Electrolysis uses electricity to break down ionic compounds.
Electrolysis is used on a massive scale in the production
of aluminium, chlorine and sodium hydroxide. To give you an idea of
the scale, annual global production of these products is measured in megatons.
Electrolysis is also used for electroplating -- putting a thin
layer of metal onto another metal object. One example of this is jewelry, but you
can find others in electronic devices and many other products around you,
for example your smartphone. In this video we will be investigating
the electrolysis of aqueous solutions. We'll explore the process with copper sulfate solution.
We will learn how to predict what forms at each electrode
using the reactivity series, and we'll describe each reaction as oxidation or reduction.
We will be using harmful solutions in this experiment,
so eye protection must go on.
In this experiment I'm going to electrolyse aqueous copper sulfate solution:
this is the electrolyte.
I'm going to be using inert electrodes made from carbon. These solid electrodes will conduct the
electrical current. The electrodes must be inert so they do not react with the solution.
I must also make sure the electrodes do not touch, therefore I am using a holder.
I'm going to connect to a direct current or DC as I want the current to flow in one direction only.
I can now connect the positive terminal to one electrode,
and the negative terminal to the other electrode, to complete the circuit.
Now I will turn the power supply on and I have it set to 6 volts DC.
And it's time to observe. What can you see?
At the negative electrode we can see brown solid appearing.
At the positive electrode we can see bubbles appear.
To understand what is happening at the electrodes,
we need to think about what is actually in the beaker.
The copper sulfate is an ionic compoun,d so it contains ions. Remember, in an aqueous solution
the solvent is water and this also has ions in.
This is the formula for copper sulfate. And so it contains a copper ion
and a sulfate iron. And then the water contains a hydrogen ion and a hydroxide ion.
Ions are attracted to the opposite electrode, so the negative sulfate ions
and the negative hydroxide ions will be attracted to the positive electrode.
And the copper and hydrogen ions are both positive and will be attracted to the negative electrode.
Only one of these ions will react at each electrode.
The observations we have made help us work out which of the two ions react.
At the negative electrode we observed a brown solid, not a gas, so we can deduce that the
copper ion is the one that reacted. There is a simple rule we can use to work out which positive
ion will react at the negative electrode. We need to look at the reactivity series.
If the metal in the aqueous solution is less reactive than hydrogen, then that metal ion
will react at the electrode and the metal solid will appear. If the metal is more reactive than
hydrogen, it will stay in the solution and the hydrogen ion will react at the electrode,
and we would see hydrogen gas forming. This explains why we can see copper coating the
negative electrode. Copper is less reactive than hydrogen, so it is the ion that reacts.
On the screen you can see a half equation to represent this.
The copper ion gains two electrons to form
the copper atom. Now let's look at what's happening at the positive electrode.
We have two negative ions attracted to this positive electrode:
the sulfate ion and the hydroxide ion.
If we collected the bubbles and tested the gas with a glowing splint it would re-light,
telling us that it was oxygen -- so we know it is the hydroxide ion that is reacting.
Again, there is a simple rule to work out which ion will react at the positive electrode.
If there is a halide ion from a halogen present -- chloride, bromide or iodide --
then that will react at the positive electrode to form chlorine, bromine or iodine gas.
If there is no halogen, or only a very weak concentration present,
then you will get oxygen. The oxygen is formed from a
reaction of the hydroxide ion from the water. Here is a half equation reaction to show this.
Four hydroxide ions lose four electrons to make two water molecules and oxygen gas.
We can also describe these reactions using oxidation and reduction. Do you recall learning OIL RIG to
help you remember what happens to the electrons during oxidation and reduction? Oxidation is loss
of electrons reduction is gain of electrons? Let's look again at the equations at the electrodes to
see where oxidation and reduction is occurring. We can see that the copper has gained electrons
a substance that gains electrons is reduced, reduction has occurred.
The electrode where reduction occurs is called the cathode.
And at the positive electrode we can see that the hydroxide ion has lost electrons. A substance
that loses electrons is oxidised, oxidation has occurred. The electrode where oxidation occurs
is called the anode. Where do the electrons transfer from and to? To answer this question,
we need to think about how electrons are travelling around the complete circuit as the current flows.
Whilst it is the electrons that cause the flow of current on the wires,
it is the ions that move to carry electricity in solution, and complete the circuit.
We have already met the electrons at the electrodes in the half equations.
Remember, at the positive electrode, or anode, negative ions lose electrons and become
oxidised. At the negative electrode, or cathode, positive ions gain electrons to become reduced.
By investigating the electrolysis of copper sulfate solution in detail, we have now learned
some simple rules for how to work out what is produced at the positive and negative electrodes.
You should now also be able to write half equations for these reactions and know how
to work out which ion has been reduced and which has been oxidised. Use these principles
to set up an experiment that investigates the electrolysis of other ionic substances.
Think about how you could test for the products at each electrode, and also think about safety issues.
Thank you for watching, we hope you have fun with your experiments.