K12Chemistry
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Electrolysis of CuSO4
I recently followed a vid on electrolysing copper sulfate with a copper and carbon electrode. I waited until all the copper precipitaed out. I was
trying to make copper and sulfuric acid so I filtered out the contaminants and saved the sulfuric acid into the vial. I placed a small amount into a
few dimples on a dimple tray and then to one added sugar, it didn't turn into carbon. I assumed the concentration was very weak as I didnt concentrate
it. In the other dimple I added sodium hydroxide, it didn't release gas but the beads of sodium hydroxide dissolved and it went brown. I would
appreciate it if someone could tell me if what I have is sulfuric acid or not.
Thanks,
K12Chemistry
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Nicodem
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3-1-2013 at 10:26 |
Hexavalent
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Yes, you have sulfuric acid, but it is very dilute. You can boil it down to concentrate it if necessary, but it's certainly not something for the
beginner to do and something even I'm reluctant to do now.
How concentrated was your starting copper sulfate solution, and did you electrolyse until it went completely colourless?
Which of the electrodes you mentioned was the anode, and which was the cathode?
By the way, the reaction with sodium hydroxide wouldn't produce any gases; the only products would be sodium sulfate and water.
Edited to fix spelling.
[Edited on 3-1-2013 by Hexavalent]
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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K12Chemistry
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hey im back
what exactly isn't something for the beginner to do. Is it boiling down the acid or even electrolysing cuso4.
my starting copper sulfate solution was almost saturated. I had a little bit of copper sulfate which I put into a conical flask. I added a little bit
of water bit by bit until all the copper sulfate was dissolved. I used a copper cathode and a carbon anode I got from a carbon zinc battery. Copper
was collecting at the copper electrode. Yes I did electrolyse it until it was clear. Yeah i realise now that sodium hydroxide doesn't do anything, I
failed with my mental equation (2NaOH+H2SO4->Na2SO4+2H2O) WTF lol.
Anyway thanks foor the reply
[Edited on 4-1-2013 by K12Chemistry]
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Pyro
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Boiling down H2SO4 to concentrate it. it takes temperatures of over 300*C and makes lots of SO3 vapours (nasty as hell)
on another note, how about keeping the solution and keep adding CuSO4 and electrolyse it. you should get a higher concentration that way.
2NaOH+H2SO4->Na2SO4+2H2O
all above information is intellectual property of Pyro.  |
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Hexavalent
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K12, in your equation, you have not balanced, assumed that hydrogen is a product and used an incorrect formula for sodium sulfate. As sodium sulfate
is an ionic compound consisting of sodium ions - which have a +1 charge - and sulfate ions - which have a 2- charge, two Na+s are required to balance
it out. Thus, the formula is Na2SO4
1. The equation given by Pyro is the correct one, showing that two molecules of sodium hydroxide would react with one molecule of sulfuric acid,
giving one molecule of sodium sulfate and one molecule of water.
2. Hydrogen does not usually occur as a single atom, i.e. 'H' - it is most often encountered, and certainly so in this case, as a diatomic molecule -
'H2'.
[Edited on 3-1-2013 by Hexavalent]
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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K12Chemistry
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The equation was supposed to be wrong. Anyway I ran out of copper sulfqte so I have ordered some. I'm going to keep the leftover and add copper
sulfqte as suggested. You said that to boil down sulfuric acid takes temperature higher than 300 Celsius but don't you just have to heat it to 100. So
that the water evaporates also sulfuric acid decomposes at 300 Celsius
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Hexavalent
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No, it doesn't work like that. Boiling at 100oC will only work up to 70% concentration, after which one will need to heat to
300oC to attain 95%+ concentration.
See 3:18;
http://www.youtube.com/watch?v=okvvD3-DF9U
However, if you need to ask questions like "do I have sulfuric acid" and cannot balance simple equations or correctly determine reaction products, you
shouldn't be handling concentrated sulfuric acid, and certainly not boiling it. Inexperience is often a large contributor to severe accidents in
laboratories, and the last thing we need is an individual receiving third-degree thermal and chemical burns across his face, further damaging the
already dwindling reputation of our hobby.
I'm not trying to be rude or calling you stupid, just warning you not to get ahead of yourself and possibly get very badly hurt.
[Edited on 3-1-2013 by Hexavalent]
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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hissingnoise
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Anodes made from compressed amorphous carbon aren't suited to a strongly oxidising environment . . .
And carbon oxidation at the anode during electrolysis increases with acid concentration and represents a loss of sulphate so that the resulting soln.
can be expected to contain sulphurous acid (H2SO3) in addition to H2SO4.
An inert anode; Pt or PbO2 is ideal, but a proper graphite anode (gouging rod?) will be somewhat less reducing . . .
The BP of 98% H2SO4 is ~337°C!
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virgilius1979
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Don't expect your acid to be very reactive, as indeed is very dilute. When I was in high school I produced some H2SO4 with copper catode and graphite
anode (removed from a battery). The contamination with carbon particles is very messy, but it can be filtered. Try to redisolve some more CuSO4 after
the solution clears, to get more concentrated acid; although this would increase the corrosion of the carbon electrode. Also the carbon suspended
powder decreases the circuit electrical resistance and this also increases current and electrode corrosion.
I have repeated this with a PbO2 anode. I dissolved 1kg of CuSO4.5H2O in about 1,5L of water. After completion I got a solution of about 15% which was
pretty weak (not any visible reaction, except with Mg powder). At that concentration the boiling point is close to 100 degrees C. Only at about 20% it
begins to rise slowly.
By the way, you should not boil it in you kitchen, as I did, because above 85% the vapors contain not only water, but H2SO4 too, and this increases
with concentration. I had to leave the room as the air became unbreathable, even with ventilation on.
Check the graphic charts here
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SixTough
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Nice link, virgilius.
Ive concntrated mine up to 70-80% no problem. that is on a small scale though which i do not recommend as it gets consumed fast in a lab. I might be
wrong but doesn't PbO2 leave traces in acid making it a bit cloudy?
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