Anode and cathode describe the two electrodes in an electrochemical system, which converts chemical energy into electrical energy or vice versa. These electrodes are the pathways through which electrical current enters or leaves the non-metallic part of the circuit, such as an electrolyte solution. A frequent source of confusion is that the positive and negative charge designation of the anode and cathode can switch depending on the system. To understand the difference, it is necessary to look beyond the charge and focus on the underlying chemical process that defines each electrode’s role.
The Universal Difference Based on Function
The fundamental distinction between an anode and a cathode lies in the specific chemical reaction that takes place at their surface. This functional definition remains constant regardless of the electrode’s charge or the device’s operation. The anode is universally defined as the electrode where oxidation occurs, while the cathode is the electrode where reduction occurs.
Oxidation is a chemical process involving the loss of electrons. Conversely, reduction is the process where a gain of electrons takes place. A helpful mnemonic is “LEO the lion says GER,” which stands for “Loss of Electrons is Oxidation; Gain of Electrons is Reduction.”
Because oxidation at the anode involves losing electrons, the anode is where electrons leave the electrode and flow into the external circuit. The electrons then travel to the cathode, where they are consumed in the reduction reaction. Therefore, the anode is the electron source, and the cathode is the electron sink in the external circuit. This core chemical activity is the single most important concept for differentiating the two electrodes in any setting.
Anode and Cathode in Spontaneous Cells
Spontaneous electrochemical cells, often called galvanic or voltaic cells, generate electrical energy through a natural chemical reaction. Common examples include standard AA, AAA, or car batteries. In this type of cell, the anode is designated as the negative terminal, and the cathode is designated as the positive terminal.
The anode is negative because the spontaneous oxidation reaction releases electrons directly onto the electrode surface. This buildup of electrons gives the anode an excess negative charge, making it the source of the electrical current flowing through the external circuit.
The cathode is the positive terminal because it is ready to accept the incoming electrons. The reduction reaction occurs as positively charged ions migrate toward this electrode to gain the electrons arriving from the anode. This polarity arrangement is characteristic of any device that is discharging or operating in its energy-producing mode.
Anode and Cathode in Non-Spontaneous Cells
Non-spontaneous cells, known as electrolytic cells, require an external power source to drive a chemical reaction, operating in the reverse manner of galvanic cells. Examples include charging a rechargeable battery, electroplating metal objects, or producing hydrogen gas from water. In these systems, the polarity of the electrodes is reversed compared to the spontaneous cell.
In an electrolytic cell, the anode is the positive terminal, and the cathode is the negative terminal. This reversal occurs because the external power source dictates the charge, forcing the non-spontaneous reaction to proceed. The power supply pumps electrons away from the electrode connected to its positive terminal, forcing the anode to become electron-deficient and positive.
Since oxidation must still occur at the anode, the positive anode attracts negatively charged ions (anions) from the solution to supply the necessary electrons. Simultaneously, the cathode is connected to the negative terminal of the external source, making it electron-rich and negatively charged. Despite the charge flip, the fundamental chemical roles remain intact: oxidation still occurs at the positive anode, and reduction still occurs at the negative cathode.
Remembering the Roles
The key to resolving the confusion of the reversing charges is to always prioritize the chemical function over the electrical charge. A powerful mnemonic tool for remembering the constant functional roles is “An Ox” and “Red Cat.” “An Ox” means the Anode is where Oxidation occurs, and “Red Cat” means Reduction occurs at the Cathode.
Another helpful association uses vowels and consonants: Anode and Oxidation both begin with vowels, while Cathode and Reduction both begin with consonants. Remembering that electrons always flow from the anode to the cathode in the external circuit can also serve as a constant point of reference.
This functional definition also extends to non-chemical applications, such as electronic components like diodes. In a diode, the conventional current flows from the anode to the cathode. By focusing on the consistent chemical events of electron loss and gain, one can correctly identify the anode and cathode in any electrical system.