Salt Bridges in Galvanic Cells

[shinny]

Just a simple question which I can't find the answer to in my textbook. Why is a salt bridge necessary to keep the circuit running? I know that it's to keep the solution electrically neutral, but what does electrical neutrality have to do with ensuring that the circuit continues to run?

[Mao]

the salt bridge completes the circuit.

without it, electrons would simply flow out of one cell into the other, but there's nothing to complete the circuit with, hence the reaction won't actually take place.

[shinny]

Care to elaborate? I don't do physics so I don't understand the whole concept of circuits too well. Is it that the electrons have to have a complete path to move around or something? Basically, why must there be a complete circuit?

[mark_alec]

If there is not a complete circuit, then an electrical charge will build up on the electrodes, but not actually traverse the solution. So rather than having an electrolytic cell, you would have a capacitor.

[Collin Li]

the salt bridge completes the circuit.

without it, electrons would simply flow out of one cell into the other, but there's nothing to complete the circuit with, hence the reaction won't actually take place.

I'm pretty sure that is wrong.

In the same way that the electrical symbol for a cell has a gap in between it, so does the galvanic cell. The salt bridge does not complete the circuit, per se, but it maintains a balance of charge. This is necessary to prevent polarisation. To understand this, quickly perform a thought experiment -- what would happen if I let electrons leave the anode, and didn't replace the charge? That half-cell would become positively charged very quickly, and electrons would be more attracted to the anode (electrostatic attraction), rather than to the cathode (thermodynamically driven reaction). Hence, the salt bridge is a source of cations and anions which help to merely balance the charge, which allows the reaction to continue smoothly.

[shinny]

Ah yeh, thats basically what I got from Wikipedia, since yeh, if you left the circuit, the anode would become positive and vice versa, therefore removing the potential difference that existed. Ta everyone.

[Collin Li]

The balancing of charges (as a result of the salt-bridge) fixes this. However, if you draw a diagram and map out the electron flows, anion flows and cation flows, it doesn't look anything like a "complete circuit" (nothing goes into the salt-bridge and out, like you'd expect if it were acting like a complete circuit). My understanding is that complete circuits are only needed in between the cells, but not within the cells.

It's merely to balance the charge, and not to complete the circuit.

[Mao]

mmm thats a fair point, the salt bridge doesn't allow ions to flow through it, but rather provides ions that will flow into either cell.

though I do think that definition is a bit too rigorous for the purpose of VCE Chemistry.

[cara.mel]

Mao why do you delete your old posts?

[Mao]

coblin said what i wanted to say in a more precise manner =]

[and, fundamentally I was wrong, so ]

[bec]

the salt bridge completes the circuit.

without it, electrons would simply flow out of one cell into the other, but there's nothing to complete the circuit with, hence the reaction won't actually take place.

I'm pretty sure that is wrong.

In the same way that the electrical symbol for a cell has a gap in between it, so does the galvanic cell. The salt bridge does not complete the circuit, per se, but it maintains a balance of charge. This is necessary to prevent polarisation. To understand this, quickly perform a thought experiment -- what would happen if I let electrons leave the anode, and didn't replace the charge? That half-cell would become positively charged very quickly, and electrons would be more attracted to the anode (electrostatic attraction), rather than to the cathode (thermodynamically driven reaction). Hence, the salt bridge is a source of cations and anions which help to merely balance the charge, which allows the reaction to continue smoothly.

What you're saying makes complete sense, but now I'm confused because I've been taught specifically that "the salt bridge has a dual purpose: to provide ions to balance the charge, and to complete the internal circuit." I think I saw that solution in a prac exam too...are you 100% certain that it DOESN'T complete the internal circuit?

[Mao]

the salt bridge completes the circuit.

without it, electrons would simply flow out of one cell into the other, but there's nothing to complete the circuit with, hence the reaction won't actually take place.

I'm pretty sure that is wrong.

In the same way that the electrical symbol for a cell has a gap in between it, so does the galvanic cell. The salt bridge does not complete the circuit, per se, but it maintains a balance of charge. This is necessary to prevent polarisation. To understand this, quickly perform a thought experiment -- what would happen if I let electrons leave the anode, and didn't replace the charge? That half-cell would become positively charged very quickly, and electrons would be more attracted to the anode (electrostatic attraction), rather than to the cathode (thermodynamically driven reaction). Hence, the salt bridge is a source of cations and anions which help to merely balance the charge, which allows the reaction to continue smoothly.

What you're saying makes complete sense, but now I'm confused because I've been taught specifically that "the salt bridge has a dual purpose: to provide ions to balance the charge, and to complete the internal circuit." I think I saw that solution in a prac exam too...are you 100% certain that it DOESN'T complete the internal circuit?

I believe the contention is on the technicality of calling it "completing the circuit", because it just balances the charge.
but if we think of movement of charges as electricity, then it is, in a way, "completing the circuit" (the cations, for example, is flowing in the direction opposite to electrons, but movement of positive ions in one direction is equivalent as movement of negative charges in the other)

[shinny]

So its not completing the circuit in the sense that the electricity doesn't actually flow through it to allow the charge to move in a cyclical manner, but it completes the circuit in the sense that without it, charge wouldn't be able to flow through the wire otherwise?

[ganges]

Dont worry about the exact theory behind why you need a salt bridge, you dont need to get so overly technical in worrying about why the charges build up where and stuff like that..

JUst say it connects the external circuit to the internal circuit and you should get full marks. (they are usually 1 marks anyways)

[Wizard]

My teacher said that another function of the salt bridge was that it separated the anolyte and catholyte, prevnting them from mixing. How does this work?

Thanx