Page 223 - Theory and Problems of BEGINNING CHEMISTRY

212 OXIDATION AND REDUCTION [CHAP. 14

Solved Problems

INTRODUCTION
2−
14.1. (a) What is the formula of a compound of two ions: X and Y ?(b) What is the formula of a covalent
+
compound of two elements: “W” with an oxidation state of +1 and “Z” with an oxidation state of −2?
Ans. We treat the oxidation states in part b just like the charges in part a. In this manner, we can predict formulas
for ionic and covalent compounds. (a)X 2 Y (b) W 2 Z.

ASSIGNING OXIDATION NUMBERS
14.2. Show that rules 2 and 3 (Sec. 14.2) are corollaries of rule 1.
Ans. Rule 2: Uncombined elements have zero charges, and so the oxidation numbers must add up to zero. Since
all the atoms are the same, all the oxidation numbers must be the same—0. Rule 3: For monatomic ions,
the oxidation numbers of all the atoms add up to the charge on the ion. Since there is only one atom (it is
monatomic), the oxidation number of that atom must add up to the charge on the ion; that is, it is equal to
the charge on the ion.

14.3. Draw an electron dot diagram for H 2 O 2 . Assign an oxidation number to oxygen on this basis. Compare
this number with that assigned by rule 6 (Sec. 14.2).
Ans.
H O O H
Free atom 6
− Controlled − 7
Oxidation number − 1
The electrons shared between the oxygen atoms are counted one for each atom. Peroxide oxygen is assigned
an oxidation state of −1 by rule 6 also.

3−
14.4. What is the sum of the oxidation numbers of all the atoms in the following compounds or ions? (a)PO 4 ,
+ − 2−
(b)VO 2 ,(c) ClO 2 ,(d)Cr 2 O 7 ,(e) SiCl 4 , and ( f ) NaCl.
Ans. The sum equals the charge on the species in each case: (a) −3 (b) +1 (c) −1 (d) −2 (e)0 ( f )0

14.5. Determine the oxidation numbers for the underlined elements: (VO ) 3 PO .
4
2
Ans. We recognize the phosphate ion, PO 4 . Each VO 2 ion therefore must have a 1+ charge. The oxidation
3−
numbers are +5 for P and +5 for V.
−
14.6. What is the oxidation number of chlorine in each of the following? (a)Cl 2 O 3 ,(b) ClO 4 , and (c) ClF 5 .
Ans. (a) +3 (b) +7 (c) +5

14.7. Determine the oxidation number for the underlined element: (a)POCl 3 ,(b)HNO 2 ,(c)Na 2 SO 4 ,(d)PCl 5 ,
and (e)N O 3 .
2
Ans. (a) +5(b) +3(c) +6(d) +5(e) +3

−
14.8. Determine the oxidation number for the underlined element: (a)ClO ,(b)PO 3− ,(c)SO 2− , and
4 4
2+
(d)VO .
Ans. (a) +1 (b) +5 (c) +6 (d) +4
14.9. What is the oxidation number of Si in Si 6 O 18 12− ?
Ans. 6x + 18(−2) =−12 x =+4