Naked Science Forum
On the Lighter Side => New Theories => Topic started by: sorincosofret on 28/10/2008 15:17:41
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Electrolysis of Molten Sodium Chloride
Background and actual explanation
In the example we will use the most common of the salts, sodium chloride. Solid sodium chloride, in normal condition, does not conduct electricity, because there are no electrons which are free to move.
However, molten sodium chloride does. According to actual interpretation, when sodium chloride and heated and melted, the sodium ions and the chloride ions can separate from one another somewhat, and they are free to move throughout the liquid.
Let’s analyze in detail the phenomena of molten NaCl electrolysis in an electrolytic cell. The cell is driven by a battery or some other source of direct current. The battery acts as an electron pump, pushing electrons into one electrode and pulling them from the other. The electrode from which the electrons are withdrawn is labeled as positive. The one receiving the electrons is labeled as negative (fig.1).
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Figure 1 Electrolytic cell
The following equation represents the breaking apart of NaCl(l):
2NaCl(l) → 2Na(l) + Cl2 (g)
The half-reactions involved in this process are:
Reduction: 2Na+(l) + 2e- → Na(s) E° = -2.71 V
Oxidation 2Cl-(l) → Cl2 (g) + 2 e- E° =-1.36V
net voltage required = - 4.07V
The negative sign of voltage tells us that the overall reaction will NOT be spontaneous, and a minimum of 4.07 volts will be required for this reaction to occur.
Experimental part
The experiment has as purpose to measure the conductibility of molten NaCl at a potential lower then 4V, when no chemical reaction takes place at electrodes.
In this purpose a ceramic crucible is filled with NaCl and in the solid NaCl two inox electrodes are inserted. The electrodes are home made inserting two inox materials into two glass tube, which serve as mechanical protection and insulator {fig.2). In the experiment NaCl of chemical purity was used, but the results are the same with kitchen NaCl.
Electrodes are connected to a series circuit formed by a alkaline battery of 1,5 V, and ammeter. Having solid NaCl in the crucible, the ammeter registers a null current (0 mA).
The crucible is heated with a flame coming from metan gas (the flame must be blue in order to have high temperature). After 40 seconds circa, the current starts to flow into circuit, gradually, from 0,2 mA, and arriving to 40 mA after circa 1 minute. After that the increasing of current is less evident, arriving to about 45 mA in another minute.
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Figure 2. Detail of the experiment
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Figure 3 Experiment details
Experiment interpretation
It is inconceivable to explain this simple experiment in the frame of actual physics and chemistry. Because if melted NaCl is formed by ions, there is no possibility to discharge these ions at electrodes at a voltage lower then 4 V. In this case the electrolytic cell should comport like a capacitor.
The actual theoreticians should choose one option from two possible:
• A electric current can flow through a melted NaCl salt at a potential lower then potential necessary for electrode reaction
• The series of electrode potential is wrong.
In proposed theory, an electric current does not mean a charge displacement. Therefore an electric current can pass through a molted salt without having electrode processes. Of course, even there are not electrode processes, the electrolytic cell has a resistive comportment and some power is consumed into circuit. The Faradays lows of electrolysis need some structural corrections.
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Have you taken into account concentration and temperature effects? I see no indication in your analysis that you have done so and since the standard electrode potentials are valid only at 25 oC (against 800 oC in molten salt) and in (aqueous) 1 M solution (the melt will be much more concentrated than that, tens of molar at least and that's assuming the fact the ions are no longer solvated is of no importance).
Sorry, sorin. If you're still trying to bring down the physical sciences as currently understood you'll need to do better than that, you'll need to show that your results are in conflict with the full complexity of those predicted by real current understanding (rather than just those you'd predict from reading a high-school text book).
I have no idea what current theory does predict for this experiment, but equally, neither do you!
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"a minimum of 4.07 volts will be required for this reaction to occur."
Please stop posting this rubbish. It was wrong before and it's still wrong now.
"there is no possibility to discharge these ions at electrodes at a voltage lower then 4 V"
Why on earth not?
Go and look up the Nernst equation.
That 4.07V isn't an absolute limit, its the value for the potential for a particular set of conditions, none of which apply in this case.
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There are some comments coming from semi alphabetised people, but they need to take some lessons on electrochemistry.
The Nernst equation is applied to aqueous solution which is not the case in presented experiment.
Even the Nernst equation would be valid, the deviation of measured potential from standard potential are minuscul. If you vary on a large scale the concentration, temperature, etc, the modification over the cell potential are less then 0,5 V ( and this 0,5 V is an exaggerations).
I attach here some paragraphs in italic (with my comments) and some picture from a site http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/electrode.html
One implication is that the cell potential will be reduced from the standard value if the concentration of Zn2+(aq) is greater than that of Cu2+(aq) at the standard temperature. An excess concentration of Cu2+(aq) will give a higher voltage. The graph at right shows the increase in cell voltage with increasing concentration of the cation. Note that the horizontal axis is logarithmic, and that the straight line variation of the voltage represents an logarithmic variation with Q. Note that the cell potential is equal to the standard value if the concentrations are equal even if they are not equal to the standard value of 1M, since the logarithm gives the value zero.
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As is observed in the picture a logarithmic increasing of concentration has as consequence a linear modification of potential. It can be seen that a ten factor of concentration modification lead to a potential modification with .... 0.05V.
Consider a concentration of 10-5Molar for Zn2+(aq) and 0.1 Molar for Cu2+(aq) as a test case for temperature dependance. We can see that the cell potential tends to increase with temperature, or that a colder cell prodices less voltage - a commonly observed phenomenon with dry cell batteries. The variation with temperature is linear with temperature, but quite small for this cell. The large variations of practical output voltage with temperature for dry cells does not arise from the Nernst equation alone.
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As is observed from the second picture a modification of 25 degree of temperature lead to a modification of 0.01 V in the potential.
Both these consideration applies to aqueous solutions. In case of molten salt, there is a small variation due to the temperature of cell, but this is less then 0.2 V. The concentration and activity of both ions is considered equal with 1.
Go on to a low level chemist teacher in order to refresh your knowledge in the field, before making comments.
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Okay Sorin, I've read many of your posts now, and if you do not stop being rude to people who critisise your science, you will be banned from this forum. Both Rosy and Bored Chemist have displayed consistently high levels of science understanding on this forum.
I think you should understand that these people do not have it in for you, but see problems in the science contained within your posts - this is either because you are wrong, or you have not explained yourself properly.
I expect to see you being more polite in your posts in future.
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1. Are you sure stainless steel is totally inert in that case? Chlorine at hot...
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Ben,
Have you made the same recommendations for those persons who criticise me?
I have nothing against criticism, but if they send me to read ,,good" books on physics, why should I haven't the same right when they made some childish errors?
For example in the present post why a person who ,,displayed consistently high levels of science understanding" does not apply the Nernst equation and shows me the error.
In this case they can blame me based on a concrete thing. But, to blame me based on a equation which does not apply and even these equation should apply the experiment is outside the prediction of this equation, is not a constructive and useful fact. For a practitioner, the nernst equation (variation of electrode potential with external factors) has importance in low potential measurement. In a cell is quite unimportant if the cell has the potential 1.5 or 1.45 or 1.55 V.
The experiment was made far away from Nernst prediction. You can diminish the 4,07 theoretical potential with a maximum 0,5 V due to working conditions. I have worked at 1,5 V. From 1,5 to 3,5 in electrochemistry is a gap which is impossible to be explained ...
Anyone can ask a factory working in the field of molten NaCl electrolysis and will see that applied potential is more then 4,1 V ( they work ussualy at 5 up to 7 V ) and they can very easy made an experiment with a potential lower then 4 V and observe what's happen. After that if my conclusion is wrong, please come and blame me.
I'm not interested in their posts on the forum in general. Maybe they are specialists in actual orthodox theory. But it should be a common sens attitude to recognise when a ,,simple or complex experiment" can't be explained in a satisfactory mode in actual theory.
I don't think it is only my guilty for this actual situation.
My interest is only to post some experiments or interpretations and if the operators will not consider a rude comportment I will not respond to comments.
So, I will fill in the new posts to new theories sections, and from my point of view is very convenient to not receive any comments too. In this way, as Centhaur said, the posts are going to the forgotten part of the site.
As I said I have nothing against to be criticised or to be asked for details about posted materials.
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I have made the experiments only with Fe(inox) electrodes. I have read in my chemistry books that molten NaCl is electrolysed with metallic electrodes. I haven't found any detailed specification. I will search for details and I will repeat with graphite electrodes.
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I have made the experiments only with Fe(inox) electrodes. I have read in my chemistry books that molten NaCl is electrolysed with metallic electrodes. I haven't found any detailed specification. I will search for details and I will repeat with graphite electrodes.
Ok, remember however that you also must be completely sure there isn't the slightest amount of water mixed with the (assumed totally pure) compound NaCl; as every salt, it tends to absorb water vapour and it doesn't release it completely, even at high temperatures, unless you probably operate in vacuum or in a totally dry athmosphere.
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A new repetition with molten salt and electric currents.
Using graphite electrode, dry and grounded salt (reagent degree), as is indicated in fig. 1 in absence op heating the current through system is 0 micro. [ Invalid Attachment ]
Then the crucible is heated as it can be seen in the fig. 2. It is difficult to believe that NaCl 99% become conductor due to humidity at this temperature. In fact the water is eliminated at maximum 400 C, when the process of melting does not start.
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The maximum observed current was about 45 mili ampers. This current can't be attributed to water coming from air during heating.
In the folowing picture a part of crucible and ammeter are visible at few seconds after flame removing. The current is about 23 miliA.
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I have found in Electricite, G. Bruhat, septieme edition, Paris, 1959, that a half a century ago, the phenomena of solution conductibility under the value necessary for electrod reaction, was known. In this book, it is called invisible electrolysis. Of course there is no explanation for this phenomena.
During time, in order to hide the deficiencies of actual theories, these kind of things are not good looking in modern and ,,good" treatise about electricity.
Who is able to propose a theory for this ,,invisible electrolysis"?
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Here's a nice thought experiment for you. If you heat salt to a high enough temperature it boils. If you keep heating the vapour it dissociates. So, if it's hot enough it forms Na and Cl without any electric potential.
How big as shock is it that hot salt needs less voltage to decompose it than cold salt?
"The Nernst equation is applied to aqueous solution which is not the case in presented experiment. "
Actually it can be applied to non aqueous solutions, and it often is. You just need to use the right standard potentials. God knows what they are in this case.
However that's the real world so Sorin discounts it.
On the other hand, he uses standard voltages measured in aqueous solution then assumes that they will be the same in molten salt at about 800C.
A very silly assumption. Do you really thing the solvation energies of the ions will be the same in water and in molten salt?
I wasn't joking about the Nernst equation though, it shows how you can get arbitrarily large deviations from the standard potenetials (even in water). You have forgotten to look at the chlorine concentration. The concentration of chlorine in red hot salt exposed to air is very near zero and you are assuming it's 1 atmosphere. That will give you several orders of magnitude to put into the equation.
Even that would only apply if you had inert electrodes. At the anode you are almost certainly not making much Cl2 but FeCL3 which is soluble in the molten salt. It will have a very low concentration because it can diffuse away and the Fe(II) Fe(III) redox potential is known to be striongly affected by high Cl- concentrations. It's quite a challenge to get the concentration in an aqueous solution as high as molten NaCl does
"From 1,5 to 3,5 in electrochemistry is a gap which is impossible to be explained "
Impossible for you, perfectly simple for me.
The simple answer is that, as I said, the standard electrode potentials only work under standard conditions. You are about as far from those as you can get. Why do you expect to get the right voltage?
As for the latest experiment with hot, but not molten, salt. Do you remember me explaining that "insulator" isn't an absolute trem?
One good way to get a supposed insulator to conduct is to heat it.
Nothing magical, just a Boltzman distribution of electrons and simple band theory.
"Go on to a low level chemist teacher in order to refresh your knowledge in the field, before making comments."
If I were looking for a bad chemistry teacher Sorin, I'd ask you.
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Here's a nice thought experiment for you. If you heat salt to a high enough temperature it boils. If you keep heating the vapour it dissociates. So, if it's hot enough it forms Na and Cl without any electric potential.
How big as shock is it that hot salt needs less voltage to decompose it than cold salt?
This nice thought experiment does not have common points with our discussion. It is only a physical change of matter state. It is the same chemical compound.
The post is about conductibility of a ionic compound under the value of potential necessary for electrode reactions.
It is proved that solution (and molten) salt are conductors of electricity under this potential ( I will search again and post the text in french from Bruhat Cours of electricity), and the actual ,,good" treatise of electricity hide this thing because does not fit with orthodox theory.
The discussion about Nernst equation is puerile; anyone can make a phone call to a factory working in the molten salt electrolysis in order to see the practical condition of working.
Anyone will see that electrodes processes under 4 V in case of molten NaCl are impossible, but NaCl has a conductor comportment under 4 V.
After that anyone can search some ,,good" texts about electromagnetism, special electrolysis and will find the actual (missing) explanation.
The experiment was repeated with graphite (inert) electrodes, and the same phenomena was observed.
In the last photo the upper part of NaCl is not melted, but this does not mean that lower part is only heated.
Solid NaCl is poor conductor of thermic energy too.
Even the heated NaCl will be electric conductor, this represent a new problem for actual orthodox theory.
Who is proposing the mechanism for this phenomena?
Again space temporal modification and charged particle are moving in a circle ....
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"It is only a physical change of matter state. It is the same chemical compound.
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plain wrong, and the resy of the post is no better.
"The discussion about Nernst equation is puerile;"
Only if you don't understand it.
" anyone can make a phone call to a factory working in the molten salt electrolysis in order to see the practical condition of working.
Anyone will see that electrodes processes under 4 V in case of molten NaCl are impossible, but NaCl has a conductor comportment under 4 V. "
Ture, anyone can do this, and with a bit of thought they can understand it. WHy don't you?
I see you haven't answered the question about solvation of ions in molten salt.
"Again space temporal modification and charged particle are moving in a circle .... " gibberish.
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I have tried to avoid to put you in a ,,delicate" situation.
But if you insist...
There is no process of solvation when same solid is melting.
Ice does not suffer a solvation process when melt to liquid water. The same thing happened when a solid ionic salt is melting and a equilibrium of phase exist. This is happen in actual interpretation. Because NaCl solid is the same like NaCl liquid, more precisely formed by ions. In liquid form only the ion movement is a little bit different.
When chemistry will provide that a different physical or chemical phenomena occur due to a phase transition .... I will accept this fact.
I don't make any comments about the rest.
As a specialist and person with ,,consistently high levels of science understanding on this forum", I was expecting to receive a complete mathematical demonstration of Nernst equation for molten salts.
Instead I have received a comments ,, God knows how Nernst equation works in this case".
Any chemist or physicist with a low level of common sense can discern between true and false.
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The difference in energy between a sodium ion in the vapour phase and a sodium ion in the melt is a solvation energy.
I don't see how pointing out that you are wrong (again) puts me in a delicate situation.
I imagine that when you heated your crucible of salt you noticed that the flame was coloured yellow. This colour is exactly the same as the colour from a sodium lamp.
In both instances it is caused by emision from sodium atoms (not ions).
You have direct personal experience that shows that heat can dissociate NaCl into Na and Cl.
Even if you are too conceited to believe me, why can't you believe your own eyes?
As you say "Any chemist or physicist with a low level of common sense can discern between true and false."
Also you seem not to understand (there's a phrase I seem to use a lot) the difference between "You just need to use the right standard potentials. God knows what they are in this case. "
Which is perfectly reasonable unless you can show me a set of the apropriate standard potentials for a molten salt bath,
and "God knows how Nernst equation works in this case".
God may know- so do I. The equation is based on perfectly dull physical chemistry and it works just the same as it does anwhere else.
BTW, BenV, thanks for the support but I'm really more concerned about the integrity of the science on this forum than about being insulted by Sorin.
It's a bit like a frustrated small child shouting "YOU SMELL OF WEE!"; more amusing than upsetting, but not to be encouraged.
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By definition a solution and its solvation energy confirm a solution formation. A solution is a mix of at least two different chemical substances. Of course a hydrogen gas can form a solution when diffuses in water, because hydrogen as molecule is another compound, with different chemical properties related to water.
In case of NaCl melting or vaporisation there is only a phase change, without any change in chemical comportment. The same for water. Its water even is solid, liquid or gas. In this case there is a special therm in physical chemistry: enthalpy of melting, enthalpy of volatilisation.
In our example, NaCl volatilisation from a previous liquid state need a enthalpy of volatilisation. This enthalpy with opposite sign represent the energy released when vapour NaCl transforms in liquid.
There is no solvation process. Of course if ,,in a hypothetical experiment" vapour of NaCl are solved in water, there is a enthalpy of volatilisation and a enthalpy of NaCl ions solvation in water.
You have made a bad description of melting process. Even the flame has about 2000 C, working without in open system, the flame does not have enough energy to melt the entire quantity of salt from the crucible;e. About volatilisation of NaCl no way. If you compare the last three photo, you will see that in the last photo the upper part of salt from crucible form like balls, because the heat from the lower part is not enough to melt it all. In a previous picture you can see the starting and grounded material.
There is another problem when a small quantity of NaCl is introduced direct in the flame. There is a specific colour... but this explanation will be made when I will arrive to another book.
Again , the discussion is diluted ...
I think its better to post a simple message about experiment and its explanation in frame of actual theory. After that all readers have the possibility to compare your explanation with experiment result.
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"In case of NaCl melting or vaporisation there is only a phase change, without any change in chemical comportment."
Still wrong. Still at odds with the yellow flame.
"There is another problem when a small quantity of NaCl is introduced direct in the flame. There is a specific colour... but this explanation will be made when I will arrive to another book. "
Don't bother to write until you learn to read and think.
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Don't bother to write until you learn to connect two conductors.
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OK,I did that in about 1973 so I can carry on writing.
If salt doesn't dissociate how do you explain the yellow flame?
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This is another problem and will be discussed at in a treatise of physical chemistry.
It is irrelevant for the initial message the state of NaCl in gaseous form.
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My point still stands.
Heating salt can make it dissociate without regard for you saying (in the face of the evidence provided by your own eyes) that it does not.
It is, therfore, unsurprising that it takes less energy to dissociate hot salt than cold salt.
That, on its own, could explain the fact that electrolysis hapens at a lower voltage than would be expected based on observations made at room temperature.
The idea that "the voltage needed to decompose salt is 4.07 V" is nonsense.
As I pointed out (and you didn't understand) that is the voltage that would be needed for a cell that had chlorine at 1 atmosphere activity as one electrode and sodium as the other. The electrolyte would have to have both the sodium and the chloride ions present at an activity of 1 molar.
However, that cell is inpossible.
The sodium would react with the water.
Why did you think that the cell voltage would not change when ;
1 you changed the temperature
2 you changed the electrolyte
3 you changed the concentration of chlorine
4 you changed the Na+ concentration and
5 you changed the Cl- concentration.
Are you just not very observant?
I ask because I already pointerd this out to you in one of your other bizzare threads.
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You should prove that a melting salt has a potential lower then the theoretical potential. After that you should convince the actual scientific community that your discovery worth to be published. Be sure that this research will be very useful for you. The industry works at potentials of 5 to 9 V in case of molten salt electrolysis. So if you are convinced about what you are saying, make the thing!
After that you should modify a lot of books about subject. When you will be finished with this job, you can make comments.
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"You should prove that a melting salt has a potential lower then the theoretical potential."
What is the theoretical potential?
My comment on the matter of that potential was "God knows"; or were you not paying attention?
The calculated voltage is the minimum required at equilibrium. Its the voltge required to stop the reaction running backwards.
If you want the reaction to go forwards at a reasonble rate you need to use a higher voltage. This also generates enough heat to keep the salt molten. That's why industry uses 5 to 9 V
It's not that I need to rewrite the books; you need to read them.