Re: Re; dead battery

Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx> wrote in
news:y93abmy9m0t.fsf@xxxxxxxxxxxxxxxxxxxx:


Ian Gay <gay@xxxxxxx> writes:

Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx> wrote in
news:y9363xq8i0s.fsf@xxxxxxxxxxxxxxxxxxxx:
Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx> writes:

Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx> writes:
I was just reading the discussiong of manganese in the CRC
book. Apparently it was first extracted from manganese
dioxide by reducing with carbon. If the MnO2 in the battery
comes with powered carbon, as some have told me it does to
improve conductivity, then after the KOH and Zn are removed,
maybe one is nearly set up to produce elemental manganese.

It says that manganese was first separated by Gahn in 1774.
Where can I find a copy of whatever Gahn might have written
about this?

According to the CRC book, the enthalpy of formation of CO2 is
-94.051 kcal/mol while that of MnO2 is -124.29, so the reaction
MnO2 + C -> Mn + C02 -124.29 kcal/mol + 94.051 kcal/mol looks
like it would not go forward. I don't do stuff like this very
often, so maybe I'm just making a stupid mistake. If not, why
was Gahn able to isolate Mn in this way?

1. Delta H's are in general T dependent, and CRC only tells you
the 298K value.

2. This reaction has a large positive delta S, due to formation of
gaseous product. You need to consider the delta G for the
reaction, not just the delta H.

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.

Again, you have got a delta G value for 298K. Use the Gibbs-Helmholz
equation to extrapolate to a more reasonable (higher) temperature.

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