oxygen is –2 in
most of its compounds. (An exception is O in
H2O2 and other peroxides, where the
oxidation number is –1.)
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
64
Types of Chemical Reactions
Oxidation-Reduction Reactions
Oxidation Number Rules
Rule Applies to
Statement
4
Hydrogen
The oxidation number of an atom in an
element is zero.
5
Halogens
6
Compounds
and ions
Fluorine is –1 in all its compounds. Each of
the other halogens is –1 in binary compounds
unless the other element is oxygen.
The sum of the oxidation numbers of the
atoms in a compound is zero. The sum in a
polyatomic ion equals the charge on the ion.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
65
Types of Chemical Reactions
Oxidation-Reduction Reactions
Describing Oxidation-Reduction Reactions
Look again at the reaction of iron with
copper(II) sulfate.
2
2
Fe(s)  Cu (aq)  Fe (aq)  Cu(s)
We can write this reaction in terms of two halfreactions.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
66
Types of Chemical Reactions
Oxidation-Reduction Reactions
Describing Oxidation-Reduction Reactions
A half-reaction is one of the two parts of an
oxidation-reduction reaction. One involves the
loss of electrons (oxidation) and the other
involves the gain of electrons (reduction).
2

Fe(s)  Fe (aq)  2e
2

Cu (aq)  2e  Cu(s)
Chapter
oxidation half-reaction
reduction half-reaction
Copyright © by Houghton Mifflin Company. All rights reserved.
67
Types of Chemical Reactions
Oxidation-Reduction Reactions
Describing Oxidation-Reduction Reactions
An oxidizing agent is a species that oxidizes
another species; it is itself reduced.
A reducing agent is a species that reduces
another species; it is itself oxidized.
Loss of 2 e- oxidation
reducing agent
2
2
Fe(s)  Cu (aq)  Fe (aq)  Cu(s)
oxidizing agent
Gain of 2 e- reduction
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
68
Types of Chemical Reactions
Oxidation-Reduction Reactions
Some Common Oxidation-Reduction Reactions
Most of the oxidation-reduction reactions fall into
one of the following simple categories:
Combination Reactions
Decomposition Reactions
Displacement Reactions
Combustion Reactions
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
69
Figure 4.12: The burning of calcium metal in
chlorine. Photo courtesy of James Scherer.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
70
Figure 4.13: A representation of an oxidation reduction reaction.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
71
Types of Chemical Reactions
Oxidation-Reduction Reactions
Combination Reactions
A combination reaction is a reaction in which
two substances combine to form a third
substance.
2Sb(s)  3Cl 2 (g )  2SbCl 3 (l )
Antimony and chlorine combine in a fiery
reaction.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
72
Figure 4.14: Oxidation reduction reaction of mercury
(III) oxide into its elements. Photo courtesy of James Scherer.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
73
Types of Chemical Reactions
Oxidation-Reduction Reactions
Combination Reactions
Other combination reactions involve
compounds as reactants.
CaO(s)  SO 2 (g )  CaSO3 (s)
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
74
Types of Chemical Reactions
Oxidation-Reduction Reactions
Decomposition Reactions
A decomposition reaction is a reaction in which
a single compound reacts to give two or more
substances.
( NH4 )2 Cr2O7 (s)  Cr2O 3 (s)  4H 2O(g )  N 2 (g )
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
75
Types of Chemical Reactions
Oxidation-Reduction Reactions
Displacement Reactions
A displacement reaction (also called a singlereplacement reaction) is a reaction in which an
element reacts with a compound, displacing an
element from it.
Zn(s)  2HCl(aq)  ZnCl 2 (aq)  H 2 (g )
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
76
Types of Chemical Reactions
Oxidation-Reduction Reactions
Combustion Reactions
A combustion reaction is a reaction in which a
substance reacts with oxygen, usually with the
rapid release of heat to produce a flame.
2 C4 H10 (g )  13 O 2 (g )  8 CO2 (g )  10 H 2O(g )
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
77
Figure 4.15: Oxidation
reduction reaction of
zinc metal and
hydrochloric acid.
Photo courtesy of American Color.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
78
Types of Chemical Reactions
Oxidation-Reduction Reactions
Balancing Simple Oxidation-Reduction Reactions
At first glance, the equation representing the
reaction of zinc metal with silver(I) ions might
appear to be balanced.

2
Zn(s)  Ag (aq)  Zn (aq)  Ag(s)
However, a balanced equation must have a
charge balance as well as a mass balance.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
79
Types of Chemical Reactions
Oxidation-Reduction Reactions
Balancing Simple Oxidation-Reduction Reactions
Since the number of electrons lost in the
oxidation half-reaction must equal the number
gained in the reduction half-reaction,
2

Zn(s)  Zn (aq)  2e


2 Ag (aq)  2e  2 Ag(s )
oxidation half-reaction
reduction half-reaction
we must double the reaction involving the
reduction of the silver.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
80
Types of Chemical Reactions
Oxidation-Reduction Reactions
Balancing Simple Oxidation-Reduction Reactions
Adding the two half-reactions together, the
electrons cancel,
2

Zn(s)  Zn (aq)  2e


2Ag (aq)  2e  2Ag(s)
oxidation half-reaction
reduction half-reaction
Zn(s)  2Ag  (aq)  Zn(s)  2Ag(s)
which yields the balanced oxidation-reduction
reaction.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
81
Figure 4.16: Oxidation reduction
reaction of iron wool and oxygen.
Photo courtesy of James Scherer.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
82
Working with Solutions
The majority of chemical reactions
discussed here occur in aqueous solution.
When you run reactions in liquid solutions, it is
convenient to dispense the amounts of reactants
by measuring out volumes of reactant solutions.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
83
Working with Solutions
Molar Concentration
When we dissolve a substance in a liquid,
we call the substance the solute and the
liquid the solvent.
The general term concentration refers to the
quantity of solute in a standard quantity of
solution.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
84
Working with Solutions
Molar Concentration
Molar concentration, or molarity (M), is
defined as the moles of solute dissolved in
one liter (cubic decimeter) of solution.
moles of solute
Molarity (M) 
liters of solution
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
85
Working with Solutions
Molar Concentration
Let’s try an example.
A sample of 0.0341 mol iron(III) chloride,
FeCl3, was dissolved in water to give 25.0 mL
of solution. What is the molarity of the
solution?
moles of FeCl 3
Since molarity 
liters of solution
0.0341 mole of FeCl 3
then M 
 1.36 M FeCl 3
0.0250 liter of solution
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
86
Working with Solutions
Diluting Solutions
The molarity of a solution and its volume
are inversely proportional. Therefore,
adding water makes the solution less
concentrated.
This inverse relationship takes the form of:
M i  Vi  M f  V f
So, as water is added, increasing the final volume,
Vf, the final molarity, Mf, decreases.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
87
Quantitative Analysis
Analytical chemistry deals with the
determination of composition of materialsthat is, the analysis of materials.
Quantitative analysis involves the
determination of the amount of a substance or
species present in a material.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
88
Quantitative Analysis
Gravimetric Analysis
Gravimetric analysis is a type of
quantitative analysis in which the amount of
a species in a material is determined by
converting the species into a product that
can be isolated and weighed.
Precipitation reactions are often used in
gravimetric analysis.
The precipitate from these reactions is then
filtered, dried, and weighed.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
89
Quantitative Analysis
Gravimetric Analysis
Consider the problem of determining the
amount of lead in a sample of drinking
water.
Adding sodium sulfate (Na2SO4) to the sample
will precipitate lead(II) sulfate.
2

Na2SO4 (aq)  Pb (aq)  2Na (aq)  PbSO 4 (s)
The PbSO4 can then be filtered, dried, and
weighed. Figure 4.21 shows a similar
laboratory setup.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
90
Quantitative Analysis
Gravimetric Analysis
Suppose a 1.00 L sample of polluted water was
analyzed for lead(II) ion, Pb2+, by adding an
excess of sodium sulfate to it. The mass of lead(II)
sulfate that precipitated was 229.8 mg. What is the
mass of lead in a liter of the water? Express the
answer as mg of lead per liter of solution.
2

Na2SO4 (aq)  Pb (aq)  2Na (aq)  PbSO 4 (s)
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
91
Quantitative Analysis
Gravimetric Analysis
First we must obtain the mass percentage of lead
in lead(II) sulfate, by dividing the molar mass of
lead by the molar mass of PbSO4, then multiplying
by 100.
207.2 g/mol
%Pb 
 100  68.32%
303.3 g/mol
Then, calculate the amount of lead in the PbSO4
precipitated.
Amount Pb in sample  229.8 mg PbSO 4  0.6832  157.0 mg Pb
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
92
Quantitative Analysis
Volumetric Analysis
An important method for determining the amount
of a particular substance is based on measuring the
volume of the reactant solution.
Titration is a procedure for determining the
amount of substance A by adding a carefully
measured volume of a solution with known
concentration of B until the reaction of A and B
is just complete. (see Figure 4.22)
Volumetric analysis is a method of analysis
based on titration.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
93
Quantitative Analysis
Volumetric Analysis
Consider the reaction of sulfuric acid,
H2SO4, with sodium hydroxide, NaOH:
H 2SO4 (aq)  2NaOH(aq)  2H 2O(l )  Na 2SO4 (aq)
Suppose a beaker contains 35.0 mL of 0.175 M
H2SO4. How many milliliters of 0.250 M NaOH
must be added to completely react with the
sulfuric acid?
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
94
Quantitative Analysis
Volumetric Analysis
First we must convert the 0.0350 L (35.0 mL) to moles
of H2SO4 (using the molarity of the H2SO4).
Then, convert to moles of NaOH (from the
balanced chemical equation).
Finally, convert to volume of NaOH solution
(using the molarity of NaOH).
0.175 mole H 2SO 4 2 mol NaOH 1 L NaOH soln.
(0.0350L ) 



1 L H 2SO 4 solution 1 mol H 2SO 4 0.250 mol NaOH
0.0490 L NaOH solution (or 49.0 mL of NaOH solution)
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
95
Chemical Reactions
Summary
Reactions often involve ions in aqueous solution. Many of
these compounds are electrolytes.
We can represent these reactions as molecular
equations, complete ionic equations (with strong
electrolytes represented as ions), or net ionic equations
(where spectator ions have been canceled).
Most reactions are either precipitation reactions, acidbase reactions, or oxidation-reduction reactions.
Acid-base reactions are proton-transfer reactions.
Oxidation-reduction reactions involve a transfer of
electrons from one species to another.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
96
Chemical Reactions
Summary
Oxidation-reduction reactions usually fall into the
following categories: combination reactions, decomposition
reactions, displacement reactions, and combustion
reactions.
Molarity is defined as the number of moles of solute per liter
of solution. Knowing the molarity allows you to calculate the
amount of solute in a given volume of solution.
Quantitative analysis involves the determination of the
amount of a species in a material.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
97
Chemical Reactions
Summary
In gravimetric analysis, you determine the amount of a
species by converting it to a product you can weigh.
In volumetric analysis, you determine the amount of a
species by titration.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
98
Operational Skills
Using solubility rules.
Writing net ionic equations.
Deciding whether precipitation occurs.
Classifying acids and bases as weak or strong.
Writing an equation for a neutralization.
Writing an equation for a reaction with gas formation.
Assigning oxidation numbers.
Balancing simple oxidation-reduction reactions.
Calculating molarity from mass and volume.
Using molarity as a conversion factor.
Diluting a solution.
Determining the amount of a substance by gravimetric analysis.
Calculating the volume of reactant solution needed.
Calculating the quantity of a substance by titration.
Chapter
Copyright © by Houghton Mifflin Company. All rights reserved.
99