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# 18.3 Balancing equations using Half-reactions in acidic or basic environments - PowerPoint PPT Presentation

18.3 Balancing equations using Half-reactions in acidic or basic environments. Read pages 730-750. METHOD 1 – USING THE TABLE OF STANDARD REDUCTION POTENTIALS. identify reactants and products for possible half reactions. Find half reaction equations from page 11 (data booklet) that match

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### 18.3 Balancing equations using Half-reactions in acidic or basic environments

METHOD 1 – USING THE TABLE OF STANDARD basic environmentsREDUCTION POTENTIALS

• identify reactants and products for possible half reactions.

• Find half reaction equations from page 11 (data booklet) that match

• Look for a reduction half reaction and an oxidation half reaction.

• Make the number of electrons lost equal the number of electrons gained. (multiply the reactions)

• Add the oxidation and reduction half reactions. Electrons will cancel out and equation is balanced

NOTE:

• number of atoms and total charge on both sides must balance out.

• If it takes place in acidic environment add H+, if basic environment add OH- to help balance out charges and atoms

SAMPLE PROBLEM 1 basic environments

• IO3-(aq)+ Al (s) → I2 (s) + Al 3+(aq)

Find half reactions:

IO3-(aq) → I2 (s)

Al (s) → Al 3+(aq)

try to find a reduction and an oxidation half reaction that matches on page 11

Reduction:

2IO3-(aq)+ 12H+ (aq) + 10 e-→ I2 (s) + 6H2O (aq)

Oxidation:

Al (s) → Al 3+(aq) + 3 e-

Multiply by 3 matches on page 11

6IO3-(aq)+ 36H+ (aq) + 30 e-→ 3I2 (s) + 18H2O (aq)

Multiply by 10

10Al (s) → 10Al 3+(aq) + 30 e-

Finally, add them together and inspect.

6IO3-(aq)+ 36H+ (aq) + 10Al (s) → 3I2 (s) + 18H2O (aq) +10Al 3+(aq)

SAMPLE PROBLEM 2 matches on page 11

• Use half reactions found on page 11to write a balanced redox equation for the reduction of acidic MnO4-(aq) solution to Mn 2+(aq) by Ni (s).

SAMPLE PROBLEM 3 matches on page 11

• Use the half reactions found on page 11to write a balanced redox equation for the oxidation of NO (g) by an acidic Cr2O72-(aq) solution.

PRACTICE: matches on page 11

Write the balanced oxidation-reduction equation for:

• the reduction of acidic MnO4- (aq) solution to Mn 2+ (aq) by Fe 2+ (aq).

• the reduction of acidic MnO4- (aq) solution to MnO2(s) by Pb (s).

• Al + Fe 3+ → Al 3+ + Fe 2+

• the oxidation of Cl- (aq) by acidic H2O2(aq)

Homework matches on page 11

• Page732 # 17-20

• Page 734 # 21-24