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What's going on in an activated instant ice pack?


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#1 mrblond5311

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Posted 6 March 2006 - 05:54 AM

Hey,
I was just wondering what process of reactions is going on in an instant ice pack. I've included a link with all the ingredients.

"Endothermic composition and cold pack
Document: United States Patent 4081256
Abstract: A particulate composition producing an endothermic reaction when mixed with water, and a cold pack containing such a composition, and water isolated therefrom until the intended time of use. The composition consists essentially of, in parts by weight, about 65 to about 130 parts urea, about 35 to about 80 parts hydrated sodium acetate (Na C.sub.2 H.sub.3 O.sub.2 .multidot.3H.sub.2 O), about 18 to about 40 parts potassium chloride, potassium nitrate, or mixtures thereof, about 18 to about 30 parts ammonium chloride, and about 6 to about 10 parts quar gum. A cold pack comprises an outer sealed, flexible, impermeable enclosure containing the composition, and an inner sealed, flexible, impermeable, easily ruptured enclosure containing about 85 to about 140 parts by weight water. The pack reaches and maintains a temperature of about -7.degree. C for up to 30 minutes, and is moldable to conform to the surfaces being cooled."

http://freepatentson...om/4081256.html
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#2 woelen

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Posted 6 March 2006 - 07:39 AM

The solids in the inner bag dissolve in water and due to this, the water cools down. Some compounds take up quite a lot of heat while they dissolve. You can experience this effect yourself somewhat by dissolving a lot of table salt in water. Even better is dissolving ammonium nitrate (from fertilizer). You certainly will notice the cooling down. The mixture in the bags is chosen such that the maximum amount of solid can dissolve in water at a fairly constant rate, the solids being such that a lot of heat is taken up.

The opposite effect also is quite common. If you dissolve e.g. sodium hydroxide (drain cleaner), then you'll notice a strong heating of the solution.
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#3 mrblond5311

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Posted 6 March 2006 - 08:09 AM

The solids in the inner bag dissolve in water and due to this, the water cools down. Some compounds take up quite a lot of heat while they dissolve. You can experience this effect yourself somewhat by dissolving a lot of table salt in water. Even better is dissolving ammonium nitrate (from fertilizer). You certainly will notice the cooling down. The mixture in the bags is chosen such that the maximum amount of solid can dissolve in water at a fairly constant rate, the solids being such that a lot of heat is taken up.

The opposite effect also is quite common. If you dissolve e.g. sodium hydroxide (drain cleaner), then you'll notice a strong heating of the solution.



Thanks for the help. Silkworm and I were talking about this and he was wondering what is reacting with what in the bag; as far as the processes go. You have 2 ammoniums, 2 chlorides, an acetate, and a nitrate. It seems like they over did it to me. Any help will be appreciated!
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#4 silkworm

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Posted 6 March 2006 - 08:30 AM

The solids in the inner bag dissolve in water and due to this, the water cools down. Some compounds take up quite a lot of heat while they dissolve. You can experience this effect yourself somewhat by dissolving a lot of table salt in water. Even better is dissolving ammonium nitrate (from fertilizer). You certainly will notice the cooling down. The mixture in the bags is chosen such that the maximum amount of solid can dissolve in water at a fairly constant rate, the solids being such that a lot of heat is taken up.


So, what you're saying this is a physical phenomenon and no chemical reactions are taking place?

#5 RyanJ

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Posted 6 March 2006 - 11:57 AM

So, what you're saying this is a physical phenomenon and no chemical reactions are taking place?


Well, as with anything its a shady area. There are a transfer of electrons for for Annonium Nitrate to dissolve and so it ca be cosidered a reacion on some level (I think we have this debate before somewhere).


Its kind of interesting because dissolving is noramally an exothermic process :)

Cheers,

Ryan Jones
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#6 woelen

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Posted 6 March 2006 - 02:24 PM

I would call this a physical process, but indeed, it is a little bit on the border. Two kinds of energy are involved, lattice energy and hydration energy.

When a solid dissolves, then energy is needed to break down the lattice, in which the molecules (or ions) reside. On the other hand, energy is released by the hydration of these ions. An example:

NaOH contains ions Na(+) and OH(-). On dissolving you have three processes with a certain energy balance:

Na(+)/OH(-) + energy -----breakdown of lattice----> Na(+) + OH(-)
Na(+) ----solvation----> Na(+)(aq) + energy
OH(-) ----solvation---->OH(-)(aq) + energy

For NaOH, the balance is positive, especially the part of solvation (hydration) of OH(-) gives a lot of enerhy. For NaCl the balance is slightly negative. For NH4NO3 the balance is even more negative.

In the instant cold packages, I would not speak of a real chemical reaction, but one could regard the solvation process also as a chemical reaction.

================================================================================

Dissolving of some compounds, however, really is a chemical reaction. If you dissolve anhydrous CrCl3 in water, then there is a real chemical reaction. The lattice contains molecules (almost purely covalent units) of CrCl3. When these are dissolved then the following process occurs:

CrCl3 ---solvation-----> Cr(3+)(aq) + 3Cl(-)(aq)

Here, the covalent units are split in ions. This is a true chemical reaction and that also is clear from the properties of CrCl3. CrCl3 is a pink/blossom solid, while the usual hydrated form is a dark green solid. Other examples of such chemical reactions are: dissolving of H2SO4, dissolving of solid anhydrous NiSO4.
Dissolving of hydrated CrCl3.6H2O again can be regarded as a borderline process between chemical and physical change, just as dissolving of table salt.
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#7 BombFanatic

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Posted 10 March 2006 - 01:45 AM

CHEMISTRY EXPERIMENTS
MATERIALS
REQUIRED:
Ammonium Nitrate

Instant Cold! Mix just a small amount of water with Ammonium Nitrate, and the temperature immediately drops to near freezing! Make your own instant Ice Packs! An easy and surprising experiment.


I got this from United Nuclear.

http://unitednuclear.../experindex.htm
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