Balancing Redox

1
Balancing Redox

• Tales From Beyond the Stoichiometric
• The Missing Slides

2
Half-Reaction Method

• Must occur in aqueous (water) solutions
• Split in to an oxidation reaction and a reduction
  reaction and write as half-reactions
• Must first assign oxidation states to determine
  what is oxidized and what is reduced

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Example

• MnO4-(aq) Fe2(aq) ? Fe3(aq) Mn2(aq)
• Mn is reduced and Fe is oxidized
• MnO4-(aq) ? Mn2(aq)
• Fe2(aq) ? Fe3(aq)

4
Balancing Half-Reactions - Acidic

• Balance everything except oxygen and hydrogen
• Balance oxygen with H2O
• Balance hydrogen with H
• Balance charge with e-
• Balance electrons between the 2 half reactions
• Recombine and remove spectators

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Balance Atoms and Charges

• MnO4-(aq) ? Mn2(aq)
• MnO4-(aq) 8H 5e- ? Mn2(aq) 4H2O
• Fe2(aq) ? Fe3(aq) e-
• Electrons should appear with reactants for
  reduction and products for oxidation.

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Balance Electrons

• MnO4-(aq) 8H 5e- ? Mn2(aq) 4H2O
• 5(Fe2(aq) ? Fe3(aq) e-)
• 5Fe2(aq) ? 5Fe3(aq) 5e-

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Recombine

• MnO4-(aq) 8H 5e- ? Mn2(aq) 4H2O
• 5Fe2(aq) ? 5Fe3(aq) 5e-
• MnO4-(aq) 8H 5e- 5Fe2(aq) ? Mn2(aq)
  4H2O 5Fe3(aq) 5e-
• MnO4-(aq) 8H 5Fe2(aq) ? Mn2(aq) 4H2O
  5Fe3(aq)

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Balancing Half-Reactions - Basic

• Balance according to the acidic method.
• Add OH- to both sides to react H to make H2O
• Remove spectators

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Example

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

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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)