1. 18.3 Balancing equations using Half-reactions in acidic or basic environments
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2. 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
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

3. SAMPLE PROBLEM 1 IO3-(aq)+ Al (s) → I2 (s) + Al 3+(aq)
Find half reactions:
IO3-(aq) → I2 (s)
Al (s) → Al 3+(aq)

4. try to find a reduction and an oxidation half reaction that matches on page 11
2IO3-(aq)+ 12H+ (aq) + 10 e-→ I2 (s) + 6H2O (aq)
Oxidation:
Al (s) → Al 3+(aq) + 3 e-

5. Finally, add them together and inspect.
Multiply by 3
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)

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

7. SAMPLE PROBLEM 3
Use the half reactions found on page 11 to write a balanced redox equation for the oxidation of NO (g) by an acidic Cr2O7 2- (aq) solution.

8. PRACTICE: 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)

9. Homework
Page732 # 17-20
Page 734 # 21-24