Question

(a) Why is it impossible to measure the standard reduction potential of a single half-reaction? (b) Describe how the standard reduction potential of a half-reaction can be determined.

Short answer

(a) It is impossible to measure the standard reduction potential of a single half-reaction because the reduction half-reaction and the oxidation half-reaction occur simultaneously in a redox reaction, and the measured voltage or potential is a combined potential of both reactions. The standard reduction potential of a half-reaction cannot be measured in isolation, as it always depends on the other half-reaction in the overall redox reaction. (b) To determine the standard reduction potential of a half-reaction, a reference electrode with a known standard reduction potential, such as the Standard Hydrogen Electrode (SHE), is used. An electrochemical cell is set up with the reference electrode on one side and the half-reaction of interest on the other side. The cell potential (E_cell) is measured under standard conditions, and it is equal to the standard reduction potential of the half-reaction of interest when measured against the SHE. By measuring the cell potential, the standard reduction potential of the half-reaction can be determined.

Step-by-step solution

Understanding standard reduction potential

The standard reduction potential is a measure of the tendency of a chemical species to accept electrons and become reduced in a redox reaction (reduction-oxidation reaction). In a redox reaction, there are two half-reactions: a reduction half-reaction and an oxidation half-reaction. The standard reduction potential is expressed in volts (V) and is measured at standard conditions (25°C, 1 atm pressure, and 1 M concentration for all species).

Why the standard reduction potential of a single half-reaction cannot be measured

It is impossible to measure the standard reduction potential of a single half-reaction because in a redox reaction, the reduction half-reaction and the oxidation half-reaction occur simultaneously. When one species accepts electrons and undergoes reduction, another species must lose electrons and undergo oxidation. This means that the voltage or potential we measure in an electrochemical cell is a combined potential of both the reduction and oxidation reactions. The standard reduction potential of a half-reaction cannot be measured in isolation since it always depends on the other half-reaction in the overall redox reaction. In other words, there is no absolute value of the standard reduction potential for an individual species, as it is a relative quantity that is measured in reference to another reaction.

Determining the standard reduction potential of a half-reaction

To determine the standard reduction potential of a half-reaction, we use a reference electrode with a known standard reduction potential, most commonly the Standard Hydrogen Electrode (SHE). The SHE has an assigned potential of 0 V. To measure the standard reduction potential of a half-reaction, an electrochemical cell is set up with the reference electrode on one side and the half-reaction of interest on the other side. The redox potential of the cell, called the cell potential (E_cell), is measured under standard condition and is equal to the difference between the standard reduction potential of the half-reaction of interest (E) and the standard reduction potential of the reference electrode (E_ref), which is 0 V for SHE: E_cell = E - E_ref Since E_ref is 0 V for the SHE, we get: E_cell = E Thus, the cell potential is equal to the standard reduction potential of the half-reaction of interest when measured against the SHE. By measuring the cell potential, we can determine the standard reduction potential of the half-reaction.

Key concepts

Redox Reaction

A redox reaction, short for reduction-oxidation reaction, involves two processes: reduction and oxidation. In these reactions, electrons are transferred between chemical species. The species gaining electrons undergoes reduction, while the one losing electrons undergoes oxidation. This electron transfer results in changes to the oxidation states of the involved chemicals.

• Reduction: electron gain and a decrease in oxidation state.
• Oxidation: electron loss and an increase in oxidation state.

Redox reactions are fundamental in various processes, including metabolism in living organisms and energy production in batteries. An essential aspect of redox reactions is that they consist of two half-reactions, one for reduction and one for oxidation, which occur simultaneously. This simultaneous action makes it impossible to measure the standard reduction potential of a single half-reaction in a vacuum.

Electrochemical Cell

An electrochemical cell is a device that generates electrical energy from a chemical reaction or facilitates a chemical reaction through the introduction of electrical energy. These cells form the basis of batteries and include both galvanic cells and electrolytic cells.
In a galvanic cell, a spontaneous redox reaction generates electricity. It consists of two electrodes submerged in electrolyte solutions within separate half-cells. One electrode undergoes reduction (cathode), and the other undergoes oxidation (anode). The flow of electrons from one electrode to the other through an external circuit provides electrical energy.
Electrochemical cells are crucial in determining standard reduction potentials. By measuring the cell potential of an electrochemical cell, where one half-cell contains a reference electrode, we can find the standard reduction potential of an unknown reaction.

Reference Electrode

A reference electrode is a stable electrode with a known and constant potential, used in electrochemical measurements. It provides a benchmark against which the potential of a test electrode can be measured. Commonly used reference electrodes include the Standard Hydrogen Electrode (SHE), Calomel electrode, and Silver/Silver Chloride electrode.
The role of the reference electrode is vital because it helps create a standardized method for measuring potentials in electrochemical cells. When calculating the standard reduction potential of a half-reaction, we always know the potential of the reference electrode, thus allowing us to deduce the unknown potential of the other half-cell from the overall cell potential.
Having a reliable reference is crucial for consistency and accuracy in chemical analyses.

Standard Hydrogen Electrode (SHE)

The Standard Hydrogen Electrode (SHE) is the go-to reference electrode for measuring standard reduction potentials. It is assigned a potential of 0 volts at all standard conditions, acting as a fixed point from which all other potentials are measured.
The SHE consists of a platinum electrode in a hydrogen-ion (H⁺) solution, where hydrogen gas is bubbled over at a constant pressure. The fact that its potential is defined as zero means it provides a reliable standard that helps scientists and engineers determine unknown standard reduction potentials accurately.
In practice, when you compare a half-reaction against the SHE, the half-reaction's measured cell potential equals its standard reduction potential, thanks to the SHE's potential being zero. This simplicity and reliability make the SHE an indispensable tool in both research and industry.