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 both oxidation and reduction must occur simultaneously in a redox reaction, requiring a flow of electrons from an oxidation half-reaction to the reduction half-reaction to generate a measurable voltage. (b) The standard reduction potential of a half-reaction can be determined by pairing it with a reference electrode (typically the Standard Hydrogen Electrode) in an electrochemical cell and measuring the cell potential, which is the difference between the standard reduction potential of the half-reaction and the reference electrode. Since the reference electrode has a known standard potential, the standard reduction potential of the half-reaction can be calculated from the measured cell potential.

Step-by-step solution

(a) Understanding standard reduction potential and single half-reaction

Standard reduction potential is the tendency of a chemical species to either accept or lose electrons in a redox reaction. It is measured as a positive or negative voltage, with a higher positive value indicating a stronger tendency to be reduced. A half-reaction is a chemical equation that represents only reduction or oxidation, not both, happening in a redox reaction. The standard reduction potential cannot be measured for a single half-reaction because both oxidation and reduction must occur simultaneously in a redox reaction. Since a half-reaction consists only one part of the whole reaction, it is impossible to determine the reduction potential alone, as it requires a flow of electrons from an oxidation half-reaction to the reduction half-reaction to generate a measurable voltage.

(b) Determining the standard reduction potential of a half-reaction

The standard reduction potential of a half-reaction can be determined via an electrochemical cell. This process involves two steps: 1. **Pairing the half-reaction with a reference electrode:** The first step is to pair the half-reaction with a reference electrode, typically the Standard Hydrogen Electrode (SHE), which has a defined standard potential of 0 V. This allows the potential of the unknown half-reaction to be determined relative to the reference electrode. 2. **Measuring the cell potential:** The electrochemical cell is constructed with the half-reaction (as one electrode) and the reference electrode in separate compartments, connected by an electrolyte bridge and an external wire. When the cell is functioning, a voltage, known as the cell potential (E_cell), is generated. The cell potential is the difference between the standard reduction potential of the half-reaction (E) and the reference electrode (E_ref). The equation for calculating the cell potential is given by: \[E_{cell} = E - E_{ref}\] Since the standard reduction potential of the reference electrode (SHE) is known to be 0 V, the standard reduction potential of the half-reaction (E) can be determined by measuring the cell potential (E_cell): \[E = E_{cell}\] Thus, by using the electrochemical cell and measuring the cell potential, the standard reduction potential of a half-reaction can be determined.