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Chapter 20
Electrochemistry
Electrochemistry is the branch of chemistry that focuses on the electricity
related aspects of oxidation-reduction reactions.
If you separate the oxidation reaction from the reduction reaction in an
appropriate way, you can use the electrons being transferred to produce
electricity.
The oxidation reaction can be separated from the reduction reaction by using
either a salt bridge or a porous disk. Both of the devices function in the same
way-they allow ions to migrate from one solution to the other without letting the
two solution mix.
An electrode is a conductor that is used to establish contact with the non-metallic
portion of a circuit (an electrolyte solution). The electrode where the oxidation
reaction occurs is called the anode. The electrode where the reduction reaction
occurs is called the cathode.
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Diagram of an Electrochemical Cell (battery or galvanic cell)
(You should be able to completely label a diagram similar to this.)
Zn (s) +
Cu+2 (aq)
Cu (s)
+
Zn+2 (aq)
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Line Notation is a short hand way of describing a redox reaction.
Anode electrode
Anode solution
Cathode solution
Cathode electrode
An Ox and a Red Cat
Oxidation is at the Anode
Reduction is at the Cathode
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Anode electrode
Zn (s) +
Anode solution
Cu+2 (aq)
Cathode solution
Cu (s)
+
Cathode electrode
Zn+2 (aq)
Let’s write the line notation for the reaction above.
First identify the oxidation reaction (that is where the anode
will be identified).
Second identify the reduction reaction (that is where the
cathode will be identified).
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Electrical Potential (voltage or how forcefully the electrons
“want” to be transferred. The electrical potential for a battery
must be positive in order to operate.
We can use a table of standard reduction potential to
determine the direction that a galvanic cell will operate (also
the direction of the redox reaction).
For example, if we wanted to use:
Fe (s), Fe+3 (aq), Al (s), and Al+3 (aq)
what would the redox reaction look like, what would be
the cell potential, and what substance would the anode and
cathode be made of?
See page 626 to find the standard reduction potentials needed.
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Balance the Redox reaction below:
H2S (g) + MnO4-1 (aq)
S (s)
+
Mn+2 (aq)
Find xo:
Draw a cell and label all parts of the cell:
Write the line notation for the cell:
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Classwork/Homework:
Complete problems 3, 9, 16, 15, 18, 27 on pages 636-637.
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Voltaic Cell (battery) versus Electrolytic Cell (electrolysis)
If a redox reaction has a negative value for xo, then we can use
electricity to force the reaction to occur. When we use electricity
to make a redox reaction occur, we are using an Electrolytic Cell.
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