Question

Saturated solution of \(\mathrm{KNO}_{3}\) is used to make a 'salt bridge' because (a) velocity of \(\mathrm{K}^{+}\) is greater than that of \(\mathrm{NO}^{-}{ }_{3}\) (b) velocity of \(\mathrm{NO}_{3}^{-}{ }_{3}\) is greater than that of \(\mathrm{K}^{+}\) (c) velocities of \(\mathrm{K}^{+}\) and \(\mathrm{NO}^{-}{ }_{3}\) both are nearly the same (d) none of the above

Short answer

The answer is (c). Velocities of \(\mathrm{K}^{+}\) and \(\mathrm{NO}^{-}{ }_{3}\) both are nearly the same. This ensures the charge neutrality necessary for the salt bridge in electrochemical cells.

Step-by-step solution

Understanding Salt Bridges

Salt bridges are used in electrochemical cells to maintain charge neutrality. If the solution becomes charged, it will stop the reaction. \(\mathrm{KNO}_{3}\) is used because it dissociates into \(\mathrm{K}^{+}\) and \(\mathrm{NO}_{3}^{-}\).

Comparing Ion Velocities

The solution maintains neutrality by allowing the positive \(\mathrm{K}^{+}\) and negative \(\mathrm{NO}_{3}^{-}\) ions to move. If one ion moved considerably faster than the other, it would break the neutrality faster.

Picking the Correct Answer

Given our understanding of salt bridges and the importance of neutrality, we can eliminate options (a) and (b). The velocities of \(\mathrm{K}^{+}\) and \(\mathrm{NO}_{3}^{-}\) should be nearly the same to maintain balance. Therefore, option (c) is the correct answer.

Key concepts

Electrochemical Cells

Electrochemical cells are the fundamental units of batteries and various types of power-generating devices. In essence, these cells convert chemical energy into electrical energy through redox reactions. They consist of two electrodes, the anode and the cathode, which are connected by an external circuit and immersed in an electrolyte solution. As redox reactions occur, electrons flow from the anode to the cathode through the external circuit, providing electrical current.

The role of the salt bridge is crucial in maintaining the function of an electrochemical cell. It connects the two separate half-cells of the electrolyte and allows for the flow of ions to balance the charges that evolve due to the redox reactions. If the electrical neutrality is not maintained, the flow of electrons would halt, stopping the electric current. Thus, to keep the cell working properly, salt bridges use a salt solution that facilitates the mobility of ions while maintaining overall charge balance.

Ion Mobility

Ion mobility is a key factor that enables the ions to move through the electrolyte and the salt bridge in an electrochemical cell. This mobility refers to the ability of an ion to travel through a medium in response to an electric field. It is influenced by several factors, including the charge and size of the ion, as well as the viscosity of the medium through which the ion is moving.

In the context of salt bridges, ion mobility should ideally be similar for both the anion and cation to prevent the build-up of charge in any part of the cell. If one type of ion were to move significantly faster than the other, it would lead to a rapid accumulation of charge, which would eventually halt the cell's operation. Essentially, the mobility of the ions in the salt bridge governs how effectively the cell can continue its redox processes without interruption.

Electrical Neutrality

Electrical neutrality in an electrochemical system is the state where the positive and negative charges are balanced, leading to no net charge in any part of the system. This balance is critical for the ongoing chemical reactions that occur in the cell and is achieved by the movement of ions through the salt bridge.

The salt bridge acts as a gateway, allowing for the exchange of ions between the two half-cells to counteract the charge separation caused by the electron transfer. For a cell to function efficiently, it is essential that the salt bridge be composed of a salt like KNO3, which dissociates into ions K+ and NO3- that move with similar mobilities. By maintaining electrical neutrality, the salt bridge prevents the build-up of charge that would otherwise disrupt the flow of electrons and the overall reaction in the cell.

Dissociation of KNO3

The dissociation of KNO3 (potassium nitrate) is a crucial process when it comes to its use in salt bridges of electrochemical cells. When KNO3 dissolves in water, it separates into its constituent ions: positively charged potassium ions (K+) and negatively charged nitrate ions (NO3-). This process is vital for the formation of a functioning salt bridge.

The ability of KNO3 to dissociate completely and produce ions that have similar velocities ensures that one type of charge doesn't accumulate more rapidly than the other in either half-cell. This uniform movement is what maintains the electrical neutrality required for seamless electron flow within the electrochemical cell. In conclusion, the dissociation of KNO3 plays an indispensable role in supporting the cell's internal balance and overall efficiency.