Half-reactions show the oxidation or reduction reaction separated.

Cu(s) + 2 AgNO3(aq) → Cu(NO3)2(aq) + 2 Ag(s)

Oxidation: Cu → Cu2+ + 2e–

Reduction: Ag+ + 1e– → Ag

Half reactions are often shown as aqueous net ionic equations – spectator IONS not included.

+2+10 0

Balancing redox reactions 2

Two methods for balancing redox reactions: 1. Oxidation number method 2. Half-reaction method.

• Balance redox equations using the oxidation number method.

• Balance redox equations in acidic and basic solutions using the half reaction method.

Redox reactions usually require an acidic or basic solution.

• Acid / base not oxidized or reduced in reaction. • Usually converted to water.

Half-reaction method is used for balancing redox reactions in the presence of acid or base.

Cr2O72–

(aq) + SO32–

(aq) → Cr3+(aq) + SO4

2–(aq)

1: Assign ox.numbers and write half-reactions.

Balancing Redox Reactions in Acidic Solutions

+3+4-2+6 -2+6-2

Oxidation: SO32- → SO4

2- + 2e–

Reduction: Cr2O72- + 3e– → Cr3+

Not ionic (no spectators) –keep together!

2: Balance all elements except H and O.

3: Balance oxygen atoms by using H2O.

Oxidation: SO32- → SO4

2- + 2e–

Reduction: Cr2O72- + 6e– → Cr3+2

Oxidation: SO32- + H2O → SO4

2- + 2e–

Reduction: Cr2O72- + 6e– → 2 Cr3+ + 7 H2O

4: Balance hydrogen atoms using H+ ions.

5: Balance the number of electrons lost and gained.

Oxidation: SO32- + H2O → SO4

2- + 2e–

Reduction: Cr2O72- + 6e– → 2 Cr3+ + 7 H2O

+ 2 H+

14 H+ +

Oxidation: 3 x (SO32- + H2O → SO4

2- + 2e– + 2 H+)

Reduction: 14 H+ + Cr2O72- + 6e– → 2 Cr3+ + 7 H2O

3 SO32- + 3 H2O → 3 SO4

2- + 6e– + 6 H+

6: Add the two half-reactions.

Oxidation: 3 SO32- + 3 H2O → 3 SO4

2- + 6e– + 6 H+

Reduction: 14 H+ + Cr2O7

2- + 6e– → 2 Cr3+ + 7 H2O

8 H+ + Cr2O72- + 3 SO3

2- → 2 Cr3+ + 3 SO42- + 4 H2O

MnO4– + I– → MnO2 + I2

8 H+ + 2 MnO4– + 6 I– → 2 MnO2 + 3 I2 + 4 H2O

Balance the following reaction in a acidic solution.

-1+7 0+4

Oxidation: I1- → I2 + 2e–

Reduction: MnO41- + 3e– → MnO2

2

+ 2 H2O4 H+ +

3 x ()

2x (

)Oxidation: 6 I1- → 3 I2 + 6e–Reduction: 8 H+ + 2 MnO4

1- + 6e– → 2 MnO2 + 4 H2O

Balancing Redox Reactions in Basic Solutions• For basic solutions add hydroxide ions.

MnO4– + C2O4

2– → CO2 + MnO2

+4+3+7 +4

Oxidation: C2O42- → CO2 + 2e–

Reduction: MnO41- + 3e– → MnO2

2

+ 2 H2O4 H+ +

*5a: Add the same number of OH- as H+ to BOTH sides of the equation.

5b: Eliminate H+ / OH- by forming water..Cancel any waters you can to simplify each half reaction.

Oxidation C2O42- → 2 CO2 + 2e–

Reduction: 4 H+ + MnO41- + 3e– → MnO2 + 2

H2O+ 4 OH-4 OH- +

4 H2O2

Oxidation C2O42- → 2 CO2 + 2e–

Reduction: 2 H2O + MnO41- + 3e– → MnO2 + 4

OH-

3 x ()

2x (

)

4 H2O + 2 MnO4– + 3 C2O4

2– → 2 MnO2 + 6 CO2 + 8 OH–

Oxidation 3 C2O42-

→ 6 CO2 + 6e–

Reduction: 4 H2O + 2 MnO41- + 6e– → 2 MnO2 + 8

OH-

N2O + ClO– → NO2– + Cl–

Balance the following reaction in a basic solution.+3+1+1 -1

Oxidation: N2O → NO2- + 4e–

Reduction: ClO- + 2e– → Cl-

23 H2O +

2 H+ +

+ 6 H+

+ 1 H2O

5a: Add same number of OH- to BOTH side.5b:Cancel any waters you can to simplify half reactions.

2 OH– + 2 ClO– + N2O → 2 Cl– + 2 NO2– + H2O

Oxidation: N2O → NO2- + 4e–

Reduction: ClO- + 2e– → Cl-

23 H2O +

2 H+ +

+ 6 H+

+ 1 H2O

+ 6 OH-

2 OH- + 2 H2O

3

+ 2 OH-

6 OH- + 6 H2O

1

Reduction: ClO- + 2e– → Cl-1 H2O + + 2 OH-

Oxidation: N2O → NO2- + 4e–2 + 3 H2O6 OH- +

2x ( )

2 H2O + 2 ClO- + 4e– → 2 Cl- + 4 OH-

12