The process of electrolysis
What you'll learn
- Why ionic compounds conduct electricity when molten or dissolved, but not when solid.
- What electrolytes, electrodes, cathodes and anodes are.
- How positive and negative ions move during electrolysis.
- How ions are discharged at electrodes, including half equations for Higher Tier.
Starting point: ions and ionic compounds
Before electrolysis makes sense, you need to be confident with ions.
Ion
An ion is an atom or group of atoms with an electrical charge. A cation is a positive ion, and an anion is a negative ion.
An ionic compound is made from positive and negative ions. These oppositely charged ions are held together by strong electrostatic attractions — attractions between opposite charges.
In a solid ionic compound, the ions are arranged in a regular 3D structure called a giant lattice. The ions are charged, but they are locked in place, so they cannot flow through the solid.
Ionic compound
An ionic compound is a substance made from positive and negative ions held together by strong electrostatic attractions in a giant lattice.
Why some ionic substances conduct electricity
To conduct electricity, a substance must contain charged particles that can move.
In metals, the moving charged particles are electrons. In ionic substances, the moving charged particles are ions — but only when the ions are free to move.
Solid, molten and aqueous
A solid ionic compound does not conduct electricity because its ions cannot move from place to place.
A molten ionic compound has been melted. The lattice has broken down, so the ions can move through the liquid.
An aqueous solution is a substance dissolved in water. If an ionic compound dissolves in water, its ions separate and can move around in the solution.
Electrolyte
An electrolyte is a liquid or solution that conducts electricity because it contains mobile ions. Molten ionic compounds and aqueous ionic solutions are electrolytes.
Conducting electricity
Ionic compounds conduct electricity when molten or dissolved in water because their ions are free to move. Solid ionic compounds do not conduct because their ions are fixed in position.
Deciding whether an ionic substance conducts
Magnesium chloride, MgCl2, is an ionic compound. Decide whether it conducts electricity as a solid, when molten, and when dissolved in water.
- In solid MgCl2(s), the Mg²⁺ and Cl⁻ ions are held in fixed positions in the lattice, so there are no mobile charged particles. It does not conduct.
- In molten MgCl2(l), the lattice has broken down, so the Mg²⁺ and Cl⁻ ions can move through the liquid. It does conduct.
- In aqueous MgCl2(aq), the ions have separated in water and can move around the solution. It does conduct.
Solid ionic compounds
Do not say “ionic compounds conduct electricity” without a state. They conduct when molten or aqueous, but not when solid.
What electrolysis is
Electrolysis uses electricity to break down an ionic substance when it is molten or dissolved in water.
The electrolyte is connected to a direct current power supply using two conductors called electrodes. Direct current means the current flows in one direction, so one electrode stays negative and the other stays positive.
Electrolysis
Electrolysis is the process in which an electric current is passed through an electrolyte, causing ions to move to electrodes and be discharged to form elements.
Electrode
An electrode is a conducting rod or plate placed in an electrolyte so current can enter or leave the liquid or solution.
The negative electrode is called the cathode. The positive electrode is called the anode.
How ions move during electrolysis
Opposite charges attract.
Positive ions, called cations, are attracted to the negative electrode, the cathode.
Negative ions, called anions, are attracted to the positive electrode, the anode.
Ion movement
During electrolysis, cations move to the cathode and anions move to the anode.
Remembering the electrodes
For electrolysis: PANIC means Positive Anode, Negative Is Cathode. Then use opposite charges to work out where the ions move.
Electrons in the electrolyte
In the electrolyte, charge is carried by moving ions, not by electrons flowing through the liquid. Electrons move in the external wires and are transferred at the electrodes.
Discharge at the electrodes
When an ion reaches an electrode, it may gain or lose electrons. This removes its charge and forms a neutral element. This is called discharge.
Discharge
An ion is discharged when it gains or loses electrons at an electrode and becomes a neutral atom or molecule.
At the cathode, positive ions gain electrons. For example, a metal ion can become a metal atom.
At the anode, negative ions lose electrons. For example, non-metal ions can form non-metal molecules.
For Higher Tier, you need to be able to write or balance half equations for these electrode reactions.
Half equation
A half equation shows the electron gain or electron loss happening at one electrode during electrolysis.
A useful Higher Tier memory aid is OIL RIG:
- Oxidation Is Loss of electrons.
- Reduction Is Gain of electrons.
So, at the cathode, cations are usually reduced because they gain electrons. At the anode, anions are usually oxidised because they lose electrons.
Predicting ion movement and discharge in molten lead bromide
Molten lead bromide, PbBr2(l), is electrolysed. Predict what happens at each electrode.
- Identify the ions in molten lead bromide: Pb²⁺ ions and Br⁻ ions. Because it is molten, these ions are free to move.
- Use opposite charges: Pb²⁺ ions move to the negative cathode, and Br⁻ ions move to the positive anode.
- At the cathode, each Pb²⁺ ion gains 2 electrons to become lead metal: Pb²⁺(l) + 2e⁻ → Pb(l).
- At the anode, bromide ions lose electrons and pair up to form bromine molecules: 2Br⁻(l) → Br2(g) + 2e⁻.
- The overall reaction is: PbBr2(l) → Pb(l) + Br2(g).
What is being broken down?
Electrolysis separates the ions in an electrolyte by forcing them to react at different electrodes.
For a simple molten binary ionic compound — one made from only two elements — electrolysis usually produces:
- the metal at the cathode
- the non-metal at the anode
For example, molten lead bromide produces lead and bromine.
Aqueous solutions
Aqueous electrolysis can be trickier because water is present, so there may be more than two ions in the solution. Do not automatically assume the compound’s metal and non-metal will always be produced in water.
The process as a sequence
You can describe electrolysis in a clear chain:
- The ionic compound is molten or dissolved in water, so it contains mobile ions.
- A direct current is passed through the electrolyte using electrodes.
- Cations move to the cathode; anions move to the anode.
- Ions gain or lose electrons at the electrodes.
- The ions are discharged, producing elements.
In the exam
- Always check the state first: solid ionic compounds do not conduct, but molten and aqueous ionic substances can.
- Use electrode charges carefully: in electrolysis, the cathode is negative and the anode is positive.
- For Higher Tier half equations, balance both atoms and charge, and place electrons on the correct side: gained electrons on the left, lost electrons on the right.
Check yourself
- Why can molten sodium chloride conduct electricity, but solid sodium chloride cannot?
- Which electrode do positive ions move towards during electrolysis?
- In a half equation at the cathode, are electrons gained or lost?