Balancing Redox

1
Balancing Redox

• Alternate Methods

2
Oxidation Number Method

• Also called the electron transfer method
• Focuses on change in oxidation states

3
Step 1 Balance reaction and assign oxidation
states

• MnO4-(aq) Fe2(aq) ? Fe3(aq) Mn2(aq)

-2
7
2
3
2
4
Step 2 Write electrons added or removed, then
balance

• MnO4-(aq) Fe2(aq) ? Fe3(aq) Mn2(aq)
• To balance use 5 Fe2 and 5 Fe3

-2
7
2
3
2
-1 e-
5 e-
5
Step 3 In acidic solutions add H and H2O to
finish balancing, in basic solutions add H2O and
OH-

• Acidic
• MnO4-(aq) 8H 5Fe2(aq) ? 5Fe3(aq)
  Mn2(aq) 4H2O
• Basic
• MnO4-(aq) 4H2O 5Fe2(aq) ? 5Fe3(aq)
  Mn2(aq) 8OH-

6
Net Sum Method

• Use the net changes in oxidation state changes to
  find the lowest common multiple

7
Step 1 Assign oxidation states

• MnO4-(aq) Fe2(aq) ? Fe3(aq) Mn2(aq)

-2
7
2
3
2
8
Step 2 Write change in oxidation states under a
bracket

• MnO4-(aq) Fe2(aq) ? Fe3(aq) Mn2(aq)

-2
7
2
3
2
1
-5
9
Balance

• The number lost will be the coefficient of the
  reduced specie, the number gained will be the
  coefficient of the oxidized specie

10
Step 3 In acidic solutions add H and H2O to
finish balancing, in basic solutions add H2O and
OH-

• Acidic
• MnO4-(aq) 8H 5Fe2(aq) ? 5Fe3(aq)
  Mn2(aq) 4H2O
• Basic
• MnO4-(aq) 4H2O 5Fe2(aq) ? 5Fe3(aq)
  Mn2(aq) 8OH-

11
Practice

• Do the following in both acidic and basic
  solutions
• Ag(s) CN- O2 ? Ag(CN)2-(aq)
• Cl2(g) ? Cl-(aq) OCl-(aq)
• Fe(OH)2 H2O2 ? Fe(OH)3
• (H2O2 is hydrogen peroxide)