Cathode
Definition and meaning of Cathode in chemistry.
Cathode is the electrode in an electrochemical cell where reduction takes place. At this electrode, a chemical species gains electrons that are supplied through the external circuit. Every electrochemical cell has exactly one cathode and one anode.
In more detail
Reduction always happens at the cathode, and oxidation always happens at the anode. Chemistry students often remember this with the mnemonics RED CAT, reduction at the cathode, and AN OX, oxidation at the anode. The cathode's electrical charge depends on what kind of cell it is in.
In a galvanic cell, which generates electricity spontaneously like a discharging battery, the cathode is the positive terminal. In an electrolytic cell, which uses outside electricity to force a non-spontaneous reaction to occur, the cathode is the negative terminal, because it connects to the negative terminal of the power source driving the reaction.
This function-based definition, not a fixed electrical charge, is what actually defines any cathode. Conventional current flows into the cathode from the external circuit, while electrons flow into the cell there and are used up by the reduction reaction on its surface. In household batteries, the cathode is typically labeled with a plus sign, since batteries function as galvanic cells while providing power.
The word cathode comes from Greek roots meaning a way down, reflecting the historical idea of current flowing down into the electrode.
Key facts
| Field | Physical Chemistry |
|---|---|
| Half-reaction type | Reduction (electron gain) |
| Polarity in a galvanic cell | Positive electrode |
| Polarity in an electrolytic cell | Negative electrode |
| Memory aid | RED CAT (reduction at the cathode) |
In the electrolysis of molten sodium chloride, the cathode is where sodium ions gain electrons: Na+(l) + e- → Na(l). This reduction reaction produces liquid sodium metal at the cathode, while chloride ions are oxidized to chlorine gas at the anode. This process is the industrial method used to produce pure sodium metal on a large scale. Car batteries and household batteries both rely on this same reduction reaction at their cathode to deliver stored electrical energy.
Frequently asked questions
Is the cathode always the negative electrode?
No. The cathode is defined by function, where reduction occurs, not by a fixed charge. It is negative in an electrolytic cell but positive in a galvanic cell like a discharging battery.
How can I remember which electrode is the cathode?
Use the mnemonic RED CAT: reduction occurs at the cathode. Correspondingly, AN OX reminds you that oxidation occurs at the anode.
Can the same electrode switch between being the anode and cathode?
Yes. In a rechargeable battery, one electrode acts as the cathode while discharging and becomes the anode while charging, since the direction of the reaction reverses.