Re: Re; dead battery

Ian Gay <gay@xxxxxxx> writes:

Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx> wrote in
news:y9363xh3lye.fsf@xxxxxxxxxxxxxxxxxxxx:
Following Ian Gay's suggestion to use the Gibb-Helmholtz equation,
I looked some more at Chemical Principles. The long discussion
there doesn't seem to provide examples of exactly this kind of
problem, but I did find what might be a useful remark on p.655:
"Entropy, like enthalpy, is not very sensitive to temperature. The
disorder produced per unit of reaction at 298 K is approximately
the same as that produced at 1400 K."

Accordingly, when one writes G = H -TS, I'll assume (since nothing
I read made this completely clear, at least to me) that H and S
are constant. The CRC table gives delta H as -124.29 kcal/mol,
delta G as -111.18 kcal/mol and S as 12.68 cal/deg-mol for MnO2.
It gives delta H as -94.051 kcal/mol, delta G as -94.254 kcal/mol
and S as 51.06 cal/deg-mol for CO2. Solving the equation -124.29 -
.01268 T = -94.051 -.05106 T by hand (including long division),
I got a temperature of about 787.8 K, i.e. about 489.65 C. So, a
temperature of around 500 C should be enough to start producing
metallic manganese. [snip] Have I done this correctly?

No. Thermo tables give delta H and delta G of formation - the values
when compound is made from its elements. So elements have zero values
by definition. However, tables give _absolute_ entropy values, so the
elements do not have zero S. You need to include the entropies of C
and Mn in your calculation (probably a small correction ... entropy
of CO2 will be the dominant one).

Thanks, I'll take a look at the table again. I think I may also have
made a mistake in interpreting the value 787.8 I got. Instead of being
an absolute temperature, it might be the change of temperature from
298.15, in which case it would have been wrong to subtract the latter.
Also, even if it was right to subtract it, 298.15 isn't 0 C, so there
would be another correction.

If you know a book that does precisely this kind of calculation in all
detail [i.e. finding the elevated temperature at which a reaction such
as this will take place], can you please provide a reference?
--
Ignorantly,
Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx>
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.
.

Relevant Pages

  • Re: Re; dead battery
    ... above looks like it also rules out the reaction. ... "Entropy, like enthalpy, is not very sensitive to temperature. ... No. Thermo tables give delta H and delta G of formation - the values ...
    (sci.chem)
  • Re: Lizard engines and rat engines
    ... they increase the rate of entropy increase ... ... > But the rate of heat flow depends on "what is happening between the plates." ... Plot temperature of the liquid vs distance from the lower plate. ... More heat is flowing than was possible before convection cells ...
    (sci.bio.evolution)
  • Re: Slow Stealth
    ... that entropy at a higher temperature. ... You have to radiate more heat to ... Free energy has some technical meanings in thermodynamics, ... expensive to get rid of the entropy by increasing the temperature. ...
    (rec.arts.sf.science)
  • Re: global warming, program bug causes error in nasa calculations, 1998 no longer hottest year on re
    ... Here's a story of scientific investigation and discovery I'm proud to ... temperature sensor is close by. ... email to NASA GISS advising of the problem. ... stating that NASA made a correction not ...
    (uk.politics.misc)
  • Re: 2nd law of thermodynamics
    ... Being more ordered it has LESS entropy! ... energy, the entropy of this system *MUST* increase. ... not microscopic systems consisting of one or a few atoms. ... Temperature can be moved from place to place! ...
    (talk.origins)