PAUL ROBERTS: Hi, I'm Paul Roberts. I'm a chemistry teacher here at Andover College.
Today, we're going to look at the core practical electrolysis of copper sulfate solution. And
now, this is one of the core practicals that's the most challenging for students to do to
get good results on. So I'm going to give you some tips here of how you can get some
nice, clean, and accurate results.
Now, this practical is split into two parts. First of all, using the copper strips for
electrolysis. And the second part will be using the graphite rods for electrolysis.
The copper strips required for the first part of the electrolysis, their dimensions aren't
critical. Approximately 2 centimeters by 10 centimeters is sufficient. The copper strips
will need to be cleaned with emery paper. Emery paper is more effective than sandpaper.
It doesn't fall apart in your hands. And you only need to clean the very end of the copper
strip. This is something the students can do during the practical.
Once we have a nice clean strip, as such, we can give it a rinse with distilled water
to clean off any bits of copper grit that may be on there. And then for the second step,
we're going to rinse in propanone. Rinsing in propanone will remove the water and will
also take away any oils that may be on the strip from the fingers. And just wave it in
the air to dry. At this point, you'll be able to clearly smell the propanone and it will
be dry in a few seconds. When not using the propanone, it's advisable to put it back in
the bottle.
We can now start to assemble the electrolysis circuit. Now, this is the part of the experiment
that's most likely to cause problems. The issues that we have here are the wires. Now
notoriously in science labs, they have loose connections in the wires. I'm using a multimeter
here. And I'm setting my multimeter to the continuity setting. And by plugging my wire
in there, I can check-- if you hear the faint beep-- that I have good continuity in this
wire. Any point during the experiment if you have a problem or a break in the circuit,
check the continuity of the wires as your first troubleshooting part.
So to assemble the circuit, we're going to first look at the power supply. Now, you may
have two different kinds of power supply. First of all, the traditional battery pack,
making sure that all your batteries are in good condition and that you have up to 6 volts
available. The other option is using a power supply. Now, this can be set to 6 volts. And
like this power supply, it's very important that you have it switched to direct current.
If you have alternating current, electrolysis results will be inconclusive.
For this experiment, I'm going to use the battery packs. So first of all, we're going
to start to assemble the positive side of the battery pack. Starting with 6 volts, I'm
going to plug the 6 volts into an ammeter. Now, the ammeter I'm using here is an analog
ammeter. I want my ammeter range to be 1 amp range. Alternatively if you have a multimeter,
you can also use this as set on the amp range on the multimeter. Students often find it
easier to read off an analog meter, so I'm going to put the multimeter to the side now.
So out of the ammeter, I will connect the crocodile clip. And this crocodile clip will
be attached to my electrode. It's worth squeezing the crocodile clip onto the electrode, as
a loose connection can take place here. Now, this is the positive side of the circuit,
so this will be the anode. I'm labeling A on the strip.
I'm now going to assemble the negative side of the circuit. Taking this from 0 volts,
I'm going through my variable resistor. My variable resistor I've used here are two rheostats
in series. You may have a different setup for your variable resistors, but I found it
necessary to control the specific current ranges of this experiment to use two rheostats
in series. You may be using a potentiometer or a different setup.
So I now have my rheostats in series and I can connect the other electrode to the crocodile
clip, squeezing the crocodile clip on to make sure a good connection. Now, this is the negative
end of the circuit, so this would be the cathode. I'm just going to label it with a C. It's
important to use a permanent marker when labeling the cathodes so they don't rinse in water.
Now, I have my circuit set up, I can check continuity by shorting out the electrodes,
looking at the ammeter, and noticing that the ammeter increases. If there is no change
in our meter, that means there's a break in the circuit. It's probably going to be one
of the wires that is loose. So now, both the anode and cathode can be weighed. It's important
to zero the balance before we start. I'm weighing the anode here. And the students, at this
point, would record the mass. Doing separately for the cathode, zeroing the balance before
every mass. I'm recording the mass.
We can now assemble our electrolysis solution. I'm using 100 ml beaker for this-- a small
beaker. And by folding the copper over the edge of the beaker, the copper won't move
during electrolysis and accidentally short out with the other electrode. Taking care
to connect the negative side of the electrode to the electrode labeled C for cathode and
the positive for A for anode. At this point, I can see there's no short in the circuit,
as my ammeter is reading 0.
Now, I can add my copper sulfate solution. The amount of copper sulfate added isn't crucial,
as long as the copper sulfate covers the electrodes. Looking at the ammeter now, I can see a current
is flowing in the circuit. I can adjust my current to 0.2 amps, moving the rheostat.
And the ammeter is now reading 0.2 amps. At this point, the experiment will be set aside
for 20 minutes whilst the electrolysis takes place and the masses of the electrodes change.
So after 20 minutes has passed, we can stop the electrolysis by taking the electrodes
out of solution. Now, the electrodes will have changed in appearance and there will
also be some residue on them, so it's very important that we clean our electrodes first
by rinsing gently with distilled water, and then secondly, rinsing in propanone. Before
any weighing takes place, I'm just going to waft the electrode in the air until the propanone
evaporates. This will only take a few seconds. I can now zero my balance and record the final
mass.
I'm going to do the same now with the other electrode. This one's the cathode. I'm doing
the same process again. First of all, rinsing with water. I notice there's a much more pinky
appearance to this electrode. That might give you an idea of what's happened at this electrode.
I'm just rinsing with water, and then rinsing the water off and drying with propanone. Make
sure you give it a good shake. You don't want to be weighing any water or propanone. And
I can see the propanone evaporating off the electrode. The electrode is now dry. Zero
the balance. I'm weighing my cathode. I'm recording the mass.
So I'm expecting a mass change in both anode and cathode. And at this point, the students
can think about oxidation reduction processes to explain their results. So that was a electrolysis
of the copper sulfate solution with a current of 0.2 amps. The ammeter can now be adjusted
to a current of 0.4 amps using the rheostat and the electrolysis can be performed again.
As a shortcut, you can always use the final masses of the electrodes in the previous experiment
at 0.2 amps for the initial masses in the next experiment at 0.4 amps. This will save
time weighing on the balance. So we're looking to repeat this experiment a minimum of four
times, each time with a different electrolysis current. And the electrolysis current needs
to range between 0.2 and 0.5 amps. So at the end of the experiment, not only will you have
a range of electrolysis currents, you'll also have a range of changes of mass of anode and
cathode. And at this point, you'll be able to then plot a graph of change of mass of
electrode versus electrolysis current and draw your conclusions from there.
So for the electrolysis of copper sulfate solution using graphite electrodes, we're
going to use exactly the same electrolysis circuit. There's no need to change any of
the wiring or the voltage. You can just leave the current as it was before. I'm going to
once again attach the negative electrode to this graphite rod and I'm going to attach
the positive electrode to other graphite rod, and carefully insert into the copper sulfate
solution.
So after only a few moments, I'm noticing a change at the electrodes. On the negative
cathode, I'm noticing a change of color. And on the positive anode, I'm noticing some bubbles
forming. Now, you can think about your chemistry of identifying gases and try to identify what
those bubbles might be.
Now when I take the electrodes out of solution, I can see on the negative cathode, there's
a definite pink coloration. And thinking about the chemicals involved in the electrolysis
solution, try to identify what this pink color might be.
So to summarize what we've done in this core practical, we've done electrolysis of copper
sulfate using two different systems-- first with copper electrodes, where we've measured
a mass change at different currents, and then the second using the inert graphite electrodes,
where we've observed color changes and bubbles being evolved.