Electrolytic Cells use an external power supply to force a non-spontaneous redox reaction to occur.

To get an idea of what an electrolytic cell, in general is, we’ll start by looking at two half-reactions and the overall redox reaction we get by adding them. Consider the following half-reactions and overall redox reaction:

We’ll start with the half-reaction we get by reversing the reduction of chlorine on the table, to get the (click) oxidation of chloride ions Consider the following half-reactions and overall redox reaction:

Notice, because we reversed the equation, (click) the sign on the E naught is switched from positive to negative. Consider the following half-reactions and overall redox reaction:

Next, we’ll add the half-reaction for the reduction of zinc ions, the way it is on the table, with an E naught value of negative 0.76 Volts. Consider the following half-reactions and overall redox reaction:

We’ll add these two half-reactions to get the overall redox equation. Consider the following half-reactions and overall redox reaction:

Notice the electrons gained by the zinc ion, are (click) equal to the electrons lost by the chloride ions. Consider the following half-reactions and overall redox reaction: electrons gained = electrons lost

Therefore, electrons can be cancelled before adding the half-reactions. Consider the following half-reactions and overall redox reaction:

On the left side we have Zn 2+ plus 2 Cl minus Consider the following half-reactions and overall redox reaction:

And On the right side we have Zn solid plus Cl 2 gas Consider the following half-reactions and overall redox reaction:

To get the E naught value for the overall redox equation, (click) we add –1.36 volts and –0.76 volts, to give us (click) –2.12 volts Consider the following half-reactions and overall redox reaction:

A negative E naught value for the overall redox reaction, means that (click) this redox reaction is non-spontaneous as written. Consider the following half-reactions and overall redox reaction: This redox reaction is non-spontaneous

So that means, if we simply mixed Zn 2+ and Cl –, we would (click) NOT get solid zinc and chlorine gas. Consider the following half-reactions and overall redox reaction: This redox reaction is non-spontaneous

But we can Force this reaction to occur, using a process called electrolysis. Consider the following half-reactions and overall redox reaction:

Electrolysis uses an external power supply or battery to force a (click) non- spontaneous redox reaction to occur. Electrolysis uses an external power supply or battery to force a non-spontaneous redox reaction to occur.

Electrolysis takes place in an electrolytic cell. We’ll look at the simplest type of electrolytic cell. This is what we call a Type 1 Electrolytic cell. We’ll start off with (click) two inert carbon electrodes in a single container. Inert Carbon Electrode

Then we add a direct current power supply and wires. Power Supply + – Inert Carbon Electrode

Before the power supply is connected, the electrodes are both neutral. Using simplified symbols, we’ll represent a few protons by + signs and a few electrons as “e”s with a negative charge. Because they are neutral, the number of electrons is equal to the number of protons. Power Supply + – + + e–e– e–e– + + e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– Neutral e–e–

When we close the switch and connect the power supply, it takes electrons from the electrode attached to the positive terminal and pumps them on to the electrode attached to the negative terminal Power Supply + – e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– Neutral e–e– e–e– e–e– e–e– Positive Negative

The electrode on the left now has a deficiency of electrons, giving it a net positive charge Power Supply + – e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– Positive e–e– e–e– e–e– e–e– Has a deficiency of electrons

And the electrode on the left now has an excess of electrons, giving it a net negative charge Power Supply + – e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– e–e– e–e– e–e– e–e– Has a deficiency of electrons Has an excess of electrons Negative Positive

In an electrolytic cell, the electrode attached to the + terminal of the power supply is called (click) the Anode Power Supply + – e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– e–e– e–e– e–e– e–e– ANODE Positive

And the electrode attached to the negative terminal of the power supply is called (click) the Cathode. Power Supply + – e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– e–e– e–e– e–e– e–e– ANODE Positive Negative CATHODE

Just remember A plus for anode, Power Supply + – e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– e–e– e–e– e–e– e–e– ANODE Positive Negative CATHODE A+

And C minus for Cathode. Remember, this ONLY works for an Electrolytic cell, not an electrochemical cell. Power Supply + – e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– + + e–e– e–e– e–e– e–e– e–e– e–e– ANODE Positive Negative CATHODE A+C–

We’ll redistribute the electrons and stop showing the protons, for simplicity. Power Supply + – e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– – + + ANODE – CATHODE

We’ll add some molten Zinc chloride to the container Power Supply + – e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– ZnCl 2(l) – + + ANODE – CATHODE

A molten salt like zinc chloride consists of ions that are in constant random motion. Power Supply + – e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– + ANODE – CATHODE ZnCl 2(l) Zn 2+ Cl – Zn 2+ Cl – Zn 2+ Cl – Zn 2+ Cl – – +

Now we’ll focus on one zinc ion and two chloride ions. Power Supply + – e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– + ANODE – CATHODE ZnCl 2(l) Cl – Zn 2+ Cl – – +

The positive zinc ions will be attracted to the negative cathode while the negative chloride ions will be attracted to the positive anode. Power Supply + – e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– ZnCl 2(l) Cl – Zn 2+ Cl – – + + ANODE – CATHODE

Now, we’ll concentrate on the cathode and see what happens there. e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– Cl – Zn 2+ Cl – – + + ANODE – CATHODE ZnCl 2(l) Power Supply + –

– The zinc ion will gain 2 electrons from the cathode, e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– Cl – Zn 2+ Cl – + + ANODE – CATHODE ZnCl 2(l) e–e– e–e– e–e– e–e– Power Supply + – – e–e–

– And turn into a zinc atom e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– Cl – Zn 2+ Cl – + + ANODE – CATHODE ZnCl 2(l) Power Supply + – e–e– e–e– – Zn

– The equation for what just happened is Zn2+, e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– Cl – + + ANODE – CATHODE ZnCl 2(l) Power Supply + – – Zn Zn e –  Zn (s)

– Plus 2 electrons e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– Cl – + + ANODE – CATHODE ZnCl 2(l) Power Supply + – – Zn Zn e –  Zn (s)

– Gives Zinc solid e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– Cl – + + ANODE – CATHODE ZnCl 2(l) Power Supply + – – Zn Zn e –  Zn (s)

Reduction – This is an example of reduction. e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– Cl – + + ANODE – CATHODE ZnCl 2(l) Power Supply + – – Zn Zn e –  Zn (s)

Reduction – So we see that reduction of the cation Zn2+ occurs at the cathode e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– Cl – + + ANODE – CATHODE ZnCl 2(l) Power Supply + – – Zn Zn e –  Zn (s)

Reduction – Now we’ll have a look at the anode and see what happens. e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– Cl – + + ANODE – CATHODE – Zn Zn e –  Zn (s) Power Supply + – ZnCl 2(l)

Reduction – Each chloride ion will lose one electron and change into a chlorine atom (Click half way through) e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– Cl – + + ANODE – CATHODE – Zn Zn e –  Zn (s) Power Supply + – ZnCl 2(l) e–e– e–e– e–e– e–e– e–e– e–e– Cl

Reduction – The 2 chlorine atoms then join to form a molecule of Cl 2 or chlorine gas e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– + + ANODE – CATHODE – Zn Zn e –  Zn (s) Power Supply + – ZnCl 2(l) e–e– e–e– e–e– Cl

Reduction – The process taking place on the anode can be summarized by the equation (click), 2 Cl minus e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– + + ANODE – CATHODE – Zn Zn e –  Zn (s) Power Supply + – ZnCl 2(l) e–e– e–e– e–e– Cl Oxidation 2Cl –  Cl 2(g) + 2e –

Reduction – Gives Cl 2 gas e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– + + ANODE – CATHODE – Zn Zn e –  Zn (s) Power Supply + – ZnCl 2(l) e–e– e–e– e–e– Cl Oxidation 2Cl –  Cl 2(g) + 2e –

Reduction – Plus 2 electrons. e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– + + ANODE – CATHODE – Zn Zn e –  Zn (s) Power Supply + – ZnCl 2(l) e–e– e–e– e–e– Cl Oxidation 2Cl –  Cl 2(g) + 2e –

Reduction – The process of chloride ions losing electrons (click) is called oxidation e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– + + ANODE – CATHODE – Zn Zn e –  Zn (s) Power Supply + – ZnCl 2(l) e–e– e–e– e–e– Cl Oxidation 2Cl –  Cl 2(g) + 2e –

Reduction – And we see that oxidation of chloride ions to chlorine gas takes place at the anode. e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– + + ANODE – CATHODE – Zn Zn e –  Zn (s) Power Supply + – ZnCl 2(l) e–e– e–e– e–e– Cl Oxidation 2Cl –  Cl 2(g) + 2e –

Reduction – So again zinc ions are reduced at the cathode to form zinc atoms. + + ANODE – CATHODE – Zn Zn e –  Zn (s) Oxidation 2Cl –  Cl 2(g) + 2e – Power Supply + – ZnCl 2(l) Zn 2+

Reduction – And chloride ions are oxidized at the anode to form chlorine gas, Cl ANODE – CATHODE – Zn Zn e –  Zn (s) Oxidation 2Cl –  Cl 2(g) + 2e – Cl Power Supply + – ZnCl 2(l) Cl –

– Reduction – So as this cell operates, we can visualize zinc metal growing on the surface of the cathode and bubbles of chlorine gas forming on the anode Zn e –  Zn (s) Power Supply + – ZnCl 2(l) Oxidation + ANODE 2Cl –  Cl 2(g) + 2e – Zn e –  Zn (s) 2Cl –  Cl 2(g) + 2e – e–e– e–e– e–e– e–e– Zn CATHODE

Reduction – So we can say the product at the anode is chlorine gas Zn e –  Zn (s) Power Supply + – ZnCl 2(l) Oxidation + ANODE 2Cl –  Cl 2(g) + 2e – Zn e –  Zn (s) 2Cl –  Cl 2(g) + 2e – Zn – CATHODE Product at Anode

Reduction – And the product at the cathode is zinc metal. Zn e –  Zn (s) Power Supply + – ZnCl 2(l) Oxidation + ANODE 2Cl –  Cl 2(g) + 2e – Zn e –  Zn (s) 2Cl –  Cl 2(g) + 2e – Zn – CATHODE Product at Anode Product at Cathode

Now that we’ve seen how this cell works, we’ll show you a process you can use for any questions involving the electrolysis of a molten salt. We’ll use our example here of molten zinc chloride. Reduction at Cathode Oxidation at Anode Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction: Molten zinc chloride is electrolyzed.

Molten salts always consist of mobile positive and negative ions, so we write the dissociation equation for the salt forming liquid ions. We have ZnCl2 liquid Reduction at Cathode Oxidation at Anode Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction: Molten zinc chloride is electrolyzed.

Forms Zn 2+ liquid Reduction at Cathode Oxidation at Anode Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction: Molten zinc chloride is electrolyzed.

Plus 2 Cl minus liquid Reduction at Cathode Oxidation at Anode Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction: Molten zinc chloride is electrolyzed.

We write C minus for cathode, underneath the positive ion. The positive ions are attracted to the negative cathode. Reduction at Cathode Oxidation at Anode C– Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction:

And we write A+ for anode, underneath the negative ions. The negative ions are attracted to the positive anode. Reduction at Cathode Oxidation at Anode C–A+ Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction:

Reduction of Zn 2+ ions occurs at the cathode. Reduction at Cathode Oxidation at Anode C–A+ Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction:

Its half-reaction is: Zn e –  Zn (s). Reduction at Cathode Oxidation at Anode C–A+ Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction:

And the product at the cathode is zinc solid Reduction at Cathode Oxidation at Anode C–A+ Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction:

Oxidation of Cl – ions occurs at the anode Reduction at Cathode Oxidation at Anode C–A+ Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction:

Its half-reaction is: 2Cl –  Cl 2(g) + 2e –. Reduction at Cathode Oxidation at Anode C–A+ Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction:

And the product at the anode is chlorine gas Reduction at Cathode Oxidation at Anode C–A+ Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction:

We get the equation for the overall redox reaction by adding up the two half- reactions. It is (click) Zn Cl –  Zn (s) + Cl 2(g) Reduction at Cathode Oxidation at Anode C–A+ Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction:

Going through this process should greatly help you with any questions you get involving the electrolysis of molten salts, or what we call Type 1 electrolytic cells. Reduction at Cathode Oxidation at Anode C–A+ Product at Cathode is Zn (s) Product at Anode is Cl 2(g) Overall Redox Reaction: