1. Balancing Redox Equations

2. Analyze: Mg + S  MgS +2 -2 What is oxidized? What is reduced?-2 +2
Analyze: Mg + S  MgS
What is oxidized?
What is reduced?
Assign Oxidation Numbers
Figure out change in oxidation numbers
Mg: to +2 = Oxidation
S: to -2 = Reduction

3. Mg + S  MgS 2 electrons 2 electrons
Identify what species is oxidized & what species is reduced
Figure out the 2 half-reactions

4. Half-Reactions Mg + S  MgS +2 -2 Mg is oxidized: S is reduced:
+2 -2
Mg + S  MgS
Mg is oxidized:
Mg  Mg+2 + 2e-
S is reduced:
S + 2e-  S-2

5. Adjust half-reactions so that electrons lost = electrons gained
Add half-reactions:
Mg  Mg+2 + 2e-
S + 2e-  S-2
__________________________________
Mg + S + 2e-  Mg+2 +2e- + S-2
Balance elements by counting atoms

6. Zn + 2HCl  H2 + ZnCl2 +1 -1 +2 -1 Zn goes from 0 to +2 = oxidation+1 -1 +2 -1
Zn goes from 0 to +2 = oxidation
H goes from +1 to 0 = reduction
Cl goes from -1 to -1; No change

7. Zn +2HCl  H2 + ZnCl2 Zn  Zn+2 + 2e- 2H+1 + 2e-  H2
2 electrons
Zn +2HCl  H2 + ZnCl2
1 electron per H
Zn  Zn+2 + 2e-
2H+1 + 2e-  H2
______________________________________
Zn + 2H+1 +2e-  Zn+2 +2e- + H2

8. Balancing Redox Equations
Assign oxidation numbers to all atoms in equation
Determine elements changed oxidation number
Identify element oxidized & element reduced
Write half-reactions (diatomics must stay as is)
Number electrons lost & gained must be equal; multiply half-reactions if necessary
Add half-reactions; Transfer coefficients to skeleton equation
Balance rest of equation by counting atoms

9. Cu + AgNO3  Cu(NO3)2 + Ag +1 +5 -2 +2 +5 -2+1 +5 -2 +2 +5 -2
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

10. Half-Reactions +______________________ Cu  Cu+2 + 2e- Ag+1 + 1e-  Ag
Multiply by 2
+______________________
Cu + 2Ag+1 + 2e-  2Ag + Cu+2 + 2e-

11. Transfer Coefficients
Compare skeleton equation & sum of ½ rxns:
Cu + AgNO3  Ag + Cu(NO3)2
vs.
Cu + 2Ag+1 + 2e-  2Ag + Cu+2 + 2e-
Transfer coefficients!
Cu + 2AgNO3  2Ag + Cu(NO3)2

12. Cu + HNO3  Cu(NO3)2 + NO2 + H2O Assign Oxidation Numbers+1 +5 -2 +2 +5 -2 +1 -2 +4 -2
Cu + HNO3  Cu(NO3)2 + NO2 + H2O
Assign Oxidation Numbers
Identify species oxidized & reduced
Cu from 0 to +2 = oxidized
H from +1 to +1; no change
O from -2 to -2; no change
N starts as +5;
ends as +5 (no change)
and ends as +4 = reduction

13. Cu + HNO3  Cu(NO3)2 + NO2 + H2O Find change in oxidation number
Change of +2
Cu + HNO3  Cu(NO3)2 + NO2 + H2O
Change of -1
Find change in oxidation number
Write half-reactions
Cu  Cu+2 + 2e-
N+5 + 1e-  N+4

14. What’s oxidized? What’s reduced?
What is oxidized?
What is reduced?
What is the oxidizing agent?
What is the reducing agent? Cu
Can’t just say N!
It’s N in the HNO3 or N+5
N+5 Cu

15. Multiply half-reactions as necessary
Cu  Cu+2 + 2e-
2N+5 + 2e-  2N+4
[# of electrons lost = # gained]
Cu + 2HNO3  Cu(NO3)2 + 2NO2 + H2O
Add half-reactions; Transfer coefficients
Balance remaining atoms by inspection
Cu + 4HNO3  Cu(NO3)2 + 2NO2 + 2H2O