10.3 The Half-Reaction Method for Balancing Equations

SCH4U1

Dec 8th, 2009

Balancing Half-Reactions

• Half-Reaction: describes either the oxidation or reduction of a REDOX reaction.

• Oxidation: K K+ + e-

• Reduction: Cl2 + 2e- 2Cl-

• Net Reaction: K + Cl2 2KCl

Balanced Half Reactions and Net Reactions

• Net Reaction: K + Cl2 2KCl

• Balance: 2K + Cl2 2KCl

» 2 electrons lost 2 electrons gained

Balancing Half-Reactions in Acidic Media

• Write unbalanced half-reactions

• Balance all atoms EXCEPT for O and H

• Balance O by adding H2O

• Balance H by adding H+ ions

• Balance the charges by adding electrons

Balancing Half-Reactions in Acidic Media

• ClO3 - Cl-

• ClO3 - Cl- + 3H2O (add water)

• 6H+ + ClO3 - Cl- + 3H2O (add H ions)

• 6e- + 6H+ + ClO3 - Cl- + 3H2O (add electrons)

Balancing Half-Reactions in Basic Media

• Write unbalanced half-reactions• Balance all atoms EXCEPT for O and H

• Balance O by adding H2O

• Balance H by adding H+ ions• To BOTH sides, add as many OH- as H+ were

added

• Simplify OH- + H+ to H2O if possible

• Remove excess H2O from both sides

• Balance the charges by adding electrons

Balancing Half-Reactions in Basic Media

• MnO4- MnO2

• MnO4- MnO2 + 2H2O

• 4H+ + MnO4- MnO2 + 2H2O

• 4OH- + 4H+ + MnO4- MnO2 + 2H2O + 4OH-

• 4OH- + 3e- + 4H+ + MnO4- MnO2 + 2H2O +

4OH-

• 4H2O + 3e- +MnO4- MnO2 + 2H2O + 4OH-

• 2H2O + 3e- + MnO4- MnO2 + 4OH-

Balancing the Net Ionic Reaction

• Electrons are neither created nor destroyed• Electrons lost = Electrons gained Break the reaction into 2 half-reactions Balance the half-reactions separatelyMultiply both by the lowest common multipleRecombine the two half-reactionsSimplify by removing electrons and excess

species

Balancing the Net Ionic Reaction

• MnO4- + Ag Mn2+ + Ag+ (acidic conditions)

Oxidation

Ag Ag+ + e-

(balanced)

For a 5e- transfer:

5Ag 5Ag+ + 5e-

Reduction

MnO4- Mn2+

MnO4- Mn2+ + 4H2O

8H+ + MnO4- Mn2+ + 4H2O

8H+ + 5e- + MnO4- Mn2+ + 4H2O

Balancing the Net Ionic Reaction• 5Ag 5Ag+ + 5e-

• 8H+ + 5e- + MnO4- Mn2+ + 4H2O

• 5Ag + 8H+ + 5e- + MnO4- 5Ag+ + 5e- + Mn2+ +

4H2O

• 5Ag + 8H+ + MnO4- 5Ag+ + Mn2+ + 4H2O

Homework

p. 662 # 18 p. 668 # 2a, 3a p. 673 # 6a, 7b