In electrolysis, a direct current passes through an electrolyte, a molten or dissolved ionic compound, using two electrodes. The anode is connected to the positive terminal and the cathode to the negative terminal. Electrons move in the wires, ions move in the electrolyte.
This is the first lesson in electrolysis reasoning. It needs the idea of ions from bonding and structure and appears in almost every later electrolysis question.
What does each part do?
Think of the cell as a circuit with a gap in the middle that ions have to bridge.
| Part | Role | What moves |
|---|---|---|
| Power supply | Pushes electrons round the circuit | Electrons in the wires |
| Cathode (negative) | Receives electrons; positive ions arrive here and gain them | Electrons in, cations attracted |
| Anode (positive) | Loses electrons to the circuit; negative ions arrive here and give them up | Electrons out, anions attracted |
| Electrolyte | Conducts by free ions | Ions through the liquid |
Positive ions (cations) are attracted to the negative cathode. Negative ions (anions) are attracted to the positive anode. That is why the cathode is where metals or hydrogen form.
Why must the ions be free?
A solid ionic compound has ions locked in a lattice, so it does not conduct. Melting it, or dissolving it in water, lets the ions move. Only then can the circuit be completed.
Inert electrodes such as graphite or platinum carry electrons but do not react. An active electrode, such as copper in copper(II) sulfate, can take part and change mass. The lesson on aqueous and molten cases uses this difference.
Worked example
A cell contains molten zinc chloride, ZnCl₂, with graphite electrodes. Electrode X is joined to the positive terminal and electrode Y to the negative terminal. Name each electrode, say which ion moves to each and explain why the electrolyte must be molten.
Step 1, electrode names: X is joined to the positive terminal, so X is the anode. Y is the cathode.
Step 2, ion movement: the ions present are Zn²⁺ and Cl⁻. Zn²⁺ is positive, so it moves to the cathode Y. Cl⁻ is negative, so it moves to the anode X.
Step 3, reason for molten: solid zinc chloride has ions fixed in a lattice. When molten, the ions are free to move and carry the current.
Step 4, electrons: electrons travel through the wires from the anode to the cathode (via the power supply), not through the liquid.
The products are covered in predicting molten products.
The mistake to watch for
A frequent slip is to write that “the anode is negative because it attracts positive ions”, or to say electrons flow through the electrolyte.
Mistaken answer: “Zn²⁺ moves to the anode because electrons flow through the liquid.”
This mixes up the two kinds of movement and the electrode charge.
The correction is to decide the electrode from the power-supply terminal first. The anode is positive here, so it attracts the negative Cl⁻. Ions move in the liquid; electrons move in the wires.
Check yourself
1. An electrode is joined to the negative terminal of a supply. Is it the anode or cathode, and which type of ion moves towards it?
Show answer
It is the cathode, and positive ions (cations) move towards it.
2. Explain why solid sodium chloride does not conduct but molten sodium chloride does.
Show answer
In the solid, Na⁺ and Cl⁻ ions are held in fixed positions in the lattice. When molten, they are free to move and carry charge through the liquid.
3. Why are graphite electrodes described as inert?
Show answer
They conduct electrons to and from the electrolyte but do not react or dissolve during the electrolysis.
Where this leads next
Next, predict a simple molten-electrolyte product by asking which ion arrives at which electrode. The mole and equation-ratio tutor and the equation balance trainer support the later equation work.
Some students can label a diagram but lose marks when a question asks them to explain movement. In online one-to-one Chemistry tuition, your teacher can ask you to explain each ion’s path aloud and fix the step where it goes wrong.