21.2
4.5 Electrochemistry
21.1, Oxidation Numbers
Oxidation numbers are decided by the molecule
Neutral molecule = add up to zero
Charged ion = add up to the charge
Some general rules
O is usually -2
H is usually +1 (bond to metal is -1)
First two columns of metals follow the column number
Non-metals can vary (decide from everything else)
For ionic compounds split up the ions:
FeSO4 consider Fe2+ and SO42- individually
21.1 Redox Recap
OIL RIG
Oxidation involves Loss
Losing electrons
Oxidation number of atom becomes more
positive
Reduction involves Gain
Gaining electrons
Oxidation number of atom becomes negative
Balancing
More Redox
Balance the following redox reaction
NO3 + Fe NO + Fe2O3 (acidic)
And this one
BrO3 + Sb Br2 + SbO+ (basic)
21.1 Half-reaction method
In Acidic Solution
1. Write two half-reactions (one oxidation, one
reduction)
2. Balance all but Hs and Os
3. Balance Os by adding H2O
4.
5.
6.
7.
Balance Hs by adding H+
Balance charge by adding electrons (e-)
Do steps 2 through 5 for other half-reaction
Combine both half reactions, making sure ecancel out
21.1 Half-reaction method
In Basic Solution
1. Write two half-reactions (one oxidation, one
reduction)
2. Balance all but Hs and Os
3. Balance Os by adding H2O
4.
5.
6.
7.
8.
Balance Hs by adding H+
For the number of H+, add OH to both sides
Balance charge by adding electrons (e-)
Do steps 2 through 5 for other half-reaction
Combine both half reactions, making ecancel sure out
Balancing Redox
Practice!
Good website:
http://science.widener.edu/svb/tutorial/balan
ceredoxcsn7.html
21.1 Redox
Balanced reaction:
2 H+ + 2 NO3 + 2 Fe 2 NO + Fe2O3 + H2O
If you have NO3 and Fe together, where do
electrons go?
G = 283.5 kJ/mol
Is this reaction spontaneous? Will the
electrons move without a continuous force
on them?
Electrochemcial cell
Voltaic Cell
or Galvanic Cell
Spontaneous reaction
G < 0
System does work on
the surroundings
Electrolytic Cell
Non-spontaneous
reaction
G > 0
Surroundings must do
work on the system
Creating electricity is
the desired outcome:
A Battery
Usually the chemical
reaction is the desired
outcome here
21.2 Voltaic Cells
We need to separate the reduction and
oxidation steps to take advantage of the
electricity generated
Cu2+ + Zn Cu + Zn2+
21.2 Voltaic Cells
At the Anode:
Oxidation: Zn(s) Zn2+(aq) + 2 e The anode is converted to the aqueous ion
in solution
The salt bridge balances the charge, it
adds SO42- so that the solution remains
neutral
21.2 Voltaic Cells
At the Cathode:
Reduction: Cu2+(aq) + 2e- Cu(s)
The cathode is getting larger as the ions in
the solution build up around it
The salt bridge balances the charge, it
adds Na+ so that the solution remains
neutral
Things to remember
Anode is oxidation (both begin with
vowels)
Cathode is reduction
Electrode: generic term for either anode or
cathode
Salt bridge is needed (to complete the
circuit)