Anode

What is Anode?

The positive electrode of a simple electrolytic cell, joined to the positive terminal of the power supply. The product formed there is a non-metal other than hydrogen. For Extended candidates, it is also the electrode at which negative ions arriving from the electrolyte lose electrons, so oxidation happens there, and where an active electrode such as copper is itself oxidised.

This definition is part of the Electrochemistry chapter in Cambridge IGCSE Chemistry.

Anode in context

Concepts related to Anode

• Oxidation
• anions
• Active electrode

Common mistakes with Anode

• Aluminium extraction, bauxite, cryolite and carbon-anode consumption. These are examined — but in Topic 9, not here. Statement 9.6 makes bauxite and extraction by electrolysis a Core requirement, and adds cryolite, anode replacement and the electrode half-equations at Supplement. Revise them with the metals chapter; do not bring them into a Topic 4 answer.
• Electrolytic refining of copper, anode slime and purity figures.
• 2. "Positive ions go to the positive anode." Supplement 4.1.8 Repair Opposite charges attract, so positive cations move to the negative cathode and negative anions move to the positive anode. Taught in Lesson A.
• 3. "The anode is positive, in every cell, everywhere." Core 4.1.2 Repair In a simple electrolytic cell the anode is positive and the cathode is negative because an external power supply makes them so; that sign pattern belongs to this context and should not be carried unexamined into other kinds of cell. Taught in Lesson A.
• 4. "Oxidation is gain of electrons." Supplement 4.1.11 Repair Oxidation is loss of electrons and happens at the anode; reduction is gain of electrons and happens at the cathode — write the full words, then check the side your electrons are on. Taught in Lesson F.
• 6. "Chlorine is a colourless gas" — or — "concentrated sodium chloride solution gives oxygen at the anode." Core 4.1.3 Repair Chlorine is a pale yellow-green gas with a sharp choking smell, and it is the anode product of the named cell, which uses a concentrated aqueous chloride. Extended candidates add: a dilute chloride solution gives oxygen at the anode instead Supplement 4.1.10 Taught in Lesson D.
• 7. "A copper electrode behaves like a carbon one." Supplement 4.1.9 Repair Carbon and platinum electrodes take no chemical part, but a copper anode is itself oxidised and passes into the solution as copper ions, so the anode product is completely different. Taught in Lesson G.
• 8. "Connect the spoon you want to plate to the positive terminal." Core 4.1.7 Repair The cathode is the electrode where metal from the solution is deposited, so the object being plated is the negative cathode and the coating metal is the positive anode. Connected the other way round, the object is the electrode that dissolves. Extended candidates can add the electron reason: depositing a metal ion is a reduction, and reduction happens at the cathode Supplement 4.1.11 Taught in Lesson I.

Examiner tips on Anode

• The arrow that costs marks. A single electron arrow drawn straight from the anode across to the cathode, ignoring the power supply, describes a cell that does not exist. Electrons in an electrolytic cell are pushed by the supply; draw them leaving the negative terminal on one side and arriving at the positive terminal on the other.
• Where the oxygen atoms come from. Count them in the anode equation: four hydroxide ions bring four oxygen atoms, and those four end up as two in the \(\mathrm{O_2}\) molecule and two in the two water molecules. Every oxygen atom is accounted for, and none of them came from a sulfate ion.
• A careful way to say it. Write "the anode loses mass and the cathode gains mass" — that is what is observed. Do not claim the two changes are exactly equal unless a question gives you data or a calculation that establishes it.