[LMB] How demons work: what happened to the heat energy?

markus baur baur at chello.at
Wed Aug 3 21:16:03 BST 2016


Am 03.08.2016 um 21:43 schrieb Matthew George:
> The heat packs' ingredients are oxidizing iron, some fillers, and catalysts.
>
> I do wish you wouldn't switch back and forth between decimal points and
> commas.  Very misleading to casual reading.

thats what i get when i try to cater to the english 8) .. i am used to 
commas - the point is for me the deliniation for 10^3

> Isn't the heat capacity of water approximately 4.184 *J* per gram degree
> celsius, not *kJ*?  I think that would make your answer off by a factor of
> a thousand.

i just checked - and you are right, i am off by 10^3

i read 4.184 as four thousand one hundred and something (or 4.something 
kilo) - with the point delineating the 10^3

this is exactly why i was asking to check me 8)

.....


corrected version - but please keep checking - and i changed all commas 
to points

and the correction changes the reults a lot ..

the enthalpy of of Fe to Fe2O3 (main constituent of rust) is 822 kJ/mol

lets say this is a lightweight stabbing dagger weighing 111 gram (only
the iron parts)

one mol of Fe is 55,845 gram - so the dagger equals 2 mols of iron ..
total energy release is 1644 kJ

the specific heat of water is:

4.186 J / g / °C (for the liquid phase)

so the energy release of the rusting dagger is sufficient to change the
temperature of 393000 grams of water (about 393 liter) by
one degree celsisus - 1644 / 4.186 = 392.74

or change the temperature of 80kg of water (one human adult's worth) by 
about 4.9°C - and THAT change would be very noticable as it would raise 
the body temperature into lethal ranges, if you could distribute it evenly

or it would boil about 4 to 6 liters of water (depending on the starting 
temperature of the water) - again very noticeable

but nothing like these effects were reported

so we have to ask where all that energy goes ..

servus

markus

>
> Matt G.
> --
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