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194 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 19
This reaction is a redox reaction even though the product is a molecular compound and no ions are involved in the process. The more
electronegative atom (chlorine) is considered to have been reduced
by gaining electrons, and the less electronegative atom (hydrogen) is
considered to have been oxidized by losing electrons. Therefore, in
this reaction, chlorine is the oxidizing agent and hydrogen is the
reducing agent. The electronegativity values in Figure 6-18 in your
textbook may be helpful.
Practice Problems
1. For each of the following reactions, identify what is oxidized
and what is reduced. Also identify the oxidizing agent and the
reducing agent.
a. Zn ϩ Ni 2ϩ 0 Ni ϩ Zn 2ϩ
c. 2NO 0 N 2 ϩ O 2
b. 2I Ϫ ϩ Br 2 0 I 2 ϩ 2Br Ϫ
d. 2H 2 ϩ S 2 0 2H 2 S
Determining oxidation numbers When a redox reaction
occurs, there is a change in the oxidation number of each atom that
is oxidized or reduced. In order to understand these changes, you
must be able to determine the oxidation number of an element in a
compound. Chemists have developed the following rules for determining oxidation numbers.
1. The oxidation number of an uncombined atom is zero.
Therefore, free elements have an oxidation number of zero.
2. The oxidation number of a monatomic ion is equal to the
charge on the ion. For example, the oxidation number of a Fe 3ϩ
ion is ϩ3, and the oxidation number of a Cl Ϫ ion is Ϫ1.
3. The oxidation number of the more electronegative atom in a
molecule or a complex ion is the same as the charge it would
have if it were an ion. For example, in the compound phosphorus pentabromide (PBr 5 ), bromine is more electronegative than
phosphorus. Therefore, each bromine is given an oxidation
number of Ϫ1, as if it had gained an electron to complete an
octet. Phosphorus is assigned an oxidation number of ϩ5, as if
it had lost an electron to each bromine atom.
4. The most electronegative element, fluorine, always has an oxidation number of Ϫ1 when it is bonded to another element.
▲
194 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 19
This reaction is a redox reaction even though the product is a molecular compound and no ions are involved in the process. The more
electronegative atom (chlorine) is considered to have been reduced
by gaining electrons, and the less electronegative atom (hydrogen) is
considered to have been oxidized by losing electrons. Therefore, in
this reaction, chlorine is the oxidizing agent and hydrogen is the
reducing agent. The electronegativity values in Figure 6-18 in your
textbook may be helpful.
Practice Problems
1. For each of the following reactions, identify what is oxidized
and what is reduced. Also identify the oxidizing agent and the
reducing agent.
a. Zn ϩ Ni 2ϩ 0 Ni ϩ Zn 2ϩ
c. 2NO 0 N 2 ϩ O 2
b. 2I Ϫ ϩ Br 2 0 I 2 ϩ 2Br Ϫ
d. 2H 2 ϩ S 2 0 2H 2 S
Determining oxidation numbers When a redox reaction
occurs, there is a change in the oxidation number of each atom that
is oxidized or reduced. In order to understand these changes, you
must be able to determine the oxidation number of an element in a
compound. Chemists have developed the following rules for determining oxidation numbers.
1. The oxidation number of an uncombined atom is zero.
Therefore, free elements have an oxidation number of zero.
2. The oxidation number of a monatomic ion is equal to the
charge on the ion. For example, the oxidation number of a Fe 3ϩ
ion is ϩ3, and the oxidation number of a Cl Ϫ ion is Ϫ1.
3. The oxidation number of the more electronegative atom in a
molecule or a complex ion is the same as the charge it would
have if it were an ion. For example, in the compound phosphorus pentabromide (PBr 5 ), bromine is more electronegative than
phosphorus. Therefore, each bromine is given an oxidation
number of Ϫ1, as if it had gained an electron to complete an
octet. Phosphorus is assigned an oxidation number of ϩ5, as if
it had lost an electron to each bromine atom.
4. The most electronegative element, fluorine, always has an oxidation number of Ϫ1 when it is bonded to another element.
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