1. Chapter 18
Electrochemistry

2. Definition
The study of the interchange of chemical and electrical energy.
Involve oxidation-reduction reactions

3. Half-Reaction Method Used to balance redox rxns in aqueous solution
One of the rxns is the oxidation rxn
Other is the reduction rxn
The half reactions are balanced separately and then added
Differs in acidic vs. basic solution

4. In Acidic Solution Separate the oxid. and red. parts of the rxn.
For each:
a. Balance all elements but H and O
b. Balance O using H2O
c. Balance H using H+
d. Balance charge using electrons
If needed, mult. One or both half-rxns. by a whole # to make electrons equal for both
Add half-rxns and cancel same on both sides
Make sure elements/charges are balanced

5. Acronym for Balancing in Acid
Big White Hogs Eat Bagels and Cream Cheese
B = balance all but H & O
W = add water for O’s
H = Add H
E = Add electrons
B = Make sure electrons match
CC = Combine and cancel

6. Example in Acid
Balance the following equation in acid solution using the half-reaction method.
Cu(s) + HNO3(aq) -> Cu2+(aq) + NO(g)
Answer:
3Cu + 2HNO3 + 6H+ -> 3Cu2+ + 2NO + 4H2O
Cr2O72-(aq) + NO(g) -> Cr3+(aq) + NO3-(aq)
Cr2O72-(aq) + 2NO(g) + 6H+(aq) -> 2Cr3+(aq) + 2NO3-(aq) + 3H2O(l)

7. In Basic Solution Follow the steps used in acidic solution THEN…
Add OH- to match H+ on same side. Add to other side as well
Form H2O by adding OH- and H+
Cancel H2O’s on both sides
Check for element/charge balance

8. P4 + 2H2O + 4OH- -> 2PH3 + 2HPO32-
Example in Base
Balance the following equation in basic solution:
P4 -> PH3 + HPO32-
Answer
P4 + 2H2O + 4OH- -> 2PH3 + 2HPO32-