Balancing Redox Equations continued:

1

• Balancing Redox Equations (continued)

• Using O.N.s (continued).

eg Balance the following using O.N.s in acid
Zn As2O3 ? H3As Zn2

• Balance elements other than oxygen hydrogen.

2
Zn As2O3 ? H3As Zn2

• Use O.N.s to determine the total oxidation state
  of each element that changes.

Zn As2O3 ? 2 H3As Zn2
0
6
-6
2
2

• Determine number of electrons gained and lost

Zn As2O3 ? 2 H3As Zn2
0
6
-6
2
gain 12e-
lose 2e-

• Multiply half reactions to balance electrons

6
6
Zn As2O3 ? 2 H3As Zn2

• Add H2O and H to balance oxygen and hydrogen

6 Zn As2O3 ? 2 H3As 6 Zn2
3H2O
12H
12H 6 Zn As2O3 ? 2 H3As 6 Zn2
3H2O
3
eg Balance the following using O.N.s in acid
P4 ? H2PO2- H3P


P4 P4 ? 4 H2PO2- 4 H3P


gain 12e-
0
-12
P4 P4 ? 4 H2PO2- 4 H3P
0
4
lose 4e-
3P4 P4 ? 12 H2PO2- 4 H3P
24 H2O 4 P4 ? 12 H2PO2- 4 H3P
12 H
(Reduce if possible)
6 H2O P4 ? 3 H2PO2- H3P
3 H