Revision notes · Chemical changes
Electrolysis
The process of electrolysis4.4.3.1
Definition: Electrolysis is the breaking down of an ionic compound using electricity.
Passing an electric current through a molten or dissolved ionic compound decomposes it into its elements.
- •The liquid or solution that conducts electricity is called the electrolyte.
- •Positively charged ions (cations) move towards the negative electrode called the cathode.
- •Negatively charged ions (anions) move towards the positive electrode called the anode.
- •When ions reach the electrodes, they lose or gain electrons to become neutral elements.
⚠️ Common mistake: Confusing the charges of the electrodes; the anode is positive and the cathode is negative.
🧠 Remember: PANIC: Positive Anode, Negative Is Cathode.
Electrolysis of molten ionic compounds4.4.3.2
Definition: A molten ionic compound is an ionic solid that has been heated until it melts, allowing its ions to move freely.
When a simple molten ionic compound is electrolysed, the metal forms at the cathode and the non-metal forms at the anode.
- •Solid ionic compounds cannot undergo electrolysis because their ions are fixed in a lattice and cannot move.
- •Melting frees the ions so they can move to the electrodes.
- •Positive metal ions are attracted to the cathode where they gain electrons to form metal atoms.
- •Negative non-metal ions are attracted to the anode where they lose electrons to form non-metal molecules.
⚠️ Common mistake: Thinking solid ionic compounds can conduct electricity and undergo electrolysis without being molten or in solution.
🧠 Remember: Metals form at the cathode; non-metals form at the anode.
Using electrolysis to extract metals4.4.3.3
Definition: Electrolysis is used to extract metals that are more reactive than carbon from their molten ores.
Extracting reactive metals like aluminium using electrolysis requires large amounts of energy to melt the ore and produce the electric current.
- •Aluminium is extracted from bauxite ore, which is purified into aluminium oxide.
- •Aluminium oxide is dissolved in molten cryolite to lower its melting point and reduce energy costs.
- •Positive aluminium ions move to the cathode and gain electrons to form molten aluminium metal.
- •Negative oxide ions move to the anode and lose electrons to form oxygen gas, which reacts with the carbon anodes to form carbon dioxide.
⚠️ Common mistake: Thinking cryolite is a chemical reactant rather than a compound used to lower the melting point of aluminium oxide.
🧠 Remember: Cryolite lowers the melting point; carbon anodes burn away as CO2.
Electrolysis of aqueous solutions4.4.3.4
Definition: An aqueous solution is formed when a substance is dissolved in water, producing extra hydrogen (H+) and hydroxide (OH-) ions.
In aqueous electrolysis, the products at the electrodes depend on the relative reactivity of the ions present.
| Electrode | Product |
|---|---|
| Cathode | Hydrogen gas — unless the metal is less reactive than hydrogen (e.g. copper), then the metal forms instead |
| Anode | Oxygen gas — unless halide ions are present (chloride, bromide, iodide), then the halogen forms instead (chlorine, bromine, iodine) |
⚠️ Common mistake: Forgetting that water breaks down into H+ and OH- ions, which compete with the solute ions at the electrodes.
🧠 Remember: At the cathode, the least reactive element is produced.
Representation of reactions at electrodes as half equations4.4.3.5
Definition: A half equation represents the oxidation or reduction reaction occurring at a single electrode during electrolysis.
Half equations show how ions gain or lose electrons at the electrodes to form neutral atoms or molecules.
- •Reduction occurs at the negative cathode, where positive ions gain electrons.
- •Oxidation occurs at the positive anode, where negative ions lose electrons.
- •A cathode half equation showing reduction: Cu²⁺ + 2e⁻ → Cu.
- •An anode half equation showing oxidation: 2Cl⁻ → Cl₂ + 2e⁻.
⚠️ Common mistake: Writing electrons on the wrong side of the arrow in a half equation; remember electrons are gained (on the left) during reduction and lost (on the right) during oxidation.
🧠 Remember: OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons.
Feel like you’ve got it?
Practise it now →