Galvanic Cells and Salt Bridges

[patrickbotros]

I went to class and watched this video () but I still don't understand how salt bridges work. I think I memorized all of the details and steps but I don't understand why anything is happening at all! I know that Cu²⁺ gains two electrons when it touches the electrode but why? And also does the electric current cause Cu²⁺ to go to Cu, or is it the other way around? I think the reason for my confusion is I don't get what's happening in the water in each cell. Any help would be appreciated. Also: is the salt bridge made out of the ions or does it just facilitate the motion of the ions? So is the salt bridge made out of KNO₃ and KCl and NaCl or is it made out of some material that has like capillary action or something to move it?

 

[Borek]

Salt bridge is just a solution containing ions which can move - we try to made the salt bridge in such a way it is difficult for the solution inside to mix with the solutions in cells, but that's just a technical detail.

Have you ever seen what happens when you put a piece of zinc in the solution of Cu²⁺? Copper gets reduced and covers the zinc surface. This is a redox process, electrons pass from zinc to copper. Galvanic battery does exactly the same, we just do some elaborate tricks to separate the zinc oxidation and copper reduction so that each happens in different place, and the electrons required for the reaction to happen flow through an external circuit. Part of the circuit is the wire connecting electrodes, other part is the salt bridge (through which ions flow).

 

[James Pelezo]

A salt bridge is responsible for countering (balancing) changes in cation and anion concentrations in each half-cell reaction in a Galvanic/Voltaic Cell. In a Zinc/Copper Galvanic/Voltaic cell, Zinc is the anode (- chg) undergoing oxidation delivering Zn⁺² ions into solution thus increasing the concentration of cations in solution while, at the same time Copper Ions in the cathode (+) side of the cell (Cu⁺²) in solution are undergoing reduction to Cu^o(s) in basic standard state. The increase in zinc ions must be countered with the negative ion of the salt bridge which migrates toward the anode and neutralizes the gain of positive charge. The reduction of copper ions to copper metal is reducing the positive charge in solution on the cathodic side of the Galvanic Cell and the cation of the salt bridge migrates toward the cathode to counter the loss of positive charge. Such maintains a balance of charge during the discharge of the Galvanic Cell.

It is helpful to note that the processes are often referred to as uncontrolled vs controlled Galvanic process. By separating the oxidation process (Zn-side) from the reduction process (Cu-side) and connecting with a salt bridge, the movement of current can be sustained for as long as the anode of the cell (oxidation side/Zn => Zn⁺² + 2e^- ) remains present. Without the salt bridge, and inserting a Zn-bar into a solution of Cu⁺² ions, the Galvanic process would take place, but continue only until the zinc bar was so coated with reduced copper that other copper ions could not interface/come into contact with the zinc and the process would stop. Such is an uncontrolled Galvanic process.

When designing a Galvanic Cell, the ion concentration in the anode side (Zn⁺²) should be very low and the ion concentration in the cathodic side (Cu⁺²) should be very high. On closing the circuit by connecting the electrodes with metallic conductors ( e.g., copper, silver or gold wire ) the oxidation & reduction processes proceed to increase the anode's cation ion concentration attracting negative salt bridge ions (anions) to that side of the cell while the cations of the salt bridge are attracted to the cathodic side of the cell as the copper ion concentration decreases. Such is the discharge of a Galvanic Cell from maximum voltage of the cell to zero (dead battery) when the anode is completely dissolved.

A common application is 'Cathodic Protection' of in-ground metal piping. This means using/connecting a more active metal that undergoes oxidation more readily than the metals in the pipe thus reducing the potential degradation of expensive pipelines. Here's a complete lecture given on the subject ... go to ~25.45 on on the video ... Function of Salt Bridge. Hope it helps.

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