Ivor and Olive Lewis on sat 23 aug 08
< (heat) Na2O + CO2 (g)
(sodium oxide production)>>
Dear Marian,
I think you need to have a look at the Thermodynamics of this
proposition. Methinks the reaction takes place in the reverse
direction. J. W. Gibbs Rules and all that nonsense.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Neon-Cat on sat 23 aug 08
Gosh Ivor, all carbonates break down to the oxide and carbon dioxide. That's
one very good reason why we potter-types use them. The process is a
decomposition reaction. This is elementary carbonate chemistry. Why bring in
Gibbs and other nonsense? But I can work with Gibbs although by definition
Gibbs free energy involves change at constant temperature and pressure.
Constant temperature we do not have. However, ?G (delta G) for Na2CO3 is
about -1044 kJ/mol; -394 for CO2 and -414 for Na2O, thus the ?G (delta G)
for the reaction Na2CO3 ---> Na2O + CO2 (g) is +236. Ivor, this shows us
only that the reaction is not spontaneous. You saw my "heat" symbol, did you
not? We are cooking the dickens out of the Na2CO3 in the kiln. Sodium
carbonate will decompose above its melting point of 851 C (1564 F) and is on
a real roll by 1000 C (1832 F). Add a little silica to the mix, and as I
mentioned previously, the decomposition reaction will begin earlier, at
about 800 degrees C (1472 degrees F). We are having a typical well-known and
documented thermal decomposition reaction, plain and simple, as in
AB ---> A + B
MCO3 ---> MO + CO2 (where "M" is the metal)
Examples:
CaCO3 ---> CaO + CO2
?G (delta G) for the reaction is +131 at 25 degrees C.
According to you, Ivor, this reaction should never take place, yet it does,
over and over again as do these others out of examples from our ceramic
world:
MgCO3 ---> MgO + CO2 (at 900 C)
?G (delta G) is +49 at 25 degrees C.
CuCO3 ---> CuO + CO2 (at 500 C)
?G (delta G) is +370 at 25 degrees C
Li2CO3 ---> Li2O + CO2
BaCO3 ---> BaO + CO2 (at 900 C reduction)
CoCO3 ---> CoO + CO2 (at 880 C)
MnCO3 ---> MnO2 + CO2
SrCO3 ---> SrO + CO2
3FeCO3 ---> Fe2O3 +2CO2 + CO
?G (delta G) is +334 at 25 degrees C
Ivor, why do you think that sodium carbonate will behave any different than
the other carbonates? It doesn't. And, why confuse people with your
over-complicated jumble of tossed out theories? Please check facts first and
stick to fundamentals. I should not have to come back and explain simple
things to you (I should be mowing my grass before it gets too hot).
Are some of you as sick of science as I am? Then go straight to play and
make something! For inspiration (I love the vase in the photo) here's a nice
on-line Ceramics Monthly article entitled "Soda, Clay and Fire" by Gail
Nichols from April 14, 2007:
http://ceramicartsdaily.org/feature/featuredetail/20070414sodaclayfire.aspx
I am going to abandon this 'soda alternatives' thread and other related
threads now - it's causing me too much typing and I'm not utilizing soda or
salt firing techniques (at least not now or in the near future).
Ya all have fun!
Marian
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of Ivor and Olive
Lewis
Sent: Saturday, August 23, 2008 1:40 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Soda Breakdown
< (heat) Na2O + CO2 (g)
(sodium oxide production)>>
Dear Marian,
I think you need to have a look at the Thermodynamics of this
proposition. Methinks the reaction takes place in the reverse
direction. J. W. Gibbs Rules and all that nonsense.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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