Have you ever thought about how we get previous metals from ores or other mineral sources? Such as how is aluminum produced. Or what about purified gases such as oxygen for oxygen tanks? In this article we will go over the concept of electrolysis and how it can be applied to ionic compounds to produce different metals and gases.

Fig. 1: Aluminium. Wikimedia Commons.

• Electrolysis process and requirements.
• What happens at each electrode and why.
• What molten salts (ionic compounds) are.
• How is molten ionic compound electrolysis different from aqueous ionic compound electrolysis.
• What the products at each electrodes are for electrolysis reactions.
• Examples of electrolysis of molten and aqueous salts.

Electrolysis Process

What is electrolysis and what does it rely on?

But what do those words really mean? We know that the subject of electrolysis deals with physical chemistry, in particular redox chemistry. Redox (reduction and oxidation) chemistry relies on the transfer of electrons between compounds and atoms. Specifically, reduction is the gain of electrons, and oxidation is the loss of electrons.

Once we have covered the basics of reduction and oxidation, we can start thinking about their uses. Reactions that involve redox processes are usually very spontaneous ones, such as oxidations of metals and non-metal compounds and such, yet there is a way to contol these reactions. But why do we need to have "control" over them?

Redox reactions are about the flow of electrons, and if you separate the two half-reactions that make up a redox reaction, you can control the flow of electrons, in other words, the electricity that is generated in the reaction. This is what galvanic (voltaic) cells do - they take a spontaneous redox reaction, and split the two half-reactions into separate beakers, connecting them through a salt bridge (for the mobility of ions) and an external wire (for the mobility of electrons). This creates a current in the external circuit, which corresponds to the difference in reduction potential of the two electrodes of the reaction. This current corresponds to the energy being produced by the reaction, as spontaneous reactions usually produce energy.

How can we use this to our advantage though? On the other hand we have non-spontaneous redox reactions If we put enough energy into the system, we are able to make any reaction proceed right? This is what electrolytic cells do - they take a non-spontaneous redox reaction and pass a current using two electrodes, which forces a reaction to proceed. When this occurs it is called electrolysis.

Fig. 2: Experimental hydrolysis cell. Wikimedia commons.

Molten Ionic Compounds Conduct Electricity

Ionic compounds can exist in different states. The most common state you might be familiar with is the solid state. This is where the ionic compound (salt) is arranged in crystalline structures. There are two other states of ionic compounds you should be aware of which we will discuss, and those are the molten state and the aqueous state.

As you can imagine, anything in that is "melted" is essentially a liquid. This is what is referred to when discussing molten ionic compounds. When a crystalline object that is ionic is melted, the intermolecular forces keeping the structure start to degrade and fall apart. In an ionic compound, it is the electrostatic forces that create a strong and rigid crystal. When the crystalline structure is disrupted, the ions become mobile. This means that the ions can "move around" and are not constricted to a solid structure. But what implications does this have?

What happens in a solution when a salt is dissolved and the ions become mobile? When the anions (the negative ions) and cations (positive ions) are free to move around in a solution, they can adopt new physical and chemical characteristics such as conducting electricity. This is because the ions are able to be attracted to a certain pole (negative or positive) when a current is applied to the mixture, thus resulting in the movement of ions. This can complete electrical currents.

In the case of a molten ionic compound, where the ions are also mobile, electricity is also conducted. Relying on the same principle of the mobility of ions, ionic salts can also transmit electricity when in the molten state. The main difference between molten and aqueous ionic compounds is that there is no solvent in the molten state. This has crucial implications for the electrolysis of ionic salts, as based on the presence or absence of water, different products are formed, as we shall see in the next sections.

Electrolysis of Molten Ionic Compounds

So what happens in electrolysis? During electrolysis, a strong current is applied to a chemical mixture. This is done through inert electrodes, which do not react with the compounds involved in the reaction, usually made of platinum or graphite. When two electrodes are placed into the reaction vessel and connected to an outside electrical source, such as a battery, one electrode becomes the anode (the positive electrode) and the other the cathode (the negative electrode). This is determined by the flow of electrons, and in electrolysis, electrons travel from the anode to the cathode.

So what actually happens when you apply a current on a molten ionic compound? We know that when a salt is in its molten state, the ions are mobile and are free to move about, but what reaction is expected to occur?

At each electrode, one half-reaction is occurring. These are: oxidation at the anode, and reduction at the cathode. Through the process of electrolysis, the ions are being discharged by either gaining or losing an electron.

We know that in molten ionic compounds two ions exist: the anion and cation. If you perform electrolysis on a molten ionic compound, the anions will gather at the anode and will lose an electron, whilst the cations will go to the cathode and gain an electron. This means that both of the ionic species will not be ions anymore when discharged, as the negative anion will lose an electron, and the positive cation will gain an electron, both becoming neutrally charged.

Usually in the electrolysis of a molten ionic compound, as both ions are discharged, they go into the state that their original element is in. For example, most anions becoming gases, and most cations becoming metal solids or liquids depending on the conditions of the experimental setup. Let's see this more in action in the case of some examples.

Examples of Electrolysis of Molten Ionic Compounds

Take a look at the following example:

Similarly, you can apply this to other ionic salts. You can predict what the products of the electrolysis of any molten salt will be just by looking at the chemical name or compounds involved. For example: zinc chloride produces chlorine gas at the anode and zinc metal at the cathode, whilst aluminium oxide produces oxygen gas and aluminum metal.

Electrolysis of Aqueous Ionic Compounds

In aqueous environments, there are additional ions present in the solution. These are the hydroxide ion (OH^-) and H⁺. These ions will be competing to be discharged at the electrodes with the salt ions present in the solution. H⁺ will be attracted to the cathode and can be discharged as hydrogen gas via reduction, while OH^- is attracted to the anode and can be discharged as oxygen through oxidation.

Regarding the anode (the positive electrode), we have to look at the type of anion compound is present. If it is a simple anion, such as chloride (Cl^-) or bromide (Br^-) then it will be discharged into its respective gas. Yet if the anion is a more complex one, such as a sulfate or carbonate, then oxygen will be preferentially discharged.

This way there is competition between the two species at each electrode to determine which is being discharged. This is why in aqueous environments, the products of the electrolysis of ionic compounds are often harder to predict.