Electrolysis of Acids

[Sebbass69]

So, I know that acids are very conductive of electricity. However, what kinds of materials are liberated when you run a current through an acid? For example, with HCl, I imagine it would be hydrogen and chlorine, but what about sulfuric acid? How about phosphoric? I know that when ammonium sulfate is added to sulfuric acid and then electrolyzed, it forms ammonium persulfate, but what about without any substance in the acid?

[Theophrastus]

In regards to acidic, solutions, yes, the ions, are generally liberated from the solution, thus producing hydrogen gas, and the appropriate ion. To determine the products, you then need to account for any reaction that these ions may undergo with the electrodes, the solution itself, and the surrounding environment. However, I find electrolysis a messy process, as you have to be aware of contaminants and the like, and these can really screw with your results.

[UC]

So, I know that acids are very conductive of electricity. However, what kinds of materials are liberated when you run a current through an acid? For example, with HCl, I imagine it would be hydrogen and chlorine, but what about sulfuric acid? How about phosphoric? I know that when ammonium sulfate is added to sulfuric acid and then electrolyzed, it forms ammonium persulfate, but what about without any substance in the acid?[/quote

 

Electrolyzing an acid is usually only of interest when you want a conducting solution. HCl is a poor choice since the [ce] Cl^- [/ce] anion can be oxidized to chlorine gas. Sulfate and Phosphate are impossible to oxidize further, so you should only get oxygen evolution at the anode. In fact, IIRC, very dilute sulfuric acid is the standard electrolyte to use for a hoffmann apparatus.

 

Do you have a reference for the formation of ammonium persulfate?

[Sebbass69]

Yeah, I got that info from Wikipedia -

 

"Ammonium persulfate was prepared by H. Marshall by the method used for the preparation of potassium persulfate — by the electrolysis of a solution of ammonium sulfate and sulfuric acid."

- Hugh Marshall (1891). "LXXIV. Contributions from the Chemical Laboratory of the University of Edinburgh. No. V. The persulphates". J. Chem. Soc., Trans. 59: 771. doi:10.1039/CT8915900771

[Justonium]

OK, for all of the acids, hydrogen forms on the negative electrode. As for the positive electrode, different things can happen depending on how strong the acid is. If the acid is relatively weak, you will get the most stable decomposition of its negative ion. If it is a stronger acid, however, you attract the hydroxide ion from water instead to form water and oxygen gas. You might be thinking "Well what about a concentrated strong acid?" Well, the reason acids dissolved in water conduct electricity is because they dissociate into their ions, but in a concentrated acid, they are not dissociated, so the acid is an insulator. If small amounts of water are present, I would think it possible to get the negative ions of a stronger acid, however, because those acids have such strong affinities for water. They would then most likely be more willing to donate the few sulfate ions created in the dissociation with the water present, but this would be a very slow process because there would be so few ions to carry a current.

[UC]

OK, for all of the acids, hydrogen forms on the negative electrode. As for the positive electrode, different things can happen depending on how strong the acid is. If the acid is relatively weak, you will get the most stable decomposition of its negative ion. If it is a stronger acid, however, you attract the hydroxide ion from water instead to form water and oxygen gas. You might be thinking "Well what about a concentrated strong acid?" Well, the reason acids dissolved in water conduct electricity is because they dissociate into their ions, but in a concentrated acid, they are not dissociated, so the acid is an insulator. If small amounts of water are present, I would think it possible to get the negative ions of a stronger acid, however, because those acids have such strong affinities for water. They would then most likely be more willing to donate the few sulfate ions created in the dissociation with the water present, but this would be a very slow process because there would be so few ions to carry a current.

 

Strong acids are still ionized. They protonate themselves in an equilibrium, For example, you'll fine the species [ce] H3SO4^+ [/ce] and it's counterion [ce] HSO4^- [/ce]. In fact, the number of ions is ten orders of magnitude higher than in high purity water (which is poorly conductive)

 

http://en.wikipedia.org/wiki/Sulfuric_acid <- read the polarity and conductivity section.

 

Cations tend to get reduced at the cathode, while anions tend to be oxidized at the anode. Some anions are incapable of further oxidation and many cations cannot be reduced effectively in aqueous solution because [ce] H^+ [/ce] as hydronium is easier to reduce than say, sodium. (also, the whole reacts with water thing )

Edited May 25, 2009 by UC

[maha]

Electrolysis is breaking up a compound using electricity. In order for electrolysis to occur the compound must conduct (i.e. have mobile charged particles to carry the charge! here, these are the ions produced when the sulphuric acid dissociates). To electrolyse sulphuric acid, two inert electrodes (e.g. carbon or platinum electrodes) are placed in the solution and connected to a battery cell or mains supply of electricity.

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