4.4.3.1The process of electrolysis
When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or solution. These liquids and solutions are able to conduct electricity and are called electrolytes.
Passing an electric current through electrolytes causes the ions to move to the electrodes. Positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode). Ions are discharged at the electrodes producing elements. This process is called electrolysis.
- Electrolyte
- A molten or dissolved ionic compound in which the ions are free to move, so it conducts electricity.
- Cathode
- The negative electrode. Positive ions move to it.
- Anode
- The positive electrode. Negative ions move to it.
- Inert electrode
- An electrode, usually graphite or platinum, that does not react with the electrolyte or the products.
- Electrolysis
- the breaking down of an ionic substance, molten or in solution, by passing an electric current through it; the ions move to the electrodes where they are discharged as elements
The direction of travel follows from opposite charges attracting: a positive ion is pulled to the negative electrode. Naming the electrodes correctly is worth doing carefully, because nearly every mark in this sub-topic depends on it — cathode negative, anode positive, and PANIC is a useful reminder: Positive Anode, Negative Is Cathode.
4.4.3.2Electrolysis of molten ionic compounds
When a simple ionic compound, such as lead bromide, is electrolysed in the molten state using inert electrodes, the metal (lead) is produced at the cathode and the non-metal (bromine) is produced at the anode.
You should be able to predict the products of the electrolysis of binary ionic compounds in the molten state. A binary compound contains just two elements, so the prediction is always the same shape: metal at the cathode, non-metal at the anode.
Worked example
Predict the products of the electrolysis of molten aluminium oxide, Al₂O₃.
- Identify the ions: Al³⁺ and O²⁻.
- Positive ions move to the negative electrode, so Al³⁺ goes to the cathode.
- Negative ions move to the positive electrode, so O²⁻ goes to the anode.
- Each ion is discharged, forming the element.
Answer: Aluminium at the cathode and oxygen at the anode.
Common mistake
The compound must be molten, not solid. In a solid the ions are held in fixed positions in the lattice and cannot move to the electrodes, so nothing is electrolysed. A safer alternative to lead bromide for practical work is anhydrous zinc chloride.
4.4.3.3Using electrolysis to extract metals
Metals can be extracted from molten compounds using electrolysis. Electrolysis is used if the metal is too reactive to be extracted by reduction with carbon, or if the metal reacts with carbon.
Large amounts of energy are used in the extraction process, both to melt the compounds and to produce the electrical current — which is why electrolysis is the expensive option, and why it is used only when reduction with carbon will not work.
Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite, using carbon as the positive electrode (anode).
You should be able to explain why a mixture is used as the electrolyte, and why the positive electrode must be continually replaced.
Why a mixture: aluminium oxide has a very high melting point on its own. Dissolving it in molten cryolite lowers the melting point of the mixture, so less energy — and less money — is needed to keep the electrolyte molten.
Why the anode is replaced: oxygen is produced at the positive electrode, which is made of carbon. At the high temperature of the cell, that oxygen reacts with the carbon to form carbon dioxide, so the anode gradually burns away and must be replaced.
Common mistake
Cryolite lowers the melting point; it is not there to make the mixture conduct better or to react with the aluminium. Say what it does and why that saves energy.
- Extraction by electrolysis
- extracting a metal from its molten compound using electrolysis, used where the metal is too reactive to be extracted by reduction with carbon, or reacts with carbon
4.4.3.4Electrolysis of aqueous solutions
The ions discharged when an aqueous solution is electrolysed using inert electrodes depend on the relative reactivity of the elements involved.
At the negative electrode (cathode), hydrogen is produced if the metal is more reactive than hydrogen.
At the positive electrode (anode), oxygen is produced unless the solution contains halide ions, when the halogen is produced.
This happens because in the aqueous solution water molecules break down, producing hydrogen ions and hydroxide ions that are discharged. So there are always four ions competing, not two — the ones from the compound and the ones from the water.
You should be able to predict the products of the electrolysis of aqueous solutions containing a single ionic compound.
Required practical 3
Investigate what happens when aqueous solutions are electrolysed using inert electrodes
- State a hypothesis to test — for example, that a metal less reactive than hydrogen will be deposited at the cathode, while a more reactive metal will give hydrogen instead.
- Set up an electrolysis cell with two inert graphite electrodes in a small beaker or petri dish, connected to a low-voltage d.c. supply.
- Half fill the cell with the first solution and switch on. Observe both electrodes: look for a solid deposit, its colour, and any gas bubbles.
- Test any gas collected — a lighted splint gives a squeaky pop for hydrogen, a glowing splint relights in oxygen, and damp litmus paper is bleached white by chlorine.
- Rinse the electrodes and repeat with each of the other solutions, changing only the electrolyte so the comparison is fair.
- Record the products in a table and compare them with what the hypothesis predicted.
Common mistake
Do not predict sodium metal from sodium chloride solution. Sodium is more reactive than hydrogen, so hydrogen is discharged instead — sodium metal only forms when the compound is molten and no water is present.
4.4.3.5Representation of reactions at electrodes as half equationsHigher tier
During electrolysis, at the cathode (negative electrode), positively charged ions gain electrons and so the reactions are reductions. At the anode (positive electrode), negatively charged ions lose electrons and so the reactions are oxidations.
Throughout electrolysis, Higher tier students should be able to write half equations for the reactions occurring at the electrodes during electrolysis, and may be required to complete and balance supplied half equations.
Worked example
Write the half equations for the electrolysis of molten aluminium oxide, and say which is oxidation.
- At the cathode, Al³⁺ ions gain electrons. Each needs three, so Al³⁺ + 3e⁻ → Al.
- At the anode, O²⁻ ions lose electrons. Two ions are needed to make one O₂ molecule.
- Two O²⁻ ions carry 4− altogether, so four electrons are lost: 2O²⁻ − 4e⁻ → O₂.
- Check the charges balance on each side of both equations.
- Electrons lost is oxidation, and that is the anode.
Answer: Cathode: Al³⁺ + 3e⁻ → Al (reduction). Anode: 2O²⁻ − 4e⁻ → O₂ (oxidation).
Common mistake
Balance the charges as well as the atoms. In 2Br⁻ → Br₂ + 2e⁻ both sides carry 2−; an equation that balances the bromine but leaves the charges unequal has the wrong number of electrons.
Before you practise
- An electrolyte is a molten or dissolved ionic compound whose ions are free to move.
- Positive ions move to the negative cathode; negative ions move to the positive anode; ions are discharged to produce elements.
- Molten binary compound: metal at the cathode, non-metal at the anode.
- Electrolysis is used when the metal is too reactive to be extracted with carbon, or reacts with carbon — and it uses large amounts of energy.
- Aluminium: molten aluminium oxide mixed with cryolite to lower the melting point; the carbon anode burns away as oxygen and must be replaced.
- In solution: hydrogen at the cathode if the metal is more reactive than hydrogen; oxygen at the anode unless halide ions are present, when the halogen forms.
- Required practical 3: electrolyse aqueous solutions with inert electrodes and test the products, starting from a hypothesis.
- Higher tier: cathode reactions are reductions (electrons gained), anode reactions are oxidations (electrons lost).