Edexcel IGCSE Chemistry · Spec 1.55C-1.58C
Electrolysis Principles (Paper 2)
Covers define an electrolyte accurately, Explain ion movement and electron transfer and Predict products for molten and named aqueous electrolytes.
Chemistry revision video
Electrolysis Principles (Paper 2)
Prefer to watch on YouTube? Open this video on YouTube.
Explained
What an electrolyte is, and what the ions do
An electrolyte is a liquid containing mobile ions that conducts electricity and is decomposed by it. It may be a molten ionic compound, a solution of an ionic compound, or an acidic solution such as dilute sulfuric acid.
The word mobile is the whole definition. A solid ionic compound contains exactly the same ions, and does not conduct, because those ions are locked in fixed positions in the lattice. Melt it or dissolve it and the ions are free to move, and only then can a current flow.
What happens at each electrode
Positive ions, called cations, are attracted to the negative electrode, the cathode. There they gain electrons and become atoms. Gaining electrons is reduction.
Negative ions, called anions, are attracted to the positive electrode, the anode. There they lose electrons. Losing electrons is oxidation.
So reduction always happens at the cathode and oxidation always at the anode, and the two happen together because the electrons removed at one electrode are the ones supplied at the other.
How the circuit is completed
This is worth getting right, because it separates a strong answer from a vague one. In the wires, electrons carry the charge. In the electrolyte, ions carry it.
Electrons never travel through the electrolyte. They leave the anode through the wire, pass round the circuit, and enter the cathode. Inside the liquid it is the ions moving in opposite directions that completes the circuit.
Predicting the products
For a molten compound there are only two ions, so the metal forms at the cathode and the non metal at the anode.
For a solution, water supplies hydrogen ions and hydroxide ions too, so there is a competition. At the cathode the less reactive of the metal and hydrogen is produced. At the anode a concentrated halide gives the halogen, and otherwise oxygen is produced.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks why magnesium chloride conducts when molten or dissolved but not when solid, and sets it out as a five point chain: that magnesium chloride is ionic, that the ions in the solid do not move because they are in fixed positions, and that the ions in solution are free to move charge.
Its notes then contain two rulings worth reading carefully. There is no mark for the third point if electrons moving or delocalised electrons are mentioned anywhere. And a mark is lost if intermolecular forces are mentioned in connection with magnesium chloride at all.
The reasoning is exact. An ionic compound conducts because its ions move, not because electrons move through it, and reaching for delocalised electrons imports the explanation for metals into a situation where it does not apply. Similarly, an ionic lattice contains no molecules, so intermolecular forces cannot exist in it.
On the same question the mark scheme wants water described as covalent, or as a molecule, with not enough charged particles that are free to move. Mobile charged particles is the general idea; which kind of particle depends on the substance.
Why the products are what they are
An electrode reaction is just an electron transfer, so writing the half-equation forces you to think about it correctly. Sodium ions plus one electron give sodium atoms; two chloride ions give chlorine plus two electrons.
Balance the charge as well as the atoms, because the number of electrons is what makes the charges match, and a half-equation with unbalanced charge has the wrong number of them.
Spec 1.55C-1.58C
What you need to know
- Define an electrolyte accurately
- Explain ion movement and electron transfer
- Predict products for molten and named aqueous electrolytes
Active recall
Quick check
Answer each question before opening the answer.
What makes a liquid an electrolyte?
It contains mobile ions and conducts electricity
Where does reduction occur?
At the cathode
Why can aqueous products differ from molten products?
Water supplies additional ions that may be discharged
Chemistry revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.