Question

Describe the hydrogen electrode. What is the value of its standard reduction potential?

Short answer

The standard hydrogen electrode (SHE) consists of a platinum electrode in 1M acid with hydrogen gas at 1 atm and is used as a reference electrode. Its standard reduction potential is defined as 0 volts.

Step-by-step solution

Description of the Hydrogen Electrode

The hydrogen electrode, also known as the standard hydrogen electrode (SHE), is a reference electrode that is used to measure the electrode potentials of half-cells in electrochemistry. It consists of a platinum electrode coated with platinum black, which is immersed in a solution of 1M acid (usually HCl). Hydrogen gas at a pressure of one atmosphere is bubbled over the electrode at 298 K (25°C). The electrode reaction is the reduction of hydrogen ions to form hydrogen gas, or the reverse process of hydrogen gas releasing electrons and forming hydrogen ions.

Determination of Standard Reduction Potential

The standard reduction potential for the hydrogen electrode is defined as 0 volts. This is because the hydrogen electrode serves as the reference point for measuring all other electrode potentials. All other standard electrode potentials are therefore measured relative to the standard hydrogen electrode.

Key concepts

Electrode Potentials

Electrode potentials are a fundamental concept in electrochemistry that reflect the ability of an electrode to gain or lose electrons—precisely, they are measures of the tendency of an electrode to be reduced. Reduction refers to the gain of electrons by a substance, a key process in redox reactions which are the bedrock of electrochemical cells. Electrode potentials are measured in volts (V) and provide insight into the direction and magnitude of electrical potential in a half-cell reaction. When we say a half-cell, we're talking about one part of a two-part system where oxidation or reduction occurs.

Understanding electrode potentials also allows us to predict how substances will behave in electrochemical reactions. For instance, a metal with a higher tendency to be reduced—that is, with a higher reduction potential—will be more likely to gain electrons and undergo reduction. This property is central to the functioning of batteries, fuel cells, and various industrial electrochemical processes. In every electrochemical cell, one half-cell will have a more positive electrode potential and thus, under standard conditions, will undergo reduction, serving as the cell's cathode.

Electrochemistry

Electrochemistry is the study of chemical processes that cause electrons to move. This movement of electrons leads to electricity, which is why electrochemistry is essential to many forms of power generation and storage, as well as to numerous industrial processes. A quintessential application of electrochemistry is in galvanic cells, or what we commonly call batteries, where spontaneous redox reactions produce electrical energy.

Electrochemistry is not just about creating power; it's also key in preventing corrosion, refining metals, and even in neuroscience to understand electrical signals in the body. Understanding how redox reactions work, and how to control the flow of electrons in different materials under different conditions, is crucial for innovation in energy solutions and materials science. Moreover, electrochemistry is the foundation for sensors that measure the concentration of various substances, for example, glucose meters to monitor blood sugar levels.

Platinum Electrode

A platinum electrode is often used in electrochemical cells, including the standard hydrogen electrode, due to its chemical inertness and ability to facilitate the adsorption of hydrogen gas. The electrode in such cells is typically platinum metal, sometimes fashioned into a foil or a plate. A particularly effective form is 'platinum black,' a finely divided platinum powder that increases the surface area allowing for more effective interactions with hydrogen molecules.

Platinum's stable nature means that it doesn't react with the solutions or gases in the electrochemical cell, maintaining its integrity and allowing for consistent readings over time. This inertness also ensures that the electrode does not participate in the reaction but merely serves as a medium for the transfer of electrons. In this role, platinum can catalyze the oxidation or reduction of hydrogen, providing a consistent and reliable reference for measuring electrode potentials.

Reduction Potential

Reduction potential, often symbolized by 'E', is the tendency of a chemical species to acquire electrons and be reduced. This term is typically discussed in the context of a half-cell in an electrochemical cell and is measured relative to the standard hydrogen electrode (SHE), which, by convention, is set at 0 volts. The SHE serves as a baseline, allowing chemists to compare reduction potential across different species.

When two different metals or substances are compared, the one with a higher (more positive) reduction potential will undergo reduction preferentially. Conversely, the one with a lower (more negative) reduction potential will more readily lose electrons and be oxidized. Additionally, by using SHE as a standard, this allows scientists and engineers to create tables of standard reduction potentials to predict the direction of redox reactions, an essential tool in the design of batteries and other electrochemical devices.