Question

What is the function of a salt bridge? What kind of electrolyte should be used in a salt bridge?

Short answer

The salt bridge maintains electrical neutrality. Use an inert electrolyte like KNO₃ or Na₂SO₄.

Step-by-step solution

Understanding the Salt Bridge Function

A salt bridge is a key component in a galvanic (or voltaic) cell that allows for the movement of ions between two half-cells. This maintains the electrical neutrality in the internal circuit. Without a salt bridge, the flow of electrons stops as charges accumulate, preventing the redox reaction from continuing.

Selection of Electrolyte Type

The electrolyte in a salt bridge must be inert, meaning it should not react with the electrodes or the solutions in the two half-cells. Common electrolytes include potassium nitrate (KNO₃) or sodium sulfate (Na₂SO₄), as these substances provide anions and cations that do not participate in secondary reactions.

Key concepts

Galvanic Cell

A galvanic cell, also known as a voltaic cell, is a device that converts chemical energy into electrical energy. It consists of two half-cells, where redox reactions take place. Each half-cell contains an electrode submerged in an electrolyte solution.
The two half-cells are connected by a wire through which electrons flow and by a salt bridge that maintains the charge balance.
In one half-cell, oxidation occurs, releasing electrons, while in the other, reduction occurs, accepting electrons. This flow of electrons through the external wire is what generates electrical current.
Key components to remember about galvanic cells include:

• It requires two different metal electrodes, each in a separate solution.
• It utilizes redox reactions to generate electricity.
• It relies on a salt bridge to allow ion movement.

Electrolyte Selection

Choosing the right electrolyte for a salt bridge is essential for the proper functioning of a galvanic cell.
Electrolytes used in salt bridges should be inert, implying that they do not participate in any side reactions that might interfere with the cell's primary reactions.
Good examples of such electrolytes are potassium nitrate (KNO₃) and sodium sulfate (Na₂SO₄). These electrolytes provide ions that are necessary for conducting electricity without altering the chemical reactions occurring at the electrodes.
The electrolyte also needs to be soluble and capable of dissociating into ions efficiently. This ensures that ions can move freely between the two half-cells to neutralize charge imbalances.
When selecting an electrolyte, it is crucial to ensure it does not react with:

• The electrode materials
• The solutions present in the half-cells

Ion Movement

Ion movement is a fundamental aspect of the functioning of a galvanic cell. Without the movement of ions, the electrical circuit would not be complete, stopping the flow of electrons and thus, halting the generation of electricity.
The salt bridge facilitates this ion movement by allowing ions to pass between the two half-cells, balancing out any charge buildup.
In a galvanic cell, positive ions migrate toward the cathode (the site of reduction), while negative ions move toward the anode (the site of oxidation). This migration neutralizes charges that build up as the redox reactions proceed.
It's important to understand that the flow of ions through the salt bridge is crucial for:

• Maintaining charge neutrality
• Continuing the flow of electrons in the external circuit
• Preventing charge imbalance that would stop the reaction

Electrical Neutrality

Electrical neutrality in a galvanic cell is vital for the sustained flow of electrons and the proper function of the device.
As the redox reaction occurs, electrons travel from the anode to the cathode through the external circuit, potentially creating charge imbalances.
Accumulation of negative charge in the anode compartment and positive charge in the cathode compartment can stop the reaction.
The salt bridge comes into play by allowing ions to move between the compartments.
This movement helps counterbalance the charges:

• Anions from the salt bridge migrate toward the anode, neutralizing excess positive charge.
• Cations move toward the cathode, neutralizing excess negative charge.

Without maintaining electrical neutrality, the cell would soon cease to function as the electron flow would stop, cutting off the electrical current.