Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
196 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 19
(ϩ2) ϩ (n C ) ϩ 3(Ϫ2) ϭ 0
SrCO 3
2 ϩ n C ϩ (Ϫ6) ϭ 0
n C ϭ ϩ4
The oxidation number of carbon is ϩ4.
b. The dichromate ion has a charge of 2Ϫ, so rule 9 says that the
oxidation numbers add up to Ϫ2. According to rule 5, the oxidation number of oxygen in compounds is Ϫ2.
2(n Cr ) ϩ 7(Ϫ2) ϭ Ϫ2
Cr 2 O 7
2Ϫ
2(n Cr ) ϩ (Ϫ14) ϭ Ϫ2
2(n Cr ) ϭ ϩ12
n Cr ϭ ϩ6
The oxidation number of chromium is ϩ6.
Practice Problems
2. Determine the oxidation number of the boldface element in
each of these compounds.
a. Li 2 SiO 3
c. CaH 2
e. K 2 GeF 6
b. Al 4 C 3
d. BeSeO 4
f. Al(ClO 3 ) 3
3. Determine the oxidation number of the boldface element in
each of these ions.
a. PO 4
3Ϫ
c. HSO 4
Ϫ
e. PuO 2
ϩ
b. Hg 2
2ϩ
d. PtCl 6
2Ϫ
f. TeO 3
2Ϫ
Not all chemical reactions can be classified as redox reactions. For
example, in a double-replacement reaction, the positive and negative
ions of two compounds are interchanged. Most double-replacement
reactions are not redox reactions because there is no transfer of electrons between atoms. Combustion and single-replacement reactions,
however, are always redox reactions. Many synthesis and decomposition reactions are redox reactions as well.