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Hi Guys, I'm Jackie and I'm a post grad student here at Curtin. When I was going through Chemistry
in high school I had quite a few issues with electrochemistry. In particular with electrolytic
and galvanic cells so here we're at the new Curtin Resources and Chemistry Precinct and
we're going to talk to an expert - Stuart Bailey - and hopefully that will sort some
problems out.
Hi, my name is Stuart Bailey. I'm a lecturer in Chemistry at Curtin University. Today I
want to talk to you about electrochemical cells. Electrochemical cells come in 2 broad
types - galvanic cells where a chemical reaction produces electrical energy and electrolytic
cells where electrical energy produces a chemical reaction. Sometimes some people mistakenly
refer to galvanic cells as electrochemical cells but when in fact both galvanic cells
and electrolytic cells are types of electrochemical cell.
The basic principle is that there is an exchange of electrical energy - and current - with
a chemical reaction.
We can identify that in any electrochemical cell there are at least 4 components:
An Anode - this is an electrode where oxidation takes place.
A Cathode - this is an electrode where reduction takes place.
An Electrolyte - this allows current to flow internally in the cell. For example a solution
of an ionic substance. An External Circuit - this allows electrical
current to flow, producing energy in a galvanic cell or supplying energy to an electrolytic
cell. Some cells also contain a device such as a
Salt Bridge which is an iconically conducting separator which allows the anode and cathode
to contain different solutions.
The most important thing about electrochemical cells is that they separate a chemical reaction
of the oxidation reduction type so that it occurs in 2 halves - each in a different location.
We that know oxidation reduction reactions can be described in terms of 2 half reactions.
Obviously the oxidation half-reaction and the reduction half-reaction. As described
earlier oxidation takes place at the anode of a cell and reduction takes place at the
cathode of a cell. The external circuit allows electrons to travel between the 2 electrodes
and the internal electrolyte circuit allows ions to travel between the 2 electrodes. We
can illustrate this by looking at a typical galvanic cell.
I'd like to illustrate a cell which has a lead anode.
We draw the anode on the left hand side.
And a silver cathode.
We draw the cathode on the right hand side. In order to make this cell work we need to
complete the circuit by providing a salt bridge.
And by providing an external circuit.
So now we have a cell which has, on the left hand side the anode.
At the anode we have an oxidation process.
So this produces electrons at the anode.
Electrons will flow through the circuit to the cathode on the right hand side.
At the right hand side, at the an... at the cathode - the electrons are consumed in a
chemical reaction.
To allow the circuit to be complete we need to have ions flow through the solution.
Such that we have positive ions flow from the anode on the left and at the same time
we have negative ions flow from the cathode on the right.
So what is the difference between galvanic and electrolytic cells. It simply comes down
to the tendency of the chemical substances which make up the anode and the cathode to
react in one direction or the other. In short, does the cell reaction tend to liberate energy
- which is rather like an exothermic reaction. Or to consume energy - which is rather like
an endothermic reaction.
So this actually means that in principle any cell may operate as a galvanic cell or as
an electrolytic cell depending on the direction of reaction. But a given cell will only operate
spontaneously in one direction, without an external input of energy. This will be the
galvanic mode of operation. To force that cell to operate in reverse, electrical energy
must be supplied to the electrons in the circuit - as a voltage. The direction of a spontaneous
cell reaction can be predicted from half cell E naught voltages, but that is a subject for
another time.
So really, it all comes down to our way of looking at the cell. Do we want it to go in
the spontaneous direction - consuming the relevant reactants and producing the relevant
products meanwhile producing energy? Or do we want it to go in the non-spontaneous direction
- consuming the opposite reactants and producing the opposite products and consuming energy
which we must provide?
It all depends on if we want to use the cell as a source of energy, such as in a torch,
a camera, an mp3 player, a mobile phone etc. Or do we want the specific chemical products
of the cell reaction? In fact we often use certain cells repeatedly in sequence as galvanic,
then as electrolytic, then galvanic etc.
An excellent example of this is the rechargeable cells used to power electronic and other devices.
You have probably heard of Nickel-Cadmium cells, Nickel-Metal hydride cells, Lithium-Ion
cells and perhaps others. Of course we should not forget the Lead-Acid cell which is used
to make car batteries. A battery is technically a collection of cells joined together to increase
the voltage.
During the discharge cycle the cell operates in galvanic mode, producing electrical energy
which we use. When the cell is recharged it is made to operate in electrolytic mode. Which
reverses the reactions which occurred during discharge. The products of the electrolytic
recharge store the energy which is then released during the next galvanic discharge.
If we come back to the cell which I used to illustrate the galvanic operation we can see
how this cell can also be made to operate in reverse as an electrolytic cell. In this
case, the silver electrode which was previously operating as the cathode is now made to operate
in the reverse direction as the anode.
So if we come now to the lead electrode - which in the galvanic mode was operating as the
anode - it is now forced to operate in reverse direction as the cathode.
Now in the electrolytic mode, electrons are produced by the silver so we have electron
flow in the reverse direction and that completes the circuit.
In addition the ions will also move in the opposite direction to that shown for the galvanic
operation.
So in conclusion we see that all electrochemical cells contain similar components - they all
contain anode, they all contain a cathode, they contain an electrolyte and an external
circuit. The key difference is the direction of spontaneous operation which produces electrical
energy in the case of the galvanic cell or a forced reaction which consumes energy in
the electrolytic cell.
Hi guys, hopefully that helps some problems that you've been having if you've got anymore
difficulties on this kind of subject just go to the links that you've been given and
good luck with your exams and we'll see you soon.