Problem 18 - Chemistry in 30 days

Question

The function of a salt bridge is to (a) eliminate liquid-junction potential where the ions are present in large excess at the junction (b) increase the cell potential at the positive electrode (c) decrease the cell potential at the negative electrode (d) eliminate the impurities present in the electrolyte

Step-by-Step Solution

Answer

The function of a salt bridge is (a) to eliminate liquid-junction potential.

Step 1: Understanding the Role of a Salt Bridge

A salt bridge is a device used in electrochemical cells to allow for the flow of ions and maintain electrical neutrality by preventing the build-up of charge.

Step 2: Identifying the Correct Function

The salt bridge specifically works to eliminate the liquid-junction potential by allowing ions to move freely between two half-cells, thereby balancing charges without affecting the potentials at the electrodes.

Step 3: Eliminating Incorrect Options

Options (b) and (c) mention influencing cell potential at electrodes, which is incorrect because a salt bridge does not affect electrode potentials. Option (d) suggests removing impurities, which is not a function of a salt bridge. This leaves option (a) as the only viable choice.

Key Concepts

Salt Bridge

In an electrochemical cell, the salt bridge plays a crucial role in maintaining the flow of electricity. Its primary job is to allow ions to pass between the two half-cells, ensuring that the electrical neutrality of the cell is maintained. Without the salt bridge, the solution in one half-cell would become negatively charged as it gains electrons, while the other would become positively charged as it loses electrons. This would eventually stop the flow of electrons across the electrodes, halting the cell's operation.
The salt bridge typically contains a gel with inert electrolytes, such as KCl or KNO₃, which do not react with other substances in the cell. This ensures that the salt bridge fulfills its role without interfering with the cell reactions.

• Allows ion flow
• Maintains electrical neutrality
• Prevents charge build-up

Liquid-Junction Potential

Liquid-junction potential emerges when two different electrolytic solutions come into contact. It is the voltage difference that originates at the interface between these two solutions due to the unequal mobilities of ions. This potential can influence the overall measurement of cell potential if not properly managed.

A salt bridge helps eliminate this potential by offering a pathway for ions to move freely between the two solutions, thus balancing the charge distribution. By resolving imbalances in ion concentrations, the salt bridge effectively minimizes the liquid-junction potential, ensuring accurate measurements and stable cell operation.

• Occurs at electrolyte interfaces
• Caused by ion mobility differences
• Managed by salt bridges
  

Electrode Potential

Electrode potential refers to the voltage produced at an electrode as a result of its ability to gain or lose electrons. Each half-cell in an electrochemical cell has its own electrode potential, determined by the specific reactions occurring at the electrodes.

The salt bridge does not directly affect the electrode potential; its purpose is to maintain charge balance by allowing ions to flow. Thus, it ensures that the electrochemical cell runs smoothly but doesn't alter the inherent potentials of the electrodes. The electrode potential is critical in determining the overall cell potential, as it's the sum of the individual potentials of the cathode and anode.

• Voltage at an electrode
• Unaltered by salt bridges
• Determines cell potential
  

Ions Flow

The flow of ions in an electrochemical cell is fundamental to its operation. When a redox reaction occurs, electrons move from the anode to the cathode through an external circuit, but to maintain electrical neutrality within the cell, ions must move through the electrolyte.

The salt bridge facilitates the flow of ions such as anions towards the anode and cations towards the cathode. This ion migration prevents charge build-up and allows the cell to continue functioning. Without the free flow of ions, the flow of electricity would eventually cease, and the system would become inert.

• Vital for electrical neutrality
• Supported by salt bridges
• Ensures uninterrupted operation