9.2 Balancing Redox Reactions
Using the Half Reactions Method
Learning Goals …
… balance complex redox reactions using half
reactions in aqueous solutions
… balance redox reactions in acidic and basic
solutions
Balancing Redox Equations:
Half Reactions Method
Steps:
1. Write the oxidation and reduction half reactions
2. Balance elements other than O and H
3. Balance each half-reaction: add H20 to balance
oxygen; add H+ to balance the hydrogen (acidic
conditions); add H+ and OH- to balance the
hydrogen (in basic conditions)
4. Add electrons to balance charge
5. Balance the electrons (charge)
6. Add the equations together
7. Check your answer
Ex)ClO4-
+
NO2
🡪
Cl-
NO2
+ H2O🡪 NO3-
ClO4-
+ Cl- 🡪
+8H +8e
+ NO3-
4
8NO2 + 8 H2O 🡪 8 NO3ClO4- + 8 H+ 🡪
+ 8 eCl- 8NO2 + 4 H2O + 🡪
ClO84- NO3-
(acidic)
+ 2 H+
+ e-
+ 4 H 2O
x8
x1
8 +
+ 16 H + 8 e+ 4 H 2O
+ Cl- + 8 H+
Ex)ClO-
+
CrO2-
Cl2
🡪
CrO2-
🡪
+ 2 H 2O
+ 4 OH-
CrO42-
ClO
2
2 2e+ 4🡪
H+ + Cl
+ 4 OH+ 4 H 2O
2
2 CrO2- + 8 OH- 🡪 2 CrO42- 2
6 ClO + 6 H2O +🡪6 e3- Cl2
22 CrO2- + 6 ClO- + 2 H🡪
2
CrO
O
4
2
+ CrO42-
(basic)
+ 4 H+ + 3 e - x 2
+ 4 OH+ 4 H 2O
2
x3
+ 2 H 2O
+ 4 OH-
+ 4 H 2O + 6 e 4
+ 12 OH+ 3 Cl2 + 4 OH-
Self Check
How prepared am I to start my homework? Can I …
… balance complex redox reactions using half reactions in
aqueous solution?
… balance redox reactions in acidic and basic solutions
HOMEWORK
p616 #1,2
p617 #7-10
p630 #44, 45