The terms anode and cathode refer to the two fundamental electrodes in any electrochemical cell or electronic device. These electrodes are the physical sites where electric current interacts with a non-metallic medium. Confusion arises because the electrical polarity associated with each name changes depending on the system’s function. To correctly identify the anode and cathode, one must determine the specific chemical process occurring at the electrode, rather than relying on a fixed positive or negative charge.
The Universal Chemical Definition
The most reliable distinction between the two electrodes is based on the chemical reaction that takes place at their surface, a definition that remains constant across all systems. The anode is universally defined as the electrode where oxidation occurs, which is the chemical process of losing electrons. Conversely, the cathode is the electrode where reduction occurs, which is the chemical process of gaining electrons.
This electron transfer process is known as a redox reaction. Since the anode releases electrons, it serves as the electron source for the external circuit. The cathode accepts those electrons, functioning as the electron sink.
A common mnemonic is “AN OX” for Anode is Oxidation, and “RED CAT” for Reduction is Cathode. Electrons always flow through the external circuit from the anode (oxidation site) to the cathode (reduction site). This movement creates the flow of electrical energy, whether it is being produced or consumed.
Anode and Cathode in Power-Generating Systems
In devices that generate electrical power spontaneously, such as a discharging battery or a galvanic cell, the roles of the anode and cathode are defined by the natural flow of the chemical reaction. These systems harness the energy released from a spontaneous chemical reaction to produce an electrical current.
In this power-generating setup, the anode is the source of electrons released by the oxidation reaction. Because the anode builds up a negative charge relative to the rest of the cell, it is designated as the negative terminal of the battery. For example, in a standard zinc-copper cell, the zinc electrode oxidizes and releases electrons, making it the negative anode.
The electrons travel through the external circuit before arriving at the cathode. At the cathode, the reduction reaction consumes these incoming electrons, resulting in an electron deficiency relative to the anode. Consequently, the cathode is the positive terminal in power-generating systems.
The internal flow of ions through the electrolyte maintains electrical neutrality. Positively charged ions (cations) migrate toward the positive cathode, while negatively charged ions (anions) migrate toward the negative anode. This spontaneous movement allows the continuous production of electricity until the reactants are depleted.
Anode and Cathode in Power-Consuming Systems
The polarity flips in systems that consume electrical power to drive a non-spontaneous chemical reaction, such as an electrolytic cell or a rechargeable battery during charging. In these power-consuming systems, an external power source forces the chemical reaction to proceed in the non-natural direction. This external source dictates the direction of electron movement.
The external power supply pulls electrons away from one electrode and pushes them onto the other. The electrode connected to the positive terminal of the external source has electrons forcibly removed. This removal still results in oxidation, meaning this electrode is the anode, but it is now designated as the positive terminal of the electrolytic cell.
Conversely, the electrode connected to the negative terminal of the external source receives electrons forcibly pushed into the cell. This influx causes reduction to occur at this site, defining it as the cathode, which is therefore designated as the negative terminal. Despite the reversal of the electrical sign compared to a battery, the fundamental chemical definitions remain fixed: oxidation still occurs at the anode, and reduction still occurs at the cathode.
The terms anode and cathode describe the chemical action occurring at the electrode’s surface. The positive and negative polarity labels describe the electrical potential and are determined by the system’s operational mode.