2. ELECTROCHEMISTRY
Deals with chemical changes produced by an electric current and with
the production of electricity by chemical reactions
All electrochemical reactions involve transfer of electrons and are
redox reactions
Electrochemical reactions take place in electrochemical cell (an
apparatus that allows a reaction to occur through an external
conductor)
3. ELECTROCHEMICAL CELLS
Two types:
Voltaic cells: produce electrical energy
Also called galvanic cells. In these cells spontaneous
chemical reactions generate electrical energy and supply it
to an external circuit.
Electrolytic cells: require electrical energy
These are cells in which an external electrical source forces
a nonspontaneous reaction to occur.
4. Electrochemical Terms – all cells
Electrode: A conductor used to establish contact with a
nonmetallic part of a circuit, such as an electrolyte
Half-cell: a metal electrode in contact with a solution of
its own ions
Anode: The electrode where oxidation takes place
Cathode: The electrode where reduction takes place
5. VOLTAIC CELLS
• Cells in which spontaneous reactions produces
electrical energy
• The two half-cells are separated so that electron
transfer occurs through an external circuit
• Each half-cell contains the oxidized and reduced
forms of a species in contact with each other
• Half-cells are linked by a piece of wire and a salt
bridge
7. The Zinc-Copper cell
Composed of two half-cells:
1. A strip of copper immersed in 1 M CuSO4
2. A strip of zinc immersed in 1 M ZnSO4
Experimentally we see:
Initial voltage is 1.10 volts
The mass of the zinc electrode decreases
The mass of the copper electrode increases
[Zn2+] increases and [Cu2+] decreases
8.
9. Zinc – Copper Cell - notation
Salt bridge
Zn Zn 2+ (1.0 M) Cu 2+ (1.0 M) Cu
Electrode
Species (with
concentrations) in
contact with electrodes
10. A salt bridge (or porous partition) has three functions:
1. The salt bridge allows for the flow of ions and
therefore electrical contact between the two half-cells
2. As a result of electrical contact, the salt bridge
maintains the electrical neutrality in each half-cell as
ions flow into and out of the salt bridge
3. The salt bridge prevents mixing of the electrode
solutions
11. Electrical neutrality
in each half cell is
important!
anions flow into the
oxidation half-cell to
counter the build-up
of positive charge
Current flows spontaneously from and vice versa
negative to the positive electrode
(oxidation electrode to reduction If this did not
electrode) happen, current
would stop flowing
In voltaic cells, voltage drops as the
reaction proceeds. When voltage = 0,
the reaction is at equilibrium
12. Voltaic Cells - Summary
Cathode:
Anode: reduction
oxidation positive
negative
Voltaic cells: Electrochemical cells in which a
spontaneous redox reaction can be harnessed to
produce an electric current.
