Question

Which of the following will act as cathode when connected to standard hydrogen electrode which has \(E^{\circ}\) value given as zero? (i) \(Z n^{2+} / Z n, E^{\circ}=-0.76 \mathrm{~V}\) (ii) \(\mathrm{Cu}^{2+} / \mathrm{Cu}, E^{\circ}=+0.34 \mathrm{~V}\) (iii) \(\mathrm{Al}^{3+} / \mathrm{Al}, E^{\circ}=-1.66 \mathrm{~V}\) (iv) \(\mathrm{Hg}^{2+} / \mathrm{Hg}, E^{\circ}=+0.885 \mathrm{~V}\) (a) (i) and (ii) (b) (ii) and (iv) (c) (i) and (iii) (d) (i), (ii), (iii) and (iv)

Short answer

(b) (ii) Cu^2+/Cu and (iv) Hg^2+/Hg will act as cathodes when connected to standard hydrogen electrode.

Step-by-step solution

Understanding Standard Electrode Potential

The standard electrode potential (E°) is a measure of the tendency of a chemical species to be reduced, compared to the standard hydrogen electrode which has an E° value of 0 V by convention. A positive E° value indicates a greater tendency to be reduced (gain electrons) than the hydrogen electrode, which means it could act as a cathode in a cell. A negative E° value indicates a lower tendency to be reduced (thus a higher tendency to be oxidized) than the hydrogen electrode, which means it would act as an anode in a cell.

Identify the Cathode in Each Pair

In an electrochemical cell, the substance with higher reduction potential (more positive E° value) will act as the cathode because it has a greater tendency to gain electrons. We compare the E° values of each given substance to the standard hydrogen electrode (SHE) to determine which will be the cathode: (i) Zn^2+/Zn, E°=-0.76 V (ii) Cu^2+/Cu, E°=+0.34 V (iii) Al^3+/Al, E°=-1.66 V (iv) Hg^2+/Hg, E°=+0.885 V.

Determine Which Will Act as Cathode

Upon comparison, we see that options (ii) Cu^2+/Cu and (iv) Hg^2+/Hg have positive E° values, which means they have a greater tendency to be reduced than the standard hydrogen electrode. Therefore, these will act as cathodes when connected to the standard hydrogen electrode.

Key concepts

Understanding an Electrochemical Cell

An electrochemical cell is a device that generates electric current from a chemical reaction or facilitates a chemical reaction through the introduction of electrical energy. It consists of two electrodes—cathode and anode—submerged in an electrolyte, with the cathode being the site of reduction (gain of electrons) and the anode being the site of oxidation (loss of electrons).

These cells operate based on redox reactions, where one species is reduced and another is oxidized. The external circuit connects the two electrodes, allowing electrons to flow from the anode to the cathode, generating an electric current. Importantly, the salt bridge or porous partition allows ions to move between the two half-cells to maintain charge balance yet prevents the direct mixing of the different electrolytes.

Electrochemical cells can be classified in two main types: galvanic (or voltaic) cells, which produce electrical energy from spontaneous chemical reactions, and electrolytic cells, which consume electrical energy to drive non-spontaneous reactions.

Determining the Cathode in an Electrochemical Reaction

Cathode determination in an electrochemical cell is critical for understanding the flow of electrons and the direction of the chemical reaction. The cathode is the electrode where reduction occurs, meaning it is the site where electrons are gained by the chemical species. During this process, the oxidizing agent is reduced.

To identify the cathode, compare the reduction potentials of the species involved. The substance with the more positive electrode potential relative to the standard hydrogen electrode (which is set at 0 volts) will act as the cathode. In essence, this substance has a higher tendency to attract and gain electrons.

For instance, in a cell comprising zinc and copper half-cells, copper has a higher reduction potential (+0.34 V) than zinc (-0.76 V). Thus, the copper electrode would serve as the cathode, as it is more likely to undergo reduction compared to zinc, which will serve as the anode.

The Role of Reduction Potential in Electrochemistry

Reduction potential, often represented as E°, is a measure of the propensity of a chemical species to acquire electrons and thus be reduced. It is measured in volts (V) and indicates a substance's ability to serve as an oxidizing agent—the higher the reduction potential, the stronger the oxidizing agent.

A positive standard electrode potential denotes that the substance has a greater tendency to be reduced than the standard hydrogen electrode, whereas a negative potential suggests a lower tendency to gain electrons. For example, a species with E° of +0.8 V can be expected to be a good oxidizing agent and likely to act as a cathode.

In context, a Zn^2+/Zn half-cell with E° = -0.76 V is less likely to gain electrons than the standard hydrogen electrode, while Cu^2+/Cu with E° = +0.34 V is more likely to be reduced. Hence, the half-cell with the higher (more positive) reduction potential is the preferred site for reduction, indicating its role as the cathode in the electrochemical pair.