1. 18.2-18.3 Electrochemistry- Balancing Redox Equations
By: Maisy Holehouse

2. Balancing Oxidation-Reduction Equations
Oxidation State- the charge of an an atom that has lost or gained electrons electrons
Oxidation - loss of electrons, also can mean an increase in oxidation state
Reduction - gain of electrons, also can mean a decrease in oxidation state
Leo the Lion goes Ger (loss electrons oxidation, gain electrons reduction)

3. Continued ...
Remember: Mass and charge must be conserved in a chemical equation
Half- reaction method of Balancing: To balance an equation, first we must separate the original reaction into two separate equations, the oxidation reaction and the reduction reaction.
There are slightly different steps if you are balancing in an
acidic or basic solution

4. √

5. Assigning Oxidation Numbers:
Please look at the sheet that is being passed out

6. Steps to Balance an equation
Assign Oxidation States
Separate the entire reaction into two half reactions
Balance each of the half reactions by mass
Add in O to make the reaction equal, then add in enough H20’s for each oxygen, then add the corresponding H+’s on the opposite side. (For basic solutions: after adding the H+’s, add enough OH - to neutralize the effect of the H+)
Balance each of the half reactions by charge
Make the number of electrons in both half reactions equal
Add two half reactions together

7. Balancing Redox Reactions in Acidic Solutions
Fe2+ (aq) + MnO4- (aq) -> Fe3+ (aq) + Mn2+ (aq)
Fe2+ (aq) + MnO4- (aq) -> Fe3+ (aq) + Mn2+ (aq)
Reduction
oxidation
Oxidation: Fe2+ (aq) -> Fe3+ (aq) + 1e-
Reduction: MnO4- (aq) -> Mn2+ (aq) + 4 H2O (l)
8 H+ (aq) + MnO4- (aq) -> Mn2+ (aq) + 4 H2O (l)
5 e- + 8 H+ (aq) + MnO4- (aq) -> Mn2+ (aq) + 4 H2O (l)

8. Continued … 5 [Fe2+ (aq) -> Fe3+ (aq) + 1e-]
Combine:
5 Fe2+ (aq) -> 5 Fe3+ (aq) + 5 e-
5 e- + 8 H+ (aq) + MnO4- (aq) -> Mn2+ (aq) + 4 H2O (l)
Final:
5 Fe2+ (aq) + 8 H (aq) + MnO4- (aq) -> 5 Fe3+ (aq) + Mn2+ (aq) + 4 H2O (l)

9. Balancing Redox Reactions in Basic Solutions
2I- (aq) + MnO4- (aq) -> I2 (aq) + MnO2 (s)
Reduction
Oxidation
Oxidation: 2 I- (aq) -> I2 (aq)
Reduction: MnO4- (aq) -> MnO2 (s) + 2H2O
4 H+ (aq) + MnO4- (aq) -> MnO2 (s) + 2H2O
4 H+ (aq) + 4 OH- (aq) + MnO4- (aq) -> MnO2 (s) + 2H2O (l) + 4 OH- (aq)
H2O (l)

10. Continued … Balance by charge: 2 I- (aq) -> I2 (aq) + 2 e-
4 H2O (l) + MnO4- (aq) + 3 e- -> MnO2 (s) + 2H2O (l) + 4 OH- (aq)
Make electron numbers equal:
3 [2 I- (aq) -> I2 (aq) + 2 e-] = 6 I- (aq) -> I2 (aq) + 6 e-
2 [4 H2O (l) + MnO4- (aq) + 3 e- -> MnO2 (s) + 2H2O (l) + 4 OH- (aq)] = 8 H2O (l) + 2 MnO4- (aq) + 6 e- -> 2 MnO2 (s) + 4 H2O (l) + 8 OH- (aq)
Add together:
6 I- (aq) -> I2 (aq) + 6 e-
4 8 H2O (l) + 2 MnO4- (aq) + 6 e- -> 2 MnO2 (s) + 4 H2O (l) + 8 OH- (aq)
6 I- (aq) + 4 H2O (l) + 2 MnO4- (aq) -> 2 MnO2 (s) + I2 (aq) + 8 OH- (aq)

11. Sample Problem Balance the reaction in an Acidic solution:
Al (s) + Cu2+ (aq) -> Al3+ (aq) + Cu (s)
Answer:
2 Al (s) + 3 Cu2+ (aq) -> 2 Al3+ (aq) + 3 Cu (s)

12. Voltaic Cell:

13. Voltaic Cells: Generating Electricity from Spontaneous Chemical Reactions
Electrical Current: Flow of electric charge
Electrochemical cell: The way electricity is generated through redox reactions
Voltaic Cell: an electrochemical cell that produces electrical current from a spontaneous reaction
Electrolytic cell: an electrochemical cell that consumes energy to make a nonspontaneous chemical reaction occur.
Half-Cell: one half, or one jar of the voltaic cell
Electrodes: metal strips placed into the half cells that are conductive surfaces, to let electrons flow through
Potential Difference: A measure in the difference of potential energy per unit of charge
Electromotive Force: Potential difference/ motion of electrons.

15. In a Voltaic Cell the potential difference between the two electrodes is the cell potential. (Ecell) or cell emf, the more willing a substance is to be either oxidized or reduced, the higher the cell potential. (found in appendix 2)
Cell potential: Measure of ability of redox reaction to happen spontaneously, negative means the forward reaction is not possible.
Voltaic cells are dependent on temperature and concentration (Standards: 25C, 1M solutions, 1 atm gas) This is called the standard cell potential (E*cell)
Electrode with oxidation: anode (-) ; Electrode with reduction: cathode (+)
Electrons flow anode to cathode, negative to positive (red cat/ an ox)
Cations ions flow out of the anode (because electrons are flowing out, they are not there to neutralize the ions), there is a build up of positive charge in the solution, and vise versa for the cathode, electrons flow into it, there is a buildup of negative charge.
To allow the reaction to continue, there is a salt bride suspended, one end into each half cell. This salt bridge has ions, the cations go to the half cell with the cathode, or the greater amount of negative charge; the anions flow to the cell with the anode, to neutralize the build up of positive charge.

17. Zinc and Copper Questions
A galvanic reaction is going to take place between zinc and copper. The Ecell=1.10 V
Can anyone tell me, If Zinc is more willing to give up electrons (more easily oxidized)
Which one is being oxidized? Reduced? (and which one is the anion/ cation)
Zn: oxidized/anion, Cu: reduced/cation
The half reactions for each
(next slide)
The equation, with charges
Zn (s) + Cu2+ (aq) -> Cu (s) + Zn2+
The directions of the flow of the electrons (From which element to which element)

19. Math of Electrochemistry
Amperes (A/ amps): the unit in which electrical current is measured, one amp represents the flow of one coulomb (C) per second.
1A = 1 C/s
An electron has a charge of x C.
1 A = x 1018/s
this flow of electrons is caused by Potential difference, which is the potential energy difference measurement, and the unit for this is Volts (V)
1 V = 1 J/C
This means the difference of one volt shows the charge of a coulomb has an energy difference of one joule between two electrodes.

20. Electrochemical cell Notation
Cell Diagram/ Line notation:
Oxidation on the left, and reduction on the right, with a double vertical line in between (salt bridge and indication the separation)
Ex:Zn2+/Cu2+ Cu2+-> Zn(s) Zn2+ (aq) Cu2+ (aq) Cu (s)
Compounds/elements in different phases are separated by a vertical line (a boundary between phases)
In some, the reactants/ products are in the same phase, so we use a comma to separate them
Inert Electrode: Used as an anode or cathode, and helps with electron transfer but it not part of the actual reaction (Pt is commonly used)

22. Sample Problems: Balance in a basic Solution:
Al (s) + MnO4- (aq) -> MnO2 (s) + Al(OH)4- (aq)
Answer:
Al (s) + MnO4- (aq) + 2H2O -> MnO2 (s) + Al(OH)4- (aq)

23. Sample Problem:
Draw a voltaic cell for each reaction, Label anode and cathode, show the half-reaction at each electrode. And the direction flow:
2 Ag+ (aq) +Pb (s) -> 2 Ag (s) + Pb2+ (s)
Answer:

26. College Board: