Salt Bridge: Role in Half Cells Voltage

In summary, the conversation discusses the role of the salt bridge in a voltaic cell and the impact of removing it. It is explained that removing the salt bridge will instantly cut the circuit and stop all chemical reactions. The conversation also touches on the concept of standard conditions and how the concentration of cations in the anode and cathode cells will eventually become equal, causing the battery to stop working. There is also a cautionary note about discarding dead batteries and the potential for them to ignite a fire if the electrodes come into contact. The differences between concentration cells and batteries are also briefly mentioned.
  • #1
AnkurGarg
18
0
In our textbook,it is written that-Salt bridge connects the solutions in 2 half cells and complete the circuit.If it is removed from a cell,then the voltage of the cell drops to zero.

Doubt-What I think should come is-if salt bridge is removed,the positive ions accumulated in anodic half solution ,will not allow more electrons to move towards the the cathode and thus cell will stop working after some time without its voltage being reduced to zero ...As it happens in a normal cell(If i am right ,In a normal cell with a salt bridge,electrons move till electrode potential in both the half cells become equal..)

(Since here electrons won't move the potential difference in 2 half cells will always exist??)
 
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  • #2
AnkurGarg said:
thus cell will stop working after some time without its voltage being reduced to zero
If you remove the salt bridge, the circuit of the voltaic cell will be cut and the voltage will go to zero instantly. Just as if you cut one of the metallic wire leads connecting the electrodes. Oxidation and reduction reactions will cease. In other words, the cation concentration in the anode cell's electrolyte solution will cease to increase as will the cation concentration in the cathode's cell electrolyte solution will cease to decrease. Opening or cutting the circuit by removing the salt bridge stops all Galvanic chemical reactions instantly. There will be no 'stop working after some time', the cell will cease to operate the instant the circuit is cut. No leakage, no dribbling, no draining, no nothing.

AnkurGarg said:
In a normal cell with a salt bridge,electrons move till electrode potential in both the half cells become equal..)
Only partially true... When a battery/Galvanic system is designed, one of the principle objectives of assembly is to start with a very low concentration of electrolyte in the anode cell and a very high concentration of electrolyte in the cathodic cell. As the cell discharges, the concentration of cations in the anode cell solution increase while at the same time the cation concentration in the cathodic cell solution decrease. This means that cell voltage during discharge decreases as a function of changing cation concentrations in the two electrode cells. At some point in time, the concentration of cations in the anode side will equal the concentration of cations in the cathodic side of the cell. This is usually referred to as as 'Standard Cell Conditions'. However, flow of charge will continue with the anode cations continuing to increase and the cathodic ions continuing to decrease. This will continue until all of the anode's metallic electrode dissolves into solution (or reaches a galvanic limit depending on the composition of the anodic cell) and then the battery will be dead.

Also, and as a good-will/safety FYI, always tape over the electrodes of a dead batteries to be discarded with a non-conducting tape. Electrical tape, masking tape, duct tape, etc. so that the electrodes will not accidentally come into contact with a conducting metal. Especially if several dead batteries are being discarded together in a waste sack or container. There have been reports of electrodes of discarded batteries coming into contact and creating sufficient heat to ignite a fire. Considerable damage has resulted.
 
  • #3
Actually no, just because you remove the salt bridge the potential difference between half cells doesn't drop to zero. However, once you start to draw a current, charges build almost immediately lowering the potential difference to zero. It can be even difficult to observe that initial non zero potential, as measuring the potential difference with a voltmeter is nothing else but measuring the current over a high resistance (this is why good voltmeters have a very high resistance - the higher, the better). So while from practical point of view potential drops immediately, it is not a true in general.

James Pelezo said:
When a battery/Galvanic system is designed, one of the principle objectives of assembly is to start with a very low concentration of electrolyte in the anode cell and a very high concentration of electrolyte in the cathodic cell.

Your wording is confusing, you probably mean concentration batteries, but what you wrote suggests these are rules for any batteries. Nope.

Also, be very careful with the word 'electrolyte' in this context, as what you wrote can be confusing. I guess you mean 'substances reacting in the half cells', but the solution can also contain an inert electrolyte that is there just to lower the resistance - and not only its concentration is high, it doesn't change during the battery work.

At some point in time, the concentration of cations in the anode side will equal the concentration of cations in the cathodic side of the cell.

Again - be careful with wording. Cations of what? Are we talking about concentration cells, or about batteries in general?

This is usually referred to as as 'Standard Cell Conditions'.

I have never heard the term, and quick googling doesn't show it as commonly used. The only similar and commonly used term that I can think of is 'standard conditions' - but it doesn't mean 'identical concentration of cations' but 'activities of all substances involved equal to 1'. Technically sometimes it is enough that concentrations of some substances involved are identical, as their activities will then cancel out, but this is not exactly the same.

However, flow of charge will continue with the anode cations continuing to increase and the cathodic ions continuing to decrease. This will continue until all of the anode's metallic electrode dissolves into solution (or reaches a galvanic limit depending on the composition of the anodic cell) and then the battery will be dead.

No. Concentration batteries are powered by the concentration gradient. Once the concentration in both cells becomes equal, potential difference drops to zero and battery stops to work, no matter what the size of the metallic electrode.
 
  • #4
Borek said:
Are we talking about concentration cells, or about batteries in general?
As far as the chemistry, what's the difference;i.e., aren't oxidation-reduction reactions required to generate a current flow. Sure the assembly of a concentration cell and battery are in different configurations, but the chemistry and the need for separation of electrodes is basically the same.

Borek said:
suggests these are rules for any batteries. Nope.
How are batteries exempt from needing/following the Galvanic process to operate? I don't understand that one.

Borek said:
Are we talking about concentration cells, or about batteries in general?
Both. The degradation of cell voltage (concentration cell or battery) depends on the concentration of electrolytes at specified time in the life of the cell or battery. Such is predicted by the Nernst Equation for electrogalvanic systems.

Borek said:
'standard conditions' - but it doesn't mean 'identical concentration of cations'
OK, to rephrase, Standard Galvanic Cell is one in which the concentration of reducing agent and concentration of oxidizing agent such that from the Nernst Equation, EMFnon-std = EMF(Std) - (0.0592/n)log([reducing agent/oxidizing agent]) the ratio of reducing agent to oxidizing agent reduces to 1 and the voltage adjustment factor becomes zero and non-standard EMF = standard EMF. Most references, that I've reviewed, say that 'Standard Cell Conditions' are 25oC, 1-Atm, and concentrations of electrolytes are 1.00 Molar each. Such is the condition implied (and, sometimes written) on tables of standard electrode potentials. When a voltage value is calculated using a Reduction Potential Table, Isn't the value determined called 'Standard Cell Potential'?

Borek said:
No. Concentration batteries are powered by the concentration gradient. Once the concentration in both cells becomes equal, potential difference drops to zero and battery stops to work, no matter what the size of the metallic electrode.

I agree, the driving force is the concentration gradient ratio between oxidizing agent and reducing agent. However, according to the Nernst Equation, Voltage continues to change (decrease) below the point where Reducing Agent [RA] = Oxidizing Agent [OA]. Forgive me, but are we talking about different issues in the Galvanic Process?
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Gad! This is fun!
 

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  • #5
James, I have no time now for a long discussion. Problem with your posts is that you use lousy language - quite often what you write is ambiguous and can be confusing to people that don't know enough to understand you relate to a specific case, yet you present it as if you were talking about a general case.

One simple example: yes, if concentrations of all ions are 1 M they are equal to each other. But what you wrote earlier:

James Pelezo said:
the concentration of cations in the anode side will equal the concentration of cations in the cathodic side of the cell

doesn't imply all concentrations equal to 1 M, and is misleading.

James Pelezo said:
However, according to the Nernst Equation, Voltage continues to change (decrease) below the point where Reducing Agent [RA] = Oxidizing Agent [OA].

Again, problem is with misleading statement for your earlier post:

James Pelezo said:
However, flow of charge will continue with the anode cations continuing to increase and the cathodic ions continuing to decrease. This will continue until all of the anode's metallic electrode dissolves into solution (or reaches a galvanic limit depending on the composition of the anodic cell) and then the battery will be dead.

This suggests changes in concentration after we got to the equal concentrations point are spontaneous. They are not.
 
  • #6
Thread closed for moderation.
 

Related to Salt Bridge: Role in Half Cells Voltage

1. What is a salt bridge and why is it important in half cells voltage?

A salt bridge is a connection between two half cells in an electrochemical cell that allows for the flow of ions. It is important in half cells voltage because it helps maintain electrical neutrality and allows for the completion of the circuit.

2. How does a salt bridge affect the overall voltage of a half cell?

A salt bridge does not directly affect the overall voltage of a half cell. However, it plays a crucial role in maintaining a constant voltage by allowing the movement of ions between the two half cells, which balances out any buildup of charge and prevents the cell from reaching equilibrium too quickly.

3. What materials are commonly used to make a salt bridge?

Typically, a salt bridge is made of an inert material such as glass or ceramic, soaked in an electrolyte solution. Common electrolyte solutions used include potassium chloride, sodium chloride, and potassium nitrate.

4. How does the length of a salt bridge affect its performance?

The length of a salt bridge does not significantly impact its performance. As long as the bridge is long enough to connect the two half cells and allow for the movement of ions, it will function effectively. However, a shorter salt bridge may have a faster diffusion rate of ions, resulting in a more efficient cell.

5. Can a salt bridge be reused in different electrochemical cell reactions?

Yes, a salt bridge can be reused in different electrochemical cell reactions as long as it is thoroughly rinsed with the appropriate electrolyte solution between uses. This ensures that the ions from the previous reaction do not contaminate the new reaction and affect its results.

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