Sciencemadness Discussion Board

galvanic cell

aeacfm - 26-9-2010 at 02:18

simply , why the two electrolyes in the galvanic cell must be separated by slat bridge or diaphragm or other devices ?
or why must there be two solutions not one , cant we produce electrical energy using one solution ?

unionised - 26-9-2010 at 04:24

"cant we produce electrical energy using one solution ? "!
We can.
For example
http://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery

bquirky - 26-9-2010 at 04:45

the two solution example is just used to keep the termodynamisists happy. becuse it can be in theory mostly reversed and more redaly relates to the two half reactions chemists use to descirbe electrolytic reactions.


an electrical current can be produced by any two different metals in an ionic conductor (electrolyte)

the catch with that is you start getting gass escaping as a by product of the raction. which messesup the ability of the metal plates to contact the electrolyte . This is called polorisation

Practical batterys do away with both of these situations by using chemicals that react with the gasses otherwise produced in a way non detrimental to the operation of the cell.

An example of this is Manganise dioxide used in alkaline cells to react with the hydrogen liberated by the oxidisation of the zinc


There is a allmost limitless number of reactions that chould be used to create an electrical current in a cell. it comes down to an engeerneering and materials problem to create a pratical cell.

Many a PHD Thisis has been writen on the subject :)

aeacfm - 26-9-2010 at 23:36

Quote: Originally posted by bquirky  
the two solution example is just used to keep the termodynamisists happy. becuse it can be in theory mostly reversed and more redaly relates to the two half reactions chemists use to descirbe electrolytic reactions.


an electrical current can be produced by any two different metals in an ionic conductor (electrolyte)

the catch with that is you start getting gass escaping as a by product of the raction. which messesup the ability of the metal plates to contact the electrolyte . This is called polorisation

Practical batterys do away with both of these situations by using chemicals that react with the gasses otherwise produced in a way non detrimental to the operation of the cell.

An example of this is Manganise dioxide used in alkaline cells to react with the hydrogen liberated by the oxidisation of the zinc


There is a allmost limitless number of reactions that chould be used to create an electrical current in a cell. it comes down to an engeerneering and materials problem to create a pratical cell.

Many a PHD Thisis has been writen on the subject :)

many thanks for your reply
but i am abit confused could you please explain more , or you can gice me link about that

psychokinetic - 27-9-2010 at 01:02

You can, but it makes theory difficult.

Keeping them separated means you can keep track of what's going where, and what it is doing.

As was said before, it becomes electrolysis - ions get separated and gases escape. If you put both half cells into the same solution, you've now got a positive and negative electrode sitting in a solution full of negative ions. What's to stop the 'wrong' positive ions buggering off to the anode?

[Edited on 27-9-2010 by psychokinetic]

aeacfm - 27-9-2010 at 03:00

Quote: Originally posted by psychokinetic  
You can, but it makes theory difficult.

Keeping them separated means you can keep track of what's going where, and what it is doing.

As was said before, it becomes electrolysis - ions get separated and gases escape. If you put both half cells into the same solution, you've now got a positive and negative electrode sitting in a solution full of negative ions. What's to stop the 'wrong' positive ions buggering off to the anode?

[Edited on 27-9-2010 by psychokinetic]

i am sorry i still can't get it

chief - 27-9-2010 at 06:17

I lately wondered about the feasibility of an molten NaOH-cell with molten Al as electrode ...
==> woud need to be fired up once ...

Al has a high energy-density ... ; such a cell, when run under enough power ... might just sustain the heat ... and deliver e- in the kW-range ... ?

12AX7 - 27-9-2010 at 07:12

NaOH reacts by the spontaneous reaction
6 NaOH + 4 Al = 2 Al2O3 +3 H2(g) + 6 Na(g)

Sodium sulfur cell is better, at the expense of preheat and relatively high resistance.

Tim

chief - 27-9-2010 at 08:08

NaOH reacts with Al when in water ...

Are you sure that Al and molten NaOH, without any water, would react too ?
==> Why doesn't everyone make Na this way then ... ?

I would have bet that Al would react ... with oxygene etc. ... and the NaOH might serve as electrolyte ... with _some_ capability of reacting the Al ...
==> The molten NaOH would have a low resistivity ... allowing for high power from even smaller cells ... ; the heat of reaction would sustain the temperature ...

12AX7 - 27-9-2010 at 08:52

NaCl would be a reasonable inert, ionic solvent; Na3AlF6 better, since Al2O3 is soluble and therefore the reaction could potentially be reversible. A cathode reaction is required, of course.

Hot cells are a very bad idea. They have to be well insulated to last any amount of time, and the available power is necessarily quite low, otherwise they overheat from power loss. You think Li Ion leakage is bad?...

Tim

chief - 27-9-2010 at 09:04

NaCl melts at too high a temperature ...
==> with NaOH stainless steel could be used as vessel ... for quite a while ...

Also the NaOH-melt could recycle itself ... by just reacting with more air and maybe some H2O (in case this would get lost ) ...
==> Aluminum- oxide/hydroxide etc. would be produced ...

Out of the possibilities therefore NaOH would be the best way, _IF_ feasible ... ; also it is not too exotic ...

[Edited on 27-9-2010 by chief]

froot - 27-9-2010 at 09:46

Well if there's a brainstorm going on, then why not....

If we're going to toil with problem riddled cell designs we might as well go for gold. How about making a cell using the opposite ends of the list of electrode potentials... Li and F - or how close can we get to that?

Maybe the r/t liquid Na/K amalgum electrode behind a proton exchange membrane with a flourine accepting electrode of some sort. I don't think this is impossible.

So then, ok great say by some wonderful feat of improvisation we got that right, now how do we make it secondary cell?

chief - 27-9-2010 at 10:58

Who needs a secondary cell ?
Aluminum has a lot of electrical energy stored in it just by the fabrication: I goes straight for the 3+=state upon oxidation and weights only a few g/mol ... : This gives it a high energy-density ... (at its place in the scale of voltages ...)

Also it is not as dangerous as Fluorine or Li or whatever ...
==> So a good everyday-cell could run on Al, just one-way ... ; Electrical Energy could be shipped as Aluminum-Plates or bars or whatever ...

NaOH as electrolyte is probably the most fundamental thing thinkable of ... (for a stationary cell) : Almost nothing is more straight from the viewpoint of manufacture ...

Only question still is: How about the combination of _molten_ NaOH & Al ...

aeacfm - 28-9-2010 at 04:15

hey guys
every body here want to prove some thing to him self and forgot what i was asking for ??????????????????????????

please explain to me according to what i asked

i asked why we separate the two compartments of the galvanic cell ? some body said to make thermodynamists happy and to know what's going there , and i cant any thing from that .

also i the question in other words about if we can produce emf from two electrodes in one solution ? and the whole people talked about the molten cell
please know that i am a beginer in chemistry and my level may be not like you so can any body help or...................

[Edited on 28-9-2010 by aeacfm]

chief - 28-9-2010 at 05:14

Because it's more easily explainable to schoolboys & girls that way ...
==? It's the theoretic simplicity of the setup ...

=================

Anyhow I reflected upon the "spontaneous reaction" of NaOH and Al
==> The NaOH is capable of partly dissolving the Al2O3 oxide-layer of the Al ... which accounts for the relative inertness of the material ...

By affecting this oxide-layer ... it becomes possible for the Al to react with the H2O of the NaOH-solution ...
==> So Al itself might well react not-so-spontaniously with water-free molten NaOH ...

aeacfm - 28-9-2010 at 06:21

ok make it more simple as i see that either you cant under stand my condition or i asked about strange thing

now , consider two strips one of Zn , the other of Cu imersed in any solution you like (ZnSO4, CuSO4 , Water, acid ,.... any one of them which you like ) and i connected the two strips with a wire and but a galvanometer ....

now will the galvanometer give any reading ?
why ?
will it depend up on the electrolyte , electrode , or both or what?

thanks in advance

bquirky - 28-9-2010 at 06:36

The key concept here is the standard electrode potential

http://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)


At its simplest any metal will generate a galvanic potential relitive to any other metal in a standard pradictable way allmost irespective of the electrolyte used.

electrolyte selection becomes important when you want to prevent side reactions between the electrode and the electrolyte that do not add to the electrical power generated

The prime example of this is a Zinc Annode in a CuSO4 solution.. metalic copper will allmost instantly form on the zinc annode fouling it up.






aeacfm - 28-9-2010 at 20:12

ok , here what is the role of chemical potential and free energy ???????

[Edited on 29-9-2010 by aeacfm]

psychokinetic - 29-9-2010 at 12:05

*sigh*

I'm sure it has been answered at least threefold by now.

As to the last bit: Chemical potential is the potential of the chemical to move/act/do.
Free energy is the change is workable energy, which is negative or else it isn't happening.