Balancing Redox Equations

1
Balancing Redox Equations
2
analyze Mg S ? MgS

• 1. assign oxidation numbers to all elements
• 2. figure out change in oxidation numbers
• what is oxidized?
• what is reduced?

Mg 0 to 2 oxidation
S 0 to -2 reduction
3
Mg S ? MgS
2 -2
0
0

• half-reactions

Mg is oxidized 0 to 2
Mg ? Mg2 2e-
S is reduced 0 to -2
S 2e- ? S-2
4
electrons lost electrons gained
add half-reactions

• Mg ? Mg2 2e-
• S 2e- ? S-2

5
Zn 2HCl ? H2 ZnCl2

• Zn goes from 0 to 2 oxidation
• H goes from 1 to 0 reduction
• Cl goes from -1 to -1 no change

6
Zn 2HCl ? H2 ZnCl2
0 1 -1 0
2 -1

• Zn ? Zn2 2e-
• 2H1 2e- ? H2

______________________________________
Zn 2H1 2e- ? Zn2 2e- H2
7
Balancing Redox Equations

1. assign oxidation to all elements
2. determine elements that changed oxidation number
3. identify element oxidized element reduced
4. write half-reactions (diatomics must stay as is)
5. electrons lost/gained must be equal
   (multiply half-reactions if
   necessary)
6. add half-reactions
   transfer co-efficients to skeleton
   equation
7. balance rest of equation by counting atoms

8
Cu AgNO3 ? Cu(NO3)2 Ag

• Cu goes from 0 to 2 oxidation
• Ag goes from 1 to 0 reduction
• N goes from 5 to 5 no change
• O goes from -2 to -2 no change

9
Half-Reactions

• Cu ? Cu2 2e-
• Ag1 1e- ? Ag

multiply by 2

• Cu ? Cu2 2e-
• 2Ag1 2e- ? 2Ag

______________________
10
Transfer Co-efficients

• compare skeleton equation sum of ½ rxns
• Cu AgNO3 ? Ag Cu(NO3)2
• vs.
• Cu 2Ag1 2e- ? 2Ag Cu2 2e-
• transfer co-efficients!
• Cu 2AgNO3 ? 2Ag Cu(NO3)2

11
Cu HNO3 ? Cu(NO3)2 NO2 H2O
1
4

• Cu 0 to 2 oxidized
• H 1 to 1 no change
• O -2 to -2 no change
• N starts as 5
• ends as 5 no change Cu(NO3)2
• ends as 4 reduction NO2

cant just say N! sayN in HNO3 or N5

• half-reactions
• Cu ? Cu2 2e-
• N5 1e- ? N4

12

• half-reactions
• Cu ? Cu2 2e-
• N5 1e- ? N4

multiply half-reactions as necessary of
electrons lost gained
Cu ? Cu2 2e- 2 (N5 e- ? N4)
balance remaining atoms by inspection