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Offline shalikadm

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Electrolysis of dilute NaCl
« on: July 24, 2012, 05:08:19 AM »
In our electro-chemistry lessons we were told that if we electrolyze dilute NaOH,the concentrations of Na+and Cl- will not changes..yes it's true as the cathode introduce OH-to the solution as anode removes OH- from solution ..so the concentrations don't vary..

But in our industrial chemistry lessons,it says something different..Well its about Chloralkali process(diaphragm cell process) which prepares NaOH from NaCl(aq).
This is quoted from my book.
Quote
(7)Electrolysis is stopped when the concentration of NaOH is at a higher point.
If it was hydrolyzed too long O2 will bubble off instead of Cl2.
(Then the concentration of OH-will drop down and reduces amount NaOH product)
(8.)Then the aqueous solution of NaCl and NaOH is electrolyzed again.
As OH- concentration is low at first, OH- will not be discharged till Cl- concentration gets low..I understand it..
Ok now let's take that Cl- concentration is low right now..then OH- will start to discharge but according to elecro-chemistry lessons the concentration of OH- will not get vary...
Ok lets imagine that it reduces the amount of OH-...then what happens to Na+ in the solution..It must go with some anion..How..who will give anoins to Na+..low concentration of Cl- is with a same concentration of Na+...So what will happen to newer Na+..?
Also the 7th and 8th points in my book equated about..seems so bizarre to me...First it says no,then it says yes.. ???
Please someone help me understand....!

Offline Caustikola

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Re: Electrolysis of dilute NaCl
« Reply #1 on: July 24, 2012, 10:33:03 AM »
It depends on the potential of each electrode reaction, (nernst equation) .find the redox potential for each electrode reaction using arbitrary concentrations and see for yourself when each reaction is favored over the other
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Offline shalikadm

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Re: Electrolysis of dilute NaCl
« Reply #2 on: July 24, 2012, 11:06:04 AM »
It depends on the potential of each electrode reaction, (nernst equation) .find the redox potential for each electrode reaction using arbitrary concentrations and see for yourself when each reaction is favored over the other
Actually the problem isn't that !
My problem is...Does the concentration of OH- vary when we electrolyze ?
If so what happens to Na+(it can't live without an anion) ?
At the same time we don't have nernst equation in our syllabus...

Offline Caustikola

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Re: Electrolysis of dilute NaCl
« Reply #3 on: July 24, 2012, 12:05:11 PM »
The reaction at the anode is 2H2O -> 02 + 4H+ + 2e-
The hydroxide ions produced by water ionization are too few to be discharged at the anode, hence OH- is unaffected.
At the cathode
 2e- + 2H2O -> H2 + 2OH-
the OH- Produced in the cathode half reaction balances the H+ produced in the anode half reaction, hence the solution is electrically neutral and OH- concentration dosen't vary during the electrolysis
You have to note that OH- IS NOT DISCHARGED, Except you actually mix NaOH and NaCl solutions. The ions produced by water autoionization are too few to be discharged preferentially at electrodes
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Offline shalikadm

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Re: Electrolysis of dilute NaCl
« Reply #4 on: July 24, 2012, 11:41:41 PM »
the OH- Produced in the cathode half reaction balances the H+ produced in the anode half reaction, hence the solution is electrically neutral and OH- concentration doesn't vary during the electrolysis
You have to note that OH- IS NOT DISCHARGED,
Yes...I understand it...But why my book says "concentration  of OH- will drop down"...How can that happen ?...(you also agree that it doesn't vary...)

Offline shalikadm

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Re: Electrolysis of dilute NaCl
« Reply #5 on: July 25, 2012, 09:41:39 AM »
.at the anode (+ve)..the half reaction is 4OH- >O2+2H2O+4e-..ie oxygen is pref released comp2 chlorine...at the cathode (-ve) u get..2H*+2e->H2..so to balance up the No of electrons involved u double up..i guess..so your half equn at the cathode shuud b..
4H*+4e-=2H2
we can also write like this..
cathode reaction :rarrow: 2H2O+2e->H2+2OH-
anode reaction :rarrow: 4OH-->O2+H2O+4e
So the overall reaction is 2H2O->2H2+O2
So the concentration of OH- doesn't vary..what are u trying to show.it isn't clear to me...?

Offline Caustikola

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Re: Electrolysis of dilute NaCl
« Reply #6 on: July 25, 2012, 03:38:23 PM »
I think I see your question now. Your industrial chem lesson is talking about the diaphragm cell, while your electrochemistry lesson is dealing with electrolysis of dilute NaCl. The diaphragm cell is different from a cell used to electrolyse dilute NaCl. In the diaphragm cell, there are two cells, separated from each other by a membrane material, or a bridge that permits only the passage of cations. The Sodium ion move through this bridge under the influence of the current. Now, the diaphragm cell is strictly used to electrolyse concentrated solution of NaCl.

This is what happens in the diaphragm cell:
the cell is divided into two compartments, Concentrated NaCl solution is made the anode compartment, while Pure water (H2O) is made the cathode compartment.
The membrane that separates these two compartment permits the passage of only Na+
At the anode,
Cl- is discharged
2Cl- =>Cl2 + 2e-

At the cathode,
H2O is reduced,
2H2O + 2e- => H2 + 2OH-
This is the source of the OH-
and of course its concentration increases as the electrolysis progresses

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Offline shalikadm

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Re: Electrolysis of dilute NaCl
« Reply #7 on: July 26, 2012, 05:13:58 AM »
Ok then again what it's meant to me..

At first the cl- concentration is high..so at anode compartment cl- discharges giving cl2...so the Na+ deserted by cl- has nothing to do in anode compartment and go thru the diaphragm and find the partner OH- given by the discharged H2O at the cathode compartment..It be comes NaOH..

Now Cl- concentration is low as it was discharged and gave cl2...Those cl- are with their partner Na+ ions...

so in the anode compartment there is NaCl and H2O
and in the cathode compartment there is NaOH and H2O

As the cl- concentration is low at the anode compartment,then the H20 which is higher in the electro-chemical series than cl- , starts to discharge giving off O2 to the air and H+ to the solution...It also has nothing to do at this compartment as Cl- is with Na+ ions...so as its an cation it will go thru the diaphragm and reach the solution at cathode compartment..

cathode compartment is also doing the job of discharging H2O at hence giving H2 to the air and OH- to the solution and those OH- ions are the lucky one to go with the H+ ions came thru the diaphragm.So H+ and OH- combine and neutralize to H2O..So the NaOH in the cathode compartment doesn't vary...H+ will eat only those new OH- given by the cathode reaction happening after cl- concentration is low...

So it all happens the same thing ...please make this clear..I'm eating a lot of time at this problem...thanks !

Offline Caustikola

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Re: Electrolysis of dilute NaCl
« Reply #8 on: July 26, 2012, 06:58:42 AM »


As the cl- concentration is low at the anode compartment,then the H20 which is higher in the electro-chemical series than cl- , starts to discharge giving off O2 to the air and H+ to the solution...It also has nothing to do at this compartment as Cl- is with Na+ ions...so as its an cation it will go thru the diaphragm and reach the solution at cathode compartment..

actually the Cl- never really gets low enough for H2O to be discharged when electrolysing concentrated NaCl in the anode compartment. this is because the chlorine gas evolved dosent just go off into the atmosphere. it reacts with the H2O present to produce hypochlorite ions and Chloride ions
Cl2 + H20 => 2H+ + ClO- + Cl-  
I dont want to go into much detail in this equation, but as you can see, it restores Cl- to the solution, hence H2O is never discharged.
 Also, the ion sensitive membrane used in dividing the anode and cathode compartments permits the passage of only Na+ from the anode compartment to the cathode compartment, hence H+ CANNOT move to the cathode compartment
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Offline shalikadm

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Re: Electrolysis of dilute NaCl
« Reply #9 on: July 26, 2012, 10:43:56 PM »

actually the Cl- never really gets low enough for H2O to be discharged when electrolysing concentrated NaCl in the anode compartment. this is because the chlorine gas evolved dosent just go off into the atmosphere. it reacts with the H2O present to produce hypochlorite ions and Chloride ions
it restores Cl- to the solution, hence H2O is never discharged.
If the diaphragm only lets Na+ cations to go thru,how OH- would go and get discharged at anode compartment..We can see cathode is negative and it also doesn't left OH- to be discharged..
An you also says that the Cl- concentration never gets low and H2O never discharged..So how it says that O2 will bubbled off instead Cl2..?
see the 7th point of my tute on industrial chemistry.
Quote
(7)Electrolysis is stopped when the concentration of NaOH is at a higher point.
If it was hydrolyzed too long O2 will bubble off instead of Cl2.
(Then the concentration of OH-will drop down and reduces amount NaOH product)
(8.)Then the aqueous solution of NaCl and NaOH is electrolyzed again.
Is there's something wrong in my tute..I can't still understand the 7th and 8th point in my tute..

If H2O never get discharged and Cl- never gets law..Why my tute says that we have to stop electrolysis when the concentration of NaOH is at a higher level ?
Completely confuse me !

Offline Caustikola

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Re: Electrolysis of dilute NaCl
« Reply #10 on: July 27, 2012, 11:10:04 AM »
OH- dosent pass from the anode to the cathode. Like I said earlier, the OH- concentration in water is very low (10^ -7M). The OH- in the cathode comes from the cathode reaction.
2H2O + 2e- => H2 + 2OH-

Also, it could be that your textbook ignores side reactions, to give you a platform to understand the basic chemistry. H2O isn't discharged, normally because of chlorine dispropotionation.
 
 
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Offline shalikadm

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Re: Electrolysis of dilute NaCl
« Reply #11 on: July 28, 2012, 12:38:53 AM »
Also, it could be that your textbook ignores side reactions, to give you a platform to understand the basic chemistry.
In Industry of making NaOH(using diaphragm cell),does they really stop electrolysis when the OH- concentration is high ?
actually the Cl- never really gets low enough for H2O to be discharged when electrolysing concentrated NaCl in the anode compartment. this is because the chlorine gas evolved dosent just go off into the atmosphere. it reacts with the H2O present to produce hypochlorite ions and Chloride ions
Cl2 + H20 => 2H+ + ClO- + Cl- 
Are you sure that every Cl2 molecule will react with water ?
But in our books and in the net they shows that cl2 is gone to atmosphere.

From all these thing I can imagine only that we can do the electrolysis any longer
cl- never get low...OH- never get low...I can't understand this... why we stop electrolysis?

Offline Caustikola

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Re: Electrolysis of dilute NaCl
« Reply #12 on: July 28, 2012, 07:29:07 AM »
Also, it could be that your textbook ignores side reactions, to give you a platform to understand the basic chemistry.
In Industry of making NaOH(using diaphragm cell),does they really stop electrolysis when the OH- concentration is high ?
actually the Cl- never really gets low enough for H2O to be discharged when electrolysing concentrated NaCl in the anode compartment. this is because the chlorine gas evolved dosent just go off into the atmosphere. it reacts with the H2O present to produce hypochlorite ions and Chloride ions
Cl2 + H20 => 2H+ + ClO- + Cl- 
Are you sure that every Cl2 molecule will react with water ?
But in our books and in the net they shows that cl2 is gone to atmosphere.

From all these thing I can imagine only that we can do the electrolysis any longer
cl- never get low...OH- never get low...I can't understand this... why we stop electrolysis?
The Cl- may get low, because not every Cl2 molecule will react with water, infact at a second thought, I would think majority of the Cl2 produced, especially at the earlier stages will bubble off to the atmosphere. Because of the already high concentration of Cl-
Cl2 + H2O <==> HClO + H+ + Cl-
Cl- is already very high, so le chatelier principle can predict that the much of the Cl2 wont react with H2O
 And on the possible stopping of the electrolysis, if the Cl- gets sufficiently low,
H2O will be discharged, but this would take very long to happen, in practice
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Offline shalikadm

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Re: Electrolysis of dilute NaCl
« Reply #13 on: July 28, 2012, 11:17:19 AM »
The Cl- may get low, because not every Cl2 molecule will react with water, infact at a second thought, I would think majority of the Cl2 produced, especially at the earlier stages will bubble off to the atmosphere. Because of the already high concentration of Cl-
Cl2 + H2O <==> HClO + H+ + Cl-
Cl- is already very high, so le chatelier principle can predict that the much of the Cl2 wont react with H2O
 And on the possible stopping of the electrolysis, if the Cl- gets sufficiently low,
H2O will be discharged, but this would take very long to happen, in practice
That was the thing that I wanted to know...But you said that Cl- will never get varied...That's why it made me more confused....any way now the 7th point of my text book is clear now...now the 8th one...what make us able to electrolyze the aqueous solution of NaCl and NaOH again ?...Do we have to add more NaCl to the anode compartment ?
Quote
(8.)Then the aqueous solution of NaCl and NaOH is electrolyzed again

Offline Caustikola

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Re: Electrolysis of dilute NaCl
« Reply #14 on: July 28, 2012, 12:49:27 PM »
Yes, adding more NaCl is the solution.
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