Standard electrode potential

Standard electrode potential is a vital measurement in the field of electrochemistry. It measures the reducing power of any element or compound. This value tells scientists how much an electrode wants to gain electrons. The International Union of Pure and Applied Chemistry, or IUPAC, defines this value specifically. They define it as the standard electromotive force of a cell. In this specific cell, molecular hydrogen is oxidized to solvated protons at the left-hand electrode. Understanding these potentials is essential for studying how electricity moves through chemical systems.

To understand this potential, we must look at redox reactions. Every electrochemical cell, like a galvanic cell, relies on these reactions. A redox reaction consists of two separate half-reactions. The first is oxidation, which occurs at the anode where a substance loses electrons. The second is reduction, which occurs at the cathode where a substance gains electrons. Electricity is produced because of the difference in electric potential between the two metal electrodes. This potential exists in relation to the electrolyte, which is the substance the electrodes sit in.

Measuring these potentials is quite difficult because you cannot measure them in isolation. An electrode's potential changes based on temperature, concentration, and pressure. Because of this, scientists use a reference electrode to find a value. This reference is called the Standard Hydrogen Electrode, or SHE. The IUPAC defines the potential of the SHE as exactly 0.00 V. To find an unknown potential, you pair the unknown electrode with the SHE. You can also pair it with another electrode that has a known potential. In a lab, the unknown electrode connects to the positive terminal of an electrometer. The SHE connects to the negative terminal.

Scientists usually express these values as standard reduction potentials. This is because the oxidation potential is simply the negative of the reduction potential. These potentials are independent of the number of electrons transferred during the reaction. They are expressed in volts, which measure the energy per electron transferred. This makes it easy to combine two different electrode potentials to find the overall cell potential. Even if the two reactions involve different numbers of electrons, the math remains straightforward. This consistency allows researchers to predict how different chemicals will interact in a circuit.

We can categorize electrodes by how they behave near chemical equilibrium. A reversible electrode is one where the potential comes from reversible changes. For this to work, the system must stay very close to chemical equilibrium. The changes applied to the system must be very small and spread over a long time. In a real laboratory, achieving perfect reversibility is very difficult. Any sudden change can push the system out of equilibrium. For example, electrodes used in electroplating are not reversible. They use a high over-potential to force metal cations to deposit onto a surface. This process is far from equilibrium and involves constant, large changes.

The standard reduction potential table shows us how different elements behave. A larger, more positive value means the element is easily reduced. This makes the element a strong oxidizing agent. For instance, fluorine (F2) has a very high potential of +2.87 V. This means it is very easy for fluorine to gain electrons. Conversely, lithium (Li+) has a very low potential of -3.05 V. This negative value means lithium is not easily reduced. Instead, lithium is a strong reducing agent because it prefers oxidation. Zinc (Zn2+) has a potential of -0.76 V. This means zinc can be oxidized by any electrode with a higher potential, such as copper (Cu2+) at +0.34 V. However, zinc can be reduced by electrodes with lower potentials, like hydrogen (H2) at -2.23 V.

These potentials are directly linked to the concept of Gibbs free energy. In a galvanic cell, a spontaneous redox reaction must produce electricity. For a reaction to be spontaneous, the Gibbs free energy must be negative. This occurs when the overall cell potential is a positive value. If the cell potential is negative, the reaction is non-spontaneous. This type of setup is called an electrolytic cell. In an electrolytic cell, energy must be consumed to make the reaction happen. By using the standard potentials of the cathode and the anode, scientists can calculate if a reaction will occur on its own.