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Sugar water as a fuel?

Sugar water as a fuel? - Chemistry Forum

Sugar water as a fuel? - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #21  
Old 08-01-2003, 08:53 PM
David Lloyd-Jones
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Default Sugar water as a fuel?



Steve Turner wrote:

Yeah, yeah, blah-blah-blah. My question is, is there something
different between the two other than the fact that enthalpy of
formation has an arbitrary baseline?

-dlj.

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  #22  
Old 08-01-2003, 10:17 PM
G. R. L. Cowan
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Default Sugar water as a fuel?



David Lloyd-Jones wrote:

Well if you take calcium chloride as an example,
its enthalpy of formation is the distance on that baseline,
perhaps in units of kJ/mol,
between (Ca + Cl2) and CaCl2.

Its enthalpy of combustion would be a different distance,
the one between (CaCl2 + (1/2) O2) and (CaO plus Cl2).


--- Graham Cowan
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how cars gain nuclear cachet
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  #23  
Old 08-02-2003, 02:06 AM
Josh Halpern
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Default Sugar water as a fuel?



Roland Paterson-Jones wrote:

Possible, but possibly very confusing. The point being that you need one
reference zero for each type of atom under the same condition. Under
this restraint it is not clear why it is better to pick the zero for water
vapor rather than liquid water, assuming that the reference temperature
is ~298 K (and yes VA it used to be the case that one had tables for
different reference temperatures viz: US and English. The US was/is
298, the brits were (I think) 292 or 293. Bruce Hamilton and possibly
Marvin Margolis, or Fred Kasner will know. Moreover, if you are
going to rely on oxides, which oxide are you going to pick. Not
obvious in most cases.

OTOH, if you stick to the normal convention of picking the most
common elemental form at room temperature, it is fairly clear.

josh halpern


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  #24  
Old 08-02-2003, 08:15 AM
Roland Paterson-Jones
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Default Sugar water as a fuel?

"Joćo Antonio" <jas_bomfim@uol.com.br> wrote in message
news:bgehlf$gmn$1@news.mc.ntu.edu.tw...
'Exergy

I think you are missing the point. The power of state functions such as
enthalpy and entropy, is that you can ignore the process involved, but yet
still obtain the correct answer.

If you want to be truly pedantic with suchrose, for example, you have to
consider all combinations of all reactions of all interim products. Remember
that long chain hydrocarbons are built up stepwise by reaction, and there
are many paths to a long chain.

So, even you are tacitly ignoring the real chemistry when you use the
summary formula:

C12H22O11 + O2 <-> CO2 + H20

You may not be aware of your simplification, but it is there, nevertheless,
and demonstrates the power of state functions such as enthalpy.


There was no deliberate redefinition of terms, but I was being loose with my
terms. I admire people who can address deeper issues and gloss over the
semantic typo's. People like your mate, who jump in and show their undoubted
mastery of the jargon without even attempting to address the actual
question, are simply wankers.

combustion".

No habla german, I'm afraid. I am african, so my problems are even greater.
We don't even have native word for enthalpy

Are you the dudes that use alcohol in your cars? No wonder Brazil is such a
festive country. Rush hour traffic must be an instant drunken orgy with all
those fumes.

Regards
Roland


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  #25  
Old 08-02-2003, 08:25 AM
Roland Paterson-Jones
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Default Sugar water as a fuel?

"Josh Halpern" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...
Roland Paterson-Jones wrote:
(1988).
C(s,graphite)

Josh: "Possible, but possibly very confusing. The point being that you need
one
reference zero for each type of atom under the same condition. Under
this restraint it is not clear why it is better to pick the zero for water
vapor rather than liquid water, assuming that the reference temperature
is ~298 K (and yes VA it used to be the case that one had tables for
different reference temperatures viz: US and English. The US was/is
298, the brits were (I think) 292 or 293. Bruce Hamilton and possibly
Marvin Margolis, or Fred Kasner will know. Moreover, if you are
going to rely on oxides, which oxide are you going to pick. Not
obvious in most cases.

OTOH, if you stick to the normal convention of picking the most
common elemental form at room temperature, it is fairly clear."

Roland: It's a question of application. In power engineering (and I know
that you true chemists spit on us), we are almost always interested in
combustion reactions. In these, it's just a pain in the but to do the
arithmetic with H2O(g) and CO2(g) which occur with monotonous regularity in
all combustion reactions.

Much handier is to regauge the enthalpy (and exergy) tables to move the C
and H zero enthalpy points from graphite and H2(g) to CO2(g) and H2O(g),
respectively.

In this way, a single glance at enthalpy tables of hydrocarbons will give
you lower heating value (i.e. heat of combustion). With C(s,graphite) and
H2(g) as zeros, you have to balance the combustion reaction, and do some
trivial, but annoying, maths.

And, sorry again for my abuse of terminology. The numbers I gave for lower
heating values etc. should be correct, however.

Roland

--
Roland and Lisa Paterson-Jones
Forest Lodge, Stirrup Lane, Hout Bay
[Only registered users see links. ]
mobile: +27 72 386 8045
e-mail: [Only registered users see links. ]


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  #26  
Old 08-02-2003, 08:42 AM
Roland Paterson-Jones
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Default Sugar water as a fuel?

"G. R. L. Cowan" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...


Nope, not necessarily. You could also use CaO as your enthalpy zero for Ca,
according to your whim.


With hydrocarbons, your oxidation and formation equations are (almost)
always the same.

You seem to be using 'enthalpy of combustion' as a formal term for heat of
oxidisation reaction. Is this standard terminology?

Roland

--
Roland and Lisa Paterson-Jones
Forest Lodge, Stirrup Lane, Hout Bay
[Only registered users see links. ]
mobile: +27 72 386 8045
e-mail: [Only registered users see links. ]


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  #27  
Old 08-02-2003, 12:49 PM
G. R. L. Cowan
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Default Sugar water as a fuel?



Roland Paterson-Jones wrote:

But I mentioned only the energy difference between two states,
which as you say is independent of where one sets the zero.


Yes.


--- Graham Cowan
[Only registered users see links. ] --
how cars gain nuclear cachet
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  #28  
Old 08-02-2003, 01:21 PM
G. R. L. Cowan
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Default Sugar water as a fuel?



Roland Paterson-Jones included:

But my whim is to be understood,
so I use the zero almost everyone uses.


This is a formation reaction for methane:

C + 2 H2 ---> CH4

By your convention, we have +393.5 kJ/mol from carbon,
+483.7 kJ/mol from hydrogen, left side sum +877.2 kJ/mol.

On the right, 802.3 kJ. So this methane-forming reaction
has delta 'H' of -74.87 kJ/mol.

Its enthalpy of combustion is, I guess you'll agree, -802.3 kJ/mol.



--- Graham Cowan
[Only registered users see links. ] --
how cars gain nuclear cachet
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  #29  
Old 08-02-2003, 02:39 PM
Josh Halpern
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Default Sugar water as a fuel?



Roland Paterson-Jones wrote:

True, but this makes it a pretty specialized tool, and pretty much
limited to hydrogen and
maybe the first two rows with p shells. If you are only interested in
heating value
then instead of screwing enthalpy around you might as well just have a
table of
combustion reaction enthalpys for C, and H. For fuels you might have to
stick
in N, and S, but those have several oxides and that alone will start to
make
trouble.

Then again, why do this at 298 K why not 100 C?or whatever the average
temperature of the flue gas is when it exits the last heat exchanger,
then you
could have a separate table for every plant. Wouldn't that be easier. It
would certainly be fun.

Standards have to combine convenience with universality, which is why
separate definitions of the zeros for enthalpies of formation are not very
useful. As is clear from this discussion, it impedes discussion.

If one wanted, one could define enthalpys of formation based on the set of
nuclear reactions that build atoms. Then the only zero would be H. Sort
of the same problem in a different direction.

Josh Halpern

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  #30  
Old 08-02-2003, 02:42 PM
Josh Halpern
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Default Sugar water as a fuel?



Roland Paterson-Jones wrote:


Not my point, my point being WHICH oxide do you choose as the zero
for that atom. In the case of say Ca, would it not make more sense
to pick a carbonate rather than the oxide using your criteria. What
about nitrogen, or sulfur, which oxide do you pick and why. Can
you state a terse criteria to students which would explain which oxide
you are picking for the zero of that element? One can do that for
elemental forms.

josh halpern


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