|The tendency for the electrons to flow from one chemical to
another, such as from the zinc metal to the copper ion as shown here, is something that
can be channeled and controlled. Channeling the flow of electrons is what we will take up
Copper-Zinc Voltaic Cell
|The apparatus shown here is used to channel electron flow in this reaction
and is called a voltaic cell. The zinc metal is on the right side and it
is providing electrons to the blue copper ions in the beaker to the left. However, to get
there the electrons have to travel through a wire. We can see that they are doing so
because the voltmeter that they must pass through shows a reading.
|This voltaic cell has copper metal and copper sulfate
solution in the left hand beaker; zinc metal and zinc sulfate in the right hand beaker;
probably potassium sulfate in the salt bridge. With the wires connected to the voltmeter,
you can see that there is a voltage reading; electricity is flowing through this cell.
It's a voltaic cell because the chemical reaction is causing the flow of electric
|Here you can see the importance of the salt bridge. In the top picture,
the salt bridge has been removed and and the voltage reading is zero showing the there is
no current flowing. In the bottom picture, the salt bridge is replaced and the voltmeter
again shows a reading.
|This same voltaic cell is diagrammed for you here (and in
example 20 in your workbook). In the diagram, copper is on the right and zinc is on the
|Zinc metal is reacting with the copper ion to form zinc ion
and copper metal. The zinc metal is on one side and the copper ion in solution is on the
other side in the other beaker. Consequently, the two chemicals are not in direct contact
with one another.
Even so, the tendency for the zinc metal to lose (or transfer)
electrons to the copper ion still exists. That transfer is made possible by connecting a
wire between the zinc metal and the copper metal. The electrons go from the zinc over to
the copper metal where they can react with the copper ions in solution.
|Over a period of time, the zinc electrode will dissolve and
increase the concentration of the zinc ion solution. The copper ion will plate onto the
copper electrode and thus the concentration of the copper ion in solution will decrease.
As this happens, the reaction slows down and the voltage decreases.
|Zinc is the anode because that is where oxidation is occurring. The
oxidation half-reaction is Zn
Zn2+ + 2e-. Those electrons go over to the copper side where they
react with copper ion and change it into copper metal (Cu2+ + 2e- Cu), which is the reduction
half-reaction that takes place at the cathode.
|The voltage for such a cell can be calculated using the standard oxidation
potential list. Working that through, as we have done before, the voltage is 1.10 volts.
|However, if you measure the voltage of a cell like this using
a voltmeter, you will likely not get that particular voltage. The reasons
|One is that quite often solutions are not kept at 25oC, particularly in this
|Another reason is that the concentrations of all the solutions are probably not one mole
per liter. |
|Also, the voltage measured by a voltmeter depends on the electrical resistance of the
cell. So the electrical resistance in the connections (especially bad ones), the wire, the
voltmeter, and the solutions will tend to cause the voltage to be less than what you
calculate it to be under standard or ideal conditions.|
|We have a cell like this set up in the lab. You should look
at it, experiment with it, remove and replace the salt bridge, check the voltage, and so
forth, when you are in the lab.
Lead Acid Battery
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