A galvanic cell (or voltaic cell) converts the energy released from a spontaneous chemical reaction directly into electrical current. This process is the fundamental mechanism behind all conventional batteries. Confusion often arises when comparing the cell’s components to standard electrical circuits because the polarity of the cathode—the electrode where reduction occurs—is often counterintuitive.
Defining Electrode Function: Oxidation and Reduction
The two conductive metal components within any electrochemical cell are called electrodes, classified by the chemical process occurring at their surface. These processes are always a pair of complementary reactions known as reduction and oxidation (redox). Oxidation is the loss of electrons by a chemical species, while reduction is the gain of electrons.
The electrode where oxidation takes place is called the anode. Conversely, the electrode where reduction takes place is called the cathode. At the cathode, positive ions from the solution accept electrons that have traveled through the external circuit, reducing them into a neutral, solid form.
A helpful mnemonic for remembering these definitions is “An Ox” (Anode is Oxidation) and “Red Cat” (Reduction is Cathode). These chemical definitions apply universally across all types of electrochemical cells, regardless of their electrical sign.
Electron Flow and Potential: Why the Cathode is Positive
In a galvanic cell, the spontaneous chemical reaction releases energy, driving the flow of electrons. The anode metal naturally loses electrons, making it the source for the external circuit. Since oxidation generates a surplus of negative charge, the anode is designated as the negative terminal of the galvanic cell.
The liberated electrons travel through the external wire to the cathode, which has a lower electrical potential. This potential difference is the driving force that allows the cell to generate a voltage. The cathode acts as the electron sink, attracting the negatively charged electrons from the anode.
This attraction is created by positive metal ions near the cathode’s surface. These ions accept the incoming electrons for reduction, giving the cathode a positive electrical potential relative to the anode. Therefore, in a galvanic cell, the cathode is the positive electrode, and the anode is the negative electrode.
Clarifying the Confusion: Galvanic vs. Electrolytic Cells
Confusion over cathode polarity stems from the electrolytic cell, which operates on the opposite principle. A galvanic cell converts spontaneous chemical energy into electrical energy, while an electrolytic cell uses an external source to force a non-spontaneous reaction.
In the electrolytic cell, the external power supply dictates the polarity, reversing the electrical sign. The electrode connected to the negative terminal accepts electrons, making it the cathode (negative electrode). Conversely, the electrode connected to the positive terminal undergoes oxidation, making it the anode (positive electrode).
Despite this polarity reversal, the fundamental chemical definitions remain consistent: oxidation occurs at the anode, and reduction occurs at the cathode in both cell types. The distinction is whether the cell is producing power spontaneously (galvanic) or consuming external power (electrolytic).