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Salt Bridge function

  1. May 20, 2015 #1
    In our text book,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??)
     
  2. jcsd
  3. May 20, 2015 #2

    James Pelezo

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    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.

    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.
     
  4. May 21, 2015 #3

    Borek

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    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.

    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.

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

    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.

    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.
     
  5. May 21, 2015 #4

    James Pelezo

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    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.

    How are batteries exempt from needing/following the Galvanic process to operate? I don't understand that one.

    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.

    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'?

    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?
    upload_2015-5-21_6-7-0.png Gad!! This is fun!!
     

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    Last edited: May 21, 2015
  6. May 21, 2015 #5

    Borek

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    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:

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

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

    This suggests changes in concentration after we got to the equal concentrations point are spontaneous. They are not.
     
  7. May 21, 2015 #6

    PeterDonis

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