In the functions of a salt bridge, which of the following is incorrect?

It keeps the emf of the cell positive

The correct answer is option 3. It does not keep the emf of the cell positive.

Let us delve into the functions of a salt bridge in an electrochemical cell in detail:

Functions of a Salt Bridge:

1. It allows ions to go from one half-cell to another:

One of the primary functions of a salt bridge is to allow the migration of ions between the two half-cells of an electrochemical cell. In an electrochemical cell, oxidation occurs at the anode, where electrons are released into the external circuit, and reduction occurs at the cathode, where electrons are gained from the external circuit. To maintain electrical neutrality and complete the circuit, ions from the electrolyte in the salt bridge move into each half-cell to balance the charge as ions are consumed or produced during the redox reactions.

2. It provides an electrical connection between two half-cells:

The salt bridge physically connects the two compartments (half-cells) of an electrochemical cell. It completes the electrical circuit between the anode and cathode, allowing electrons to flow from the anode (where oxidation occurs) through the external circuit to the cathode (where reduction occurs).

3. It keeps the emf of the cell positive:

The emf of an electrochemical cell is determined by the difference in standard reduction potentials (E°) of the half-reactions occurring at the anode and cathode. The salt bridge itself does not contribute to the emf of the cell. Instead, it facilitates the proper functioning of the cell by allowing ion migration and maintaining charge balance. The emf of the cell remains positive or negative based on the standard reduction potentials of the half-reactions involved and the concentrations of the ions in each half-cell.

4. It maintains the electrical neutrality of the solution in two half cells:

As electrons flow through the external circuit from the anode to the cathode, the anode solution accumulates positive charge (due to loss of electrons) and the cathode solution accumulates negative charge (due to gain of electrons). The salt bridge allows ions (typically from a \(KCl\) or \(NH_4NO_3\) solution) to move into each half-cell to balance these charges. For example, in the anode compartment where electrons are released, cations (positive ions) move into the solution to balance the increasing negative charge. In the cathode compartment where electrons are gained, anions (negative ions) move into the solution to balance the increasing positive charge.

In summary, a salt bridge plays crucial roles in an electrochemical cell:

It allows ions to go from one half-cell to another.

It provides an electrical connection between two half-cells.

It maintains the electrical neutrality of the solution in two half cells.

However, it does not directly influence the emf (whether positive or negative) of the cell; rather, it supports the electrochemical reactions that determine the cell's emf.