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Maya
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<<Actually, dissolving CaCl2 is exothermic. Try it out. It's part of the reason it's sold as ice-melt for driveways. I remember reading that
making a saturated solution can bring the temperature up to about 54C (not near boiling, but unexpectedly hot). Ammonium Nitrate will give you the
dead opposite. Sold as instant, one-use ice packs.>>
UC, you are wrong . CaCl2 does not melt ice because of exothermic rxn . It melts ice b/c it lowers the freezing point of the solution as it dissolves
which actually COOLS the ice even further ! But the freezing point goes down
even further the more it dissolves.
That is the principle behind determining Molar Mass by Freezing Point Depression. Basic 1st year chemistry.
http://chemistry.about.com/cs/howthingswork/a/aa120703a.htm
\"Prefiero ser yo extranjero en otras patrias, a serlo en la mia\"
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encipher
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Maya,
What UC said was technically correct. The dissolving of CaCl2 in H2O is an exothermic reaction, which releases heat. He also did state that it was
only part of the reason the ice melts. You were incorrect in saying that the dissolving of CaCl2 actually cools the ice even further. Freezing point
depression has nothing to do with how cold or hot the solution gets when a salt is dissolved, it has to do with the temperature at which the solution
freezes (which is lowered) or boils(which is raised).
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darkurza
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| Quote: | Originally posted by Maya
<<Actually, dissolving CaCl2 is exothermic. Try it out. It's part of the reason it's sold as ice-melt for driveways. I remember reading that
making a saturated solution can bring the temperature up to about 54C (not near boiling, but unexpectedly hot). Ammonium Nitrate will give you the
dead opposite. Sold as instant, one-use ice packs.>>
UC, you are wrong . CaCl2 does not melt ice because of exothermic rxn . It melts ice b/c it lowers the freezing point of the solution as it dissolves
which actually COOLS the ice even further! But the freezing point goes down
even further the more it dissolves.
That is the principle behind determining Molar Mass by Freezing Point Depression. Basic 1st year chemistry.
http://chemistry.about.com/cs/howthingswork/a/aa120703a.htm |
Maybe Calcium Chloride isn't road salt. But in Wiki, it says
'Calcium chloride reacts exothermically with water and can burn the mouth and esophagus.'
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UnintentionalChaos
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I never said anything about a reaction because no reaction is occuring. The calcium chloride is dissolving in water, BUT it first goes from anhydrous
(or dihydrate or tetrahydrate depending on how well you sealed the container) to hexahydrate, the bond forming process of which IS quite exothermic. I
am fully aware of feezing point depression, thank you very much.
Obviously, freezing point depression is why it keeps the ice melted later on, but gram for gram (and cm^3 for cm^3), you actually get more ions (and
thus a low freezing point) from normal NaCl. So why use CaCL2? First of all, you get the inital exothermic hexahydrate formation, quickly allowing the
ice to melt and the salt to disperse into solution. Second, CaCl2 is about twice as soluble in water as NaCl (at least at 20C, so I would imagine it
is still somewhat more soluble at 0) so you can, for a given mass of ice, lower the freezing point more (assuming you have all the CaCl2 you need)
than you could with NaCL. Thirdly (and don't quote me on this one), the calcium should be less harmful to plants than sodium and for anyone who is a
gardener or is a bit neurotic about their lawn, that claim makes all the difference in the world. Plus, It's sale as de-icer makes for cheap and
abundant dessicator material as well as a useful chemical.
Oh, and thanks for the support encipher, but the actual bond-breaking of dissolution (or melting, vaporization, etc.) is endothermic and will cool the
brine back down again. Try this with a salt that doesn't form hydrates first such as the very dramatic ammonium nitrate.
[Edited on 1-5-07 by UnintentionalChaos]
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Maya
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the de icer they use on planes and on roads is the hydrated form, I'm pretty positive about this.
If you are working with the anhydrous salt of course you are going to get heat from the formation of water of hydration, but I think what was intended
( as I assumed ) was the use of the fully hydrated form.
Drop that into a bucket of ice at 0 deg then mix and stick a thermometer in there and I'll wager a thousand plantains that your temp drops like a rock
[Edited on 5-1-2007 by Maya]
\"Prefiero ser yo extranjero en otras patrias, a serlo en la mia\"
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UnintentionalChaos
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I figure if it's sold as "heat-melt" and touted as getting hot as it dissolves, it cant possibly be hexahydrate. Maybe the salt you're used to is the
hexahydrate for the sole purpose of being highly soluble in water, probbaly much easier to store, and (maybe) less environmentally hazardous.
1000 plantains, eh? I wouldn't bet 244g of zinc and 6g of copper on that one ($1 US in post-1982 pennies  ) I'm not trying to argue with basic scientific principles. That would be daft (although if you are actually on to
something, you might just be the next einstein)
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mamakat
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science fair
hi my daughter and I are working on a science fair project and we want to make a heat pack that can be portable to use in emergency kits to boil
water. however we are going to be working in the kitchen and she is 11. I'm really concerned that the rest of the family might be upset if we blow up
the kitchen any very safe suggestions.
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Ozone
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mamakat,
Check out the very reaction you are looking for (page one, this thread), which Yntarius has made to work. It consists of:
1) iron powder or filings (steel is OK)
2) table salt (sodium chloride)
3) water
4) hydrogen peroxide (for this level of experiment over-the-counter (OTC) 3% is probably OK).
As I mentioned much earlier in the tread (and have been lazy to deliver) I will film this reaction tomorrow and post it so that is can be more easily
reproduced. This reaction is quite safe and easy to do (it is also a wonderful example of reduction/oxidation (REDOX) chemistry) We will just have it
get warm not boil water .
Best wishes,
O3
[Edited on 18-1-2007 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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franklyn
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Whoosh
Curious how the original proposition from the middle of last year by ' Yntarius '
enquiring about an improvised power source for a prime mover has become a
rant about melting ice
The notion of chemically boiling water for steam to drive a turbine is so
horrendously inefficient that it hurts my sensibilities merely contemplating such
a scheme.
An open Rankine cycle which is what this is known as in thermodynamics has to
operate at high temperature to produce useful power. Utility scale powerplants
operate with 400 degee steam. One would be lucky to obtain measurable torque
from the vapor of merely boiling water.
Actually 'Dr Beaker' had the right notion
| Quote: | Originally posted by Dr. Beaker
If you only need a power source to rotate a turbine then you don't really need
to use steam for it - you just need a reaction that produces gas.
Take powdered limestone (or sodium carbonate/bicarbonate) and add viniger or
diluted acetic acid. do it in a pressure proof vessels and use the gas pressure
to make electricity. |
but the wrong application.
To most efficiently couple the output one should use the pressure produced by
a confined gas producing reaction to act on a sealed liquid reservior and use
the high pressure fluid to drive a reversible positive displacement liquid pump to
drive a generator. This will produce arbitrarily high torque to the design limits of
the pump, for as long as the reservoir can supply fluid.
If a portable scheme is imagined one can envision a scheme whereby the
working fluid is captured in another empty reaction vesel to be subsequently
expelled back again into the first container in alternating cycles by a gas
generator delivering power sufficient to drive the vehicle. A similar mechanism
has been built and operated using liquid air as the gas source although it
used this directly to act on the pistons.
I am reminded of a proposed submarine that would porpoise diving at an angle
to the depth of 3000 feet where smokeless gun propellant would ignite to
purge the ballast tanks and the now bouyant submarine would rise at a angle
to the surface to repeat this process.
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