Balancing Redox Equations

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Balancing Redox Equations
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Analyze Mg S ? MgS

• What is oxidized?
• What is reduced?
• Assign Oxidation Numbers
• Figure out change in oxidation numbers

Mg 0 to 2 Oxidation S 0 to
-2 Reduction
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Mg S ? MgS

• Identify what species is oxidized what species
  is reduced
• Figure out the 2 half-reactions

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Half-Reactions

• Mg S ? MgS

S is reduced S 2e- ? S-2
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Adjust half-reactions so that electrons lost
electrons gained
Add half-reactions

• Mg ? Mg2 2e-
• S 2e- ? S-2

Balance elements by counting atoms
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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

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Zn 2HCl ? H2 ZnCl2
2 electrons

• Zn ? Zn2 2e-
• 2H1 2e- ? H2

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

1. Assign oxidation numbers to all atoms in equation
2. Determine elements changed oxidation number
3. Identify element oxidized element reduced
4. Write half-reactions (diatomics must stay as is)
5. Number electrons lost gained must be equal
   multiply half-reactions if necessary
6. Add half-reactions Transfer coefficients to
   skeleton equation
7. Balance rest of equation by counting atoms

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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

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Half-Reactions

• Cu ? Cu2 2e-
• Ag1 1e- ? Ag
• Cu ? Cu2 2e-
• 2Ag1 2e- ? 2Ag

Multiply by 2
______________________
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Transfer Coefficients

• Compare skeleton equation sum of ½ rxns
• Cu AgNO3 ? Ag Cu(NO3)2
• vs.
• Cu 2Ag1 2e- ? 2Ag Cu2 2e-
• Transfer coefficients!
• Cu 2AgNO3 ? 2Ag Cu(NO3)2

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

1. Assign Oxidation Numbers
2. 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
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Cu HNO3 ? Cu(NO3)2 NO2 H2O

• Find change in oxidation number
• Write half-reactions
• Cu ? Cu2 2e-
• N5 1e- ? N4

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Whats oxidized? Whats reduced?

• What is oxidized?
• What is reduced?
• What is the oxidizing agent?
• What is the reducing agent?

Cant just say N! Its N in the HNO3 or N5
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• Cu ? Cu2 2e-
• 2N5 2e- ? 2N4
• of electrons lost gained
• Cu 2HNO3 ? Cu(NO3)2 2NO2 H2O