Re: Re; dead battery

Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx> writes:

The same table gives the corresponding values of delta G as
-111.18 kcal/mol for MnO2 and -94.254 for CO2. So the argument
above looks like it also rules out the reaction. At the moment,
all I have are numbers, not insight. I'm going to take Chemical Principles,
by Dickerson, Gray and Haight, 3d ed, with me today and read its discussion
of free energy and its role in chemical reactions. Maybe tomorrow I'll
know how to do this.

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.

In another thread, I asked about the 19th century terminology which described
temperatures in terms such as "au rouge sombre" but never got any explicit
temperatures. I recently looked at a Keithley application note for the
Franck-Hertz experiment, which includes a short table of colors and
temperatures. It reads:

Cathode Color Temperature
First visible dull red ~ 550 C
Bright red ~ 900 C
Yellow-red ~ 1100 C
"Incipient white" ~ 1300 C

So, it seems that I would want, if I were really going to make the manganese,
to heat the MnO2 and carbon mixture "au rouge sombre".

Have I done this correctly?
--
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.
.