Download To determine a reactivity series for a range of metals using a voltmeter

To determine a reactivity series for a range of metals using a voltmeter
Background:
You have performed displacement reactions and have concluded that some metals are
more reactive than others.
You have also learned that these displacement reactions involve the oxidation of a
reactive metal and reduction of the ions of the least reactive metal.
Since oxidation and reduction in these cases involves the transfer of electrons and the
movement of electrons constitutes an electrical current, it should be possible to use
electrical methods to determine numerically if a metal is more reactive than another.
We need to determine the energy driving the reaction and the term associated with
electrical energy is the volt.
We will, therefore, set up some simple cells and connect the metals via a voltmeter to
measure the electrical potential (an energy related term measured in volts).
It is predicted that the order of the voltages measured will match the observations from
displacement reactions.
Apparatus
 Stopclock (to measure seconds).
 Voltmeter (high resistance to minimise current flow).
 2 plug to croc leads (to connect electrodes to the voltmeter).
 100cm3 beaker.
 Approximately 100cm3 of 0.5M Na2SO4 solution. (Other solutions could be
used, preferably NOT including ions of the metals being tested or chloride ions.)
Diagram
Electron flow circuit:
V
Voltmeter
Leads
Electrodes
Stopclock
Ion flow circuit:
Electrodes
Electrolyte
Optional extras:
Magnetic stirrer / glass rod
Thermometer
Method – Preliminary Work:
After following these instructions, you may think of ways to improve your experiment
in order to obtain either better results or a better understanding of the experiment. Feel
free to develop the experiment as you see fit.
o Abrade the electrodes’ surfaces using a fine paper. Take care not to leave
fingerprints or other marks on the surface.
o Connect cleaned copper and zinc electrodes to the voltmeter.
o Make sure the voltmeter is switched on before placing the two electrodes in the
electrolyte at the same time.
o Notice the behaviour of the voltmeter reading over a minute or so. Also note the
sign of the voltage.
o What happens if the electrodes are swapped round?
Results:
Metal on +ve
terminal (Red)
Measurement
Time (sec)
Voltage
Measured (V)
Copper
Metal on -ve
terminal
(Black)
Zinc
More
reactive
metal
10
15
20
30
Zinc
Metal on -ve
terminal
(Black)
Copper
More
reactive
metal
10
15
20
30
60
Observations
Metal on +ve
terminal (Red)
Measurement
Time (sec)
Voltage
Measured (V)
60
Observations
You will notice at least two important features about the voltage reading…
1. The values “jump about” a lot, and
2. The values decrease, quickly at first but then more gradually.
This poses a problem of how to take an accurate measurement. In fact it will be
impossible to get measurements which give the same value each time the experiment is
repeated (except by chance). There are many variables which are affecting your results
so we have to be very careful with our method so as not to disturb the system more than
necessary and to make our measurements as consistent as possible.
 Always start with freshly prepared, clean electrodes.
 Keep electrodes as far apart as possible.
Possible further preliminary investigations:




You have seen that the measurements decrease quickly. How can you make
readings from separate tests compatible with each other? (Note: Do not expect
them to be the same – what, in your opinion constitutes consistent results for this
experiment?)
Does stirring have any effect?
The process is temperature dependent. What reasonable precautions should you
take to make sure that measurements are as consistent as possible?
What if you connect two electrodes of the same metal?
Investigation:
You are to select pairs of metal electrodes and determine a series for the metals.
Compare this series with that of the reactivity series obtained from displacement
reactions.
In your evaluation, comment on difficulties experienced in taking measurements and
how you tackled them.
How accurate do you think your measurements are, and with this in mind, how
precisely should they be recorded?
Try to justify any assumptions made.