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 goes from 0 to +2: Oxidation
S goes from 0 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 electrons lost = electrons gained.
Add half-reactions.
Mg  Mg+2 + 2e-
S + 2e-  S-2
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Mg + S + 2e-  Mg+2 +2e- + S-2
Balance everything else by counting atoms.

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

8. Transfer the coefficients!
Zn + 2H+1  H2 + Zn+2
This is what you’ve got from adding the 2 half-reactions.
Zn + HCl  H2 + ZnCl2
This is the skeleton equation you started with.
Transfer the coefficients.
Zn + 2HCl  H2 + ZnCl2

9. Balancing Redox Equations
Assign oxidation numbers to all atoms in equation.
See which elements have changes in oxidation number.
Identify atoms that are oxidized & atoms that are reduced.
Write the half-reactions. Diatomics have to be written as diatomics.
Make the number of electrons lost & gained equal in magnitude by multiplying half-reactions as needed.
Add the half-reactions. Transfer coefficients to skeleton equation.
Balance remainder of equation by counting up atoms.

10. 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.

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

12. Transfer Coefficients
Compare skeleton equation & sum of half-reactions:
Cu + AgNO3  Ag + Cu(NO3)2
Vs.
Cu + 2Ag+1 + 2e-  2Ag + Cu+2 + 2e-
Transfer the coefficients!
Cu + 2AgNO3  2Ag + Cu(NO3)2)
Exception: Do NOT insert the coefficient of any item that appears in more than one place in the equation.

13. 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 which species are oxidized & which reduced.
Cu from 0 to +2 = oxidized
H from +1 to +1 so no change
O from -2 to -2 so no change
N all starts as +5. Some ends as +5, some as +4 = reduction

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

15. 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 the N in the HNO3.
Or the HNO3 or N+5.
N+5 Cu

16. Multiply half-reactions as necessary.
Cu  Cu+2 + 2e-
2N+5 + 2e-  2N+4
Now the # 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