In this blog post, we’ll explain in simple terms the principle behind how chemical cells generate electricity and how a metal’s ionization tendency determines the anode and cathode.
A chemical cell is a device that generates electricity through chemical reactions; the batteries we use in our daily lives are a type of chemical cell. There are various types of batteries depending on their size and shape, and they are widely used in many fields, including household, industrial, and medical applications. These batteries contain specific chemicals inside that generate an electric current. Chemical batteries use various materials and electrolytes to supply electricity, and these components significantly affect the battery’s performance and lifespan. When using a battery, the positive and negative terminals must be connected correctly. This is directly related to the battery’s efficiency and safety; if connected incorrectly, the battery may not function properly or could even be damaged. This is because, in a chemical battery, the side where reduction occurs—gaining electrons—is the positive terminal, while the side where oxidation occurs—losing electrons—is the negative terminal, and electrons move from the negative terminal to the positive terminal.
The anode and cathode of a chemical battery are determined by the ionization tendency of the metals that make up the electrodes. Ionization tendency refers to the ease with which a metal loses electrons and becomes a cation in a solution. This is a critical factor that determines the battery’s performance and has a major impact on its efficiency and lifespan. Therefore, metals with a high ionization tendency dissolve easily in an electrolyte solution that conducts electricity well, forming cations. If copper and zinc plates are placed in a dilute sulfuric acid (H₂SO₄) solution to serve as electrodes, a chemical cell is formed. Since zinc has a higher ionization tendency than copper, the zinc atoms on the surface of the zinc plate become cations, and the electrons released from the zinc atoms travel along the wire to the copper plate. Because electric current flows in the opposite direction of the electrons, the copper plate becomes the anode and the zinc plate becomes the cathode. If the zinc plate is replaced with a silver plate, the sulfuric acid solution is replaced with a sodium chloride solution, and the copper and silver plates are connected by a wire, the copper plate becomes the cathode and the silver plate becomes the anode. This is because copper has a higher tendency to ionize than silver. Thus, in a chemical cell, the anode and cathode are determined by the relative tendencies of the two metals to ionize.
Chemical cells are not merely a means of storing energy; they are also considered a key to the efficient utilization of renewable energy and the resolution of future energy problems. For example, large-scale chemical cells can be used to store electricity generated from renewable energy sources such as solar or wind power. Unlike the small dry-cell batteries we use in our daily lives, these cells have the capacity to store and supply energy on a large scale.
The ionization tendency of metals can be compared by the magnitude of the heat of reaction. To understand this, one must know the process by which a single atom is released from a metal to become a hydrated ion. Generally, metals form crystalline structures composed of many bonded metal atoms. Metals primarily lose electrons to become cations, but a single metal atom must be released from the crystal to become an individual ion. For example, in zinc metal (Zn) immersed in an electrolyte solution, a single atom breaks away, and this zinc atom loses two electrons to become a zinc ion (Zn²⁺). Since this reaction requires energy, the zinc metal absorbs heat during the process. A chemical reaction that absorbs heat in this manner is called an endothermic reaction. The zinc ion then hydrates within the electrolyte solution. In this reaction, heat is released; such a reaction is called an exothermic reaction.
In terms of the ionization tendency of metals, the heat of reaction is determined by the amount of heat absorbed or released when a chemical reaction occurs at a constant temperature. Therefore, when comparing ionization tendencies based on the heat of reaction, the values are compared by summing the heat from endothermic reactions and the heat from exothermic reactions.
Reaction enthalpies are denoted with signs; generally, the heat released during an exothermic reaction is indicated as a positive value, while the heat absorbed during an endothermic reaction is indicated as a negative value. Since the degree of ionization tendency increases as the value of the reaction enthalpy increases, it can be seen that the degree of ionization tendency is related to the magnitude of the heat released in exothermic reactions and the magnitude of the heat absorbed in endothermic reactions. The list of metallic elements ranked by their ionization tendencies is called the ionization series. By examining the ionization series, it is easy to determine which metals will serve as the anode and which as the cathode in a chemical cell. This serves as an important criterion in the design and fabrication of cells, and the selection of metals based on the ionization series significantly influences the cell’s performance and efficiency.
