Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107	Fundamentals of Chemistry	Fall 2009
Lecture Notes: Equation Balancing	© R. Paselk 2005
	Worked Examples
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Example. Balance the following equation as it occurs in acid solution:

MnO₄^- + Cl^- Mn²⁺ + Cl₂

First break the equation into two half reactions, one for Mn and one for Cl

MnO4- Mn2+ MnO4- Mn2+ MnO4- Mn2+ + 4 H2O 8 H+ + MnO4- Mn2+ + 4 H2O 5 e- + 8 H+ + MnO4- Mn2+ + 4 H2O 10 e- + 16 H+ + 2 MnO8- 2 Mn2+ + 8 H2O Cl- Cl2 2 Cl- Cl2 2 Cl- Cl2 2 Cl- Cl2 2 Cl- Cl2 + 2 e- 10 Cl- 5 Cl2 + 10 e-

10 e^- + 16 H⁺ + 2 MnO₄^- + 10 Cl^- 2 Mn²⁺ + 8 H₂O + 5 Cl₂ + 10 e^-

16 H⁺ + 2 MnO₄^- + 10 Cl^- 2 Mn²⁺ + 8 H₂O + 5 Cl₂

Redox Balancing - Example in Basic Solution

ClO₂ ClO₂^- + ClO₃^-

First break the equation into two half reactions, one for Mn and one for Cl

ClO2 ClO2- 1 e- + ClO2 ClO2- This one is easy - just one step: balance charge with an electron. ClO2 ClO3- H2O + ClO2 ClO3- 2 OH- + H2O + ClO2 ClO3- + 2 H2O 2 OH- + H2O + ClO2 ClO3- + 2 H2O + 1 e-

1 e^- + ClO₂ + 2 OH^- + H₂O + ClO₂ ClO₂^- + ClO₃^- + 2 H₂O + 1 e^-

Cancelling and gathering terms:

2 ClO₂ + 2 OH^- ClO₂^- + ClO₃^- + H₂O

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Last modified 25 October 2005