Problem 22 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

Saturated solution of \(\mathrm{KNO}_{3}\) is used to make 'salt bridge' because (a) velocity of \(\mathrm{K}^{+}\) is greater than that of \(\mathrm{NO}_{3}^{-}\) (b) velocity of \(\mathrm{NO}_{3}^{-}\) is greater than that of \(\mathrm{K}^{+}\) (c) velocities of both \(\mathrm{K}^{+}\) and \(\mathrm{NO}_{3}^{-}\) are nearly the same (d) \(\mathrm{KNO}_{3}\) is highly soluble in water

Step-by-Step Solution

Answer

The best choice for the 'salt bridge' is (c) velocities of both \text{K}^{+} and \text{NO}_{3}^{-} are nearly the same, because it allows for equal ion migration which maintains charge balance in the electrochemical cell.

Step 1: Understanding the Purpose of a Salt Bridge

A salt bridge is a device used in an electrochemical cell to maintain electrical neutrality within the internal circuit. It allows the movement of ions to compensate the charge buildup due to the redox reaction occurring at the electrodes.

Step 2: Analyzing the Options Given

Option (a) and (b) suggest different velocities for the ions which could lead to the faster migration of one over the other, creating a charge imbalance. Option (d) gives information about solubility but does not pertain to why it would be suitable for a salt bridge. Option (c) suggests that the cation and anion have similar velocities, which could help maintain charge balance.

Step 3: Understanding Ion Migration in the Salt Bridge

If the cation and anion of a salt have similar velocities, they will migrate at nearly the same rate. This helps in maintaining charge balance across the salt bridge by preventing a charge buildup.

Step 4: Selecting the Correct Option

Since a salt bridge is ideally used to maintain neutrality without favoring the migration of one ion over the other, the correct option would be the one that suggests both ions have similar velocities.

Key Concepts

Ion Migration

Understanding ion migration is fundamental in the study of electrochemical cells and is particularly crucial when it comes to salt bridges. Ion migration refers to the movement of ions through a solution, which occurs in response to an electric potential difference. In the context of an electrochemical cell, this process is essential for maintaining an ongoing flow of charge that is necessary for the cell's operation.

In a salt bridge, ions migrate to neutralize the charges that build up at the electrodes during the cell's redox reactions. The purpose of having a salt like KNO3 is to ensure that the migration rates of both K+ and NO3− ions are similar. If one ion were to migrate significantly faster than the other, this would lead to a charge imbalance and disrupt the cell's function. Therefore, when choosing a salt for the salt bridge, the equal or nearly equal velocities of its ions guarantee a smooth operation of the cell by maintaining a stable internal environment and preventing potential interruption of the redox reactions.

Electrochemical Cell Neutrality

The principle of electrochemical cell neutrality states that the total charge within an electrochemical cell must remain balanced. This balance is crucial because any discrepancy in charge could result in inefficiencies or termination of the electrochemical reaction. The salt bridge contributes to maintaining neutrality by allowing ions to flow from one side of the cell to the other, ensuring that the cell operates continuously and efficiently.

For example, during a redox reaction, electrons are transferred from the anode to the cathode, often through an external circuit. This electron flow could create an excess of positive charge at the anode and an excess of negative charge at the cathode if not properly managed. A salt bridge prevents this by permitting the flow of positive ions towards the cathode and negative ions towards the anode. The salt bridge, in essence, 'completes the circuit' internally, helping to sustain the redox reaction by neutralizing the separate charges that would otherwise build up at each electrode.

Redox Reaction

A redox reaction is a chemical reaction involving the transfer of electrons between two species; it is a combination of two processes: reduction and oxidation. Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons. In the setting of an electrochemical cell, redox reactions are the core events that generate electrical energy.

During these reactions, electrons move from the reducing agent to the oxidizing agent, traveling through an external circuit and thus creating an electric current. The redox reactions at the individual electrodes of an electrochemical cell result in a flow of electrons and, necessarily, a corresponding migration of ions in the solution to balance the charge. The role of the salt bridge is to provide a pathway for this ion migration, which maintains the overall electrical neutrality of the cell and permits the continuous flow of electrons, or what is commonly known as electricity. Without a salt bridge or a similar mechanism, the charge imbalance would soon halt the redox reactions, stopping the production of electrical energy, thus illustrating the interdependence of the three core concepts: ion migration, electrochemical cell neutrality, and redox reactions.