The cathode is the electrode where reduction occurs in an electrochemical cell, meaning it's the site where electrons are gained. In a galvanic cell it's the positive electrode; in an electrolytic cell it's connected to the negative terminal of the power source. Either way, reduction always happens there.
The cathode is the electrode where reduction happens. Period. That definition never changes, whether you're looking at a galvanic (voltaic) cell or an electrolytic cell. The classic mnemonic is "Red Cat" (Reduction at the Cathode), paired with "An Ox" (oxidation at the anode). Electrons always flow through the external wire toward the cathode, because that's where a species in solution is waiting to grab them. Cations in the salt bridge also drift toward the cathode half-cell to balance the charge buildup as positive ions get pulled out of solution.
What does change between cell types is the cathode's sign and its consequences. In a galvanic cell, the cathode is the positive electrode, and it often gains mass as metal ions plate onto it (think Ag⁺ + e⁻ → Ag(s) depositing solid silver). In an electrolytic cell, an external power source forces a thermodynamically unfavorable reaction, and the cathode is the electrode wired to the negative terminal. The reduction still happens there. In the 2021 FRQ on molten MgCl₂ electrolysis, liquid Mg formed at the cathode because Mg²⁺ gained electrons. Per EK 9.8.A.1, you should be able to describe all of this at both the macroscopic level (electrode gains mass, gas bubbles form) and the particulate level (ions gaining electrons at the electrode surface).
The cathode lives in Unit 9: Thermodynamics and Electrochemistry, threading through Topics 9.7, 9.8, and 9.10. Learning objective AP Chem 9.8.A asks you to connect each physical component of a cell to how the cell works, and the cathode is the component most exam questions hinge on. You can't write the cell potential, predict the direction of electron flow, or explain which electrode gains mass without first identifying the cathode correctly. It also anchors AP Chem 9.7.A (an external energy source drives reduction at the cathode of an electrolytic cell, like charging a battery) and Faraday's law calculations in Topic 9.10, where the moles of electrons delivered to the cathode determine exactly how much metal plates out. If electrochemistry is the story of electrons moving, the cathode is where the story ends.
Keep studying AP Chemistry Unit 9
Reduction and Half-cell Reactions (Unit 9)
The cathode is defined by its half-reaction. When you split a redox equation into half-reactions, the reduction half-reaction (electrons on the reactant side) is the one happening at the cathode. Identifying which half-reaction has the higher E°red tells you which electrode is the cathode in a galvanic cell.
Electrolytic Cells and Coupled Energy (Unit 9)
In Topic 9.7, an external voltage forces an unfavorable reaction to run. The cathode still does reduction, but now it's reducing whatever the power source pushes electrons into, like Mg²⁺ becoming liquid Mg in molten MgCl₂ electrolysis. Same job, opposite spontaneity.
Faraday's Constant and Electrolysis Math (Unit 9)
Faraday's law turns the cathode into a stoichiometry problem. Current × time gives you charge, dividing by Faraday's constant gives moles of electrons, and the cathode half-reaction converts that into grams of metal deposited. Mass-gain-at-the-cathode calculations are a classic exam setup.
Galvanic Cells and Electron Flow (Unit 9)
Electron flow is the fastest way to find the cathode on a cell diagram. Electrons leave the anode, travel through the wire, and arrive at the cathode. If a question shows you a voltmeter or asks about salt bridge ion migration, trace the electrons and the cathode reveals itself.
Cathode questions show up in both MCQs and FRQs, almost always asking you to identify and justify rather than just recite. The 2018 FRQ gave a galvanic cell with Ag(s)/Ag⁺ and Cr(s)/Cr³⁺ half-cells and expected you to use standard reduction potentials to figure out which electrode is the cathode (the higher E°red wins). The 2021 FRQ flipped to electrolysis, where molten MgCl₂ produces liquid Mg at the cathode and Cl₂ gas at the anode under an applied voltage. Multiple-choice stems typically hand you two or three half-reactions with E° values and ask which combination gives the most favorable cell, which requires assigning the cathode correctly first. Be ready to predict the direction of electron flow, state which electrode gains or loses mass, explain which way cations move through the salt bridge, and run Faraday's law math on the cathode half-reaction. Justifications must reference the half-reactions, not just the labels.
The cathode is where reduction happens; the anode is where oxidation happens. The trap is the signs. In a galvanic cell the cathode is positive and the anode is negative, but in an electrolytic cell the cathode connects to the negative terminal of the power source. Don't memorize signs. Memorize the reactions ("Red Cat, An Ox"), because those never flip.
Reduction always occurs at the cathode in both galvanic and electrolytic cells; only the electrode's sign changes between cell types.
In a galvanic cell, the half-reaction with the higher standard reduction potential happens at the cathode, and that electrode is positive.
Electrons flow through the external wire toward the cathode, and cations in the salt bridge migrate toward the cathode half-cell.
The cathode often gains mass in galvanic cells because metal cations like Ag⁺ are reduced to solid metal on its surface.
In electrolysis, the cathode is wired to the negative terminal and reduces cations, like Mg²⁺ becoming liquid Mg in molten MgCl₂.
Faraday's law lets you calculate the mass of metal deposited at the cathode from current, time, and the moles of electrons in the half-reaction.
The cathode is the electrode where reduction occurs, meaning species gain electrons there. It appears in Topics 9.7-9.10 of Unit 9, in both galvanic cells (where it's positive) and electrolytic cells (where it's connected to the negative terminal).
No. The cathode is positive in a galvanic cell but is attached to the negative terminal in an electrolytic cell. The reliable definition is the reaction, not the sign: reduction always happens at the cathode.
Use the half-reactions. Reduction (gaining electrons) happens at the cathode and oxidation at the anode, which is the "Red Cat, An Ox" mnemonic. In a galvanic cell, the half-reaction with the higher E°red is the cathode reaction. The 2018 FRQ required exactly this comparison for Ag⁺/Ag versus Cr³⁺/Cr.
The cathode typically gains mass when metal cations are reduced and plate onto it, like Ag⁺ + e⁻ → Ag(s). It can also produce a gas instead, and Faraday's law lets you calculate exactly how much product forms from the current and time.
Electrons always flow through the external wire from the anode toward the cathode, because the cathode is where they get consumed by reduction. At the same time, cations in the salt bridge migrate toward the cathode compartment to keep the solution electrically neutral.