3.22–3.24The 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.
3.25–3.26Electrolysis 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.
3.25, 3.30–3.31Electrolysis 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.
3.27–3.29Representation 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.
3.27Half equations and ionic equationsHigher tier
Higher tier students should also be able to write balanced half equations and ionic equations where appropriate.
A half equation shows what happens to one species — the electrons lost or gained — and so the electrons appear in it. An ionic equation leaves out the ions that take no part, so only the particles that actually change are written.
Worked example
Sodium reacts with chlorine to form sodium chloride. Write a half equation for each element, and then the ionic equation for the reaction of silver nitrate solution with sodium chloride solution.
- Sodium loses its single outer electron, so the electron goes on the right: Na → Na⁺ + e⁻.
- Each chlorine atom in Cl₂ gains one electron, so two electrons go on the left: Cl₂ + 2e⁻ → 2Cl⁻.
- For the silver nitrate reaction, write out every ion present: Ag⁺, NO₃⁻, Na⁺ and Cl⁻.
- The sodium and nitrate ions are in solution before and after, so they are spectator ions and are left out.
- What is left is the silver and chloride ions joining to make the solid precipitate.
Answer: Na → Na⁺ + e⁻ and Cl₂ + 2e⁻ → 2Cl⁻; ionic equation Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
Common mistake
Check that the charges balance as well as the atoms. In Cl₂ + 2e⁻ → 2Cl⁻ the left side carries 2− from the electrons and the right side carries 2− from the two chloride ions, so it balances. A half equation whose charges do not match is wrong even if every atom is accounted for.
- Half equation
- an equation showing what happens to one species in a reaction, including the electrons lost or gained
- Ionic equation
- an equation that leaves out the ions taking no part in the reaction, so only the particles that actually change are written
Before you practise
- An electrolyte is an ionic compound melted or dissolved, so its ions are free to move.
- Positive ions (cations) go to the negative cathode; negative ions (anions) go to the positive anode.
- Molten compound: the metal forms at the cathode and the non-metal at the anode.
- Aqueous solution: hydrogen forms at the cathode unless the metal is less reactive than hydrogen, and oxygen forms at the anode unless a halide is present.
- Reduction at the cathode, oxidation at the anode — remember that oxidation is loss of electrons and reduction is gain.
- In a half equation the electrons balance the charge: Na⁺ + e⁻ → Na; 2Cl⁻ → Cl₂ + 2e⁻.
- With copper electrodes the anode dissolves and the cathode gains the same mass — that is how copper is purified.