Electrochemical cells (batteries)
Introduction
Tiny Battery
Take a piece of tin foil from the cupboard in your home and gently bite
down on it with your teeth. If you have any fillings in your mouth you will
immediately feel a very unpleasant sensation. We have all done this at some
time. What is happening?
The saliva in your mouth, the filling (usually
amalgams) and the tinfoil have set up a little electrochemical cell, that
produces a tiny current that travels through your tooth to the nerve below the
filling.
History
Leigh Galvani was the first to find that different metals strips (electrodes)
when they came in contact with a conducting solution would generate an
electrical current. Alessandro Volta used this information to develop the first
electrochemical cells using metal electrodes and solutions. Volta created the
current by placing two different metals in a salt solution. His first
electrochemical cell used copper and zinc metal for the electrodes. He later
created his first battery by linking many of these cells together is series, like
boxcars in a train.
Parts of a Voltaic Cell
Each electrochemical cell consists of two solid conductors we call
electrodes, and a conducting solution like salt water. In most cells the
conducting solution is a moist paste, containing only enough solution for the
cell to operate efficiently. The electrochemical cell is actually composed to
two half cells. Each half cell consists of one conducting electrode ( a solid )
and a conducting solution.
Oxidation occurs in one half cell and this produces electrons. Reduction
occurs in the other half cell and this accepts electrons. The flow of electrons
does not start until the battery or cells are have their electrodes connected by
an external conducting wire like copper.
How Electrochemical cells work
An explanation of how the electrochemical cell works did not occur until
100 years after the first cells were created.
Chemists believe the following happens when the two half cells are
connected.
When the two half cells, consisting of a metal electrode and a conducting
solution are connected with an external wire, the strongest oxidizing agent
will undergo a reduction in one half cell and the strongest reducing agent
will undergo an oxidation in the other half cell. Tables of the strength of
oxidizing and reducing agent can be found in any good chemistry textbook.
Key Points about the electrochemical cell
1) Cathode: The electrode (solid) at which reduction occurs is termed the
cathode. The charge on the cathode is positive
2) Anode: The electrode (solid) at which the oxidation occurs is called the
anode. The charge on the anode is negative , because it producing electrons
3) Direction of current flow: The electrons produced at the anode travel
along the external wire to the cathode were they are used to reduce the
strongest oxidizing agent. This happens because the strongest oxidizing
agent has a greater attraction for electrons and literally pulls them off the
strongest reducing agent.
4) Spontaneity: The reaction in an electrochemical cell (also called a
voltaic cell) is always spontaneous.
5) Ion flow: In the operating electrochemical cell, positive ions (cations)
in the conducting solutions migrate toward the collection of electrons on the
positive electrode or cathode, while negative ions migrates (move) toward
the negative electrode termed the anode.
Representing cells
Chemists represent the half cells, using a the following notation, a vertical
line is used to indicate the separation of the solution and the electrode(solid).
A copper half cell is represented below.
Cu2+
Solution
|
Cu (s)
Electrode
Chemists represent the entire electrochemical cell by placing the
two half - cells side by side and separating them with two vertical
lines.
The oxidation half - cell is typically written first and the
solution and electrode positions are reversed. The reduction
half - cell is written second.
A zinc - copper cell is represented below:
Zn (s)
| Zn 2+ (aq)
electrode
solution
Oxidation half cell
||
Cu 2+ (aq) |
Cu (s)
solution
electrode
Reduction half cell