mycotheologist
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Electrolysis of various salts
Everyone knows what happens when you put a couple of electrodes in an NaCl solution and apply some voltage but thats the only salt I've heard about in
electrolysis discussions so far. Firstly though, heres how I think electrolysis of aqueous NaCl works so correct me if I'm wrong: A buildup of
electrons occurs at the anode which attracts the Na+ cations. Similarly, there is a deficiency of electrons (positive charge) at the
cathode which attracts the Cl- anions. The Na+ cations get reduced by the anode and turn into free sodium metal, while the
Cl- anions get oxidised at the cathode and converted into Cl2 gas.
And I might as well include all the stuff that happens after the initial redox reactions: the Na instantly reacts violently with the water to produce
NaOH. Much of the Cl2 gas reacts with the NaOH, yielding NaClO (or at high temperatures, NaClO3). So thats the electrolysis of
an NaOH solution in a nutshell. So I know what happens to Na+ cations (which will be the same for all monatomic metal cations) at the anode
and what happens to Cl- (and the other halides) at the cathode but heres some ions I'm wondering about.
Cations:
Ammonium - Will the extra proton get reduced and end up producing H2 gas and free NH3?
Anions:
1.) Acetate - What happens when the acetate anion gets oxidised?
2.) Sulphate - Does it get oxidised into SO3?
3.) Nitrate - As far as I know, NO3 doesn't exist so what happens the nitrate anion?
4.) Borohydride - Being a powerful reducing agent, would it react violently with the cathode?
5.) Permanganate - Similarly, being a powerful oxidiser, will it react violently with the anode? What products would be formed.
If a chain of reactions occur (like in the NaCl) after the redox reactions, can you explain them too so that this thread will also serve useful for
people looking into the practical applications of all this. All this stuff deeply fascinates me.
[Edited on 8-4-2012 by mycotheologist]
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barley81
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The reduction potential of sodium ions is more negative than that of water (reduced to hydrogen and hydroxide ions), so no sodium is formed. Only
hydrogen is formed. So, there is no real chain reaction.
Ethane and carbon dioxide are formed when acetate is electrolysed. <a
href="http://woelen.homescience.net/science/chem/exps/precision_electrolysis/index_acetate.html">Link to the page.</a>
Sulfate can form persulfate, but typically oxygen is evolved, and hydrogen ions are made (oxidation of water).
Nitrate (I think) would not react. Only oxygen would be evolved, and hydrogen ions would be made.
If you electrolyze a solution of permanganate, I think different things would be produced depending on the pH. I think that
MnO<sub>2</sub> would be made in neutral or basic conditions, and Mn<sup>2+</sup> would be made in acidic conditions. Someone
should try this to confirm.
Remember, reduction happens at the cathode, and oxidation happens at the anode.
@mack07
Did a question really clarify your understanding of electrolysis?
[Edited on 9-4-2012 by barley81]
[Edited on 9-4-2012 by barley81]
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Pyridinium
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Nitrate is just below water on the list of reduction potentials. It's close enough that I'm wondering if an adjustment of conditions (or electrodes)
could push it either way to yield N2O4 / NO2. Many electrode materials have an overpotential for reduction of H2O to H2, so it seems possible to
push the reduction of water down past the reduction of NO3-. In theory, anyway.
Sulfur trioxide could not be made in aqueous solution by electrolysis, as it would immediately form H2SO4.
Borohydride: With the cathode being a reducing environment itself, I would expect the more energetic reaction to occur with the anode. I have to
think about this for a while.
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woelen
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Cations:
Ammonium - This ion is reduced at the cathode: NH4(+) + e ---> NH4
The ammonium molecule is very unstable and breaks down to NH3 and H2. When a mercury cathode is used, then some of the ammonium dissolves in the metal
and you get an amalgam with NH4 dissolved in Hg. The Hg lateron slowly starts foaming, due to decomposition of NH4 to NH3 and H2.
Anions:
1.) Acetate - This is an interesting reaction. Acetate ion looses an electron and then breaks down as follows:
CH3COO(-) - e --> CH3COO --> CH3 + CO2
Two CH3 radicals combine to CH3CH3 (ethane gas). So, you get a mix of CO2 and ethane at the anode.
2.) Sulphate - This does not form SO3. SO3 and SO4(2-) have the same oxidation status (both +6 for sulphur and -2 for oxygen). Usually, in aqueous
solution, the sulfate is not oxidized at the anode, but water is oxidized more easily to oxygen and acid. Under special carefully constructed
conditions, sulfate can be oxidized to peroxodisulfate. This requires very low temperature, a rather low pH and high current density at platinum
anode. What happens then is the following (simplified):
HSO4(-) - e --> HSO4
followed by combination of two radicals: 2HSO4 --> HOS(O)2-O-O-(O)2S-OH, which is H2S2O8.
3.) Nitrate - Nothing happens to the nitrate ion at the anode. It is not oxidized further. Instead, water is oxidized to acid and oxygen, or the anode
material is oxidized. With graphite and platinum anodes you will see formation of oxygen and lowering of pH around the anode.
Nitrate can be reduced at the cathode. If e.g. a solution of KNO3 is electrolysed, then the main reaction at the cathode is formation of hydrogen and
hydroxide ion (reduction of water), but there will be a side reaction, in which nitrate ion is reduced to nitrite, hydroxylamine or even ammonia).
4.) Borohydride - I would expect formation of hydrogen at the anode when an aqueous solution of e.g. NaBH4 is electrolysed. I also expect a side
reaction in which the borohydride ion is oxidized to borate.
The equation is: BH4(-) - e --> BH4. Here BH4 is unstable and breaks down to BH3 and H2. BH3 reacts with water, giving more H2 and H3BO3. I am
quite sure that side reactions occur in which some of the BH3 and BH4(-) are oxidized further and this would lead to formation of less hydrogen than
what you expect on the basis of the process I described above.
At the cathode I do not expect any reaction of BH4(-). At that place, hydrogen is formed, due to reduction of water. So, this electrolysis would be
one, in which hydrogen is evolved both at the anode and at the cathode!
Actually, now that I write about this, I might even try this experiment myself when my lab is available again (now I am in the process of rebuilding
part of the house, so for the next few weeks I cannot do any experiments).
5.) Permanganate - I am very sure that this will be reduced at the cathode (besides the common reaction in which hydrogen is formed from water). In
neutral solution I expect formation of MnO2 or hydrous Mn2O3 or a mix of both. So, I expect that the solution will turn turbid and brown over time. At
the anode simply oxygen is formed, due to oxidation of water.
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