# What makes a heat pack heat?



## sbugir (Oct 16, 2009)

Just curious as to what makes a heat pack heat...I'm going to guess it's sodium acetate, and when pressure is applied the acetate crystalizes and gives off an exothermic reaction? What do you think or for that matter know?

I've never seen or used one


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## cloud jaguar (Oct 16, 2009)

i think it is magic


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## sbugir (Oct 16, 2009)

Hee hee like I said, I've never seen one so it may be the case... Go magic  

Oh God, Phil's been reading this for over three minutes, get ready for an elaborate answer


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## PhilinYuma (Oct 16, 2009)

lemmiwinks said:


> Just curious as to what makes a heat pack heat...I'm going to guess it's sodium acetate, and when pressure is applied the acetate crystalizes and gives off an exothermic reaction? What do you think or for that matter know? I've never seen or used one


You've got it right!


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## sbugir (Oct 16, 2009)

PhilinYuma said:


> You've got it right!


hmm, then what determines the time factor? And what's with the not so elaborate explantation  ? You have me slightly worried Phil.


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## PhilinYuma (Oct 16, 2009)

lemmiwinks said:


> hmm, then what determines the time factor? And what's with the not so elaborate explantation  ? You have me slightly worried Phil.


Hah! Good try! You thought that you were going to lure me into a discussion of the Second Law of Thermodynamics, didn't you? Unfortunately for you, I am trying to make some apple turnovers with Craisins for Sunny, so you're outta luck! The time it takes for the exothermic reaction to take place is "pretty quick," and the time that it lasts depends on the size and temperature of the environment in which it takes place. I like the magic idea, though.  

Here's the recipe for the turnovers:

2 cubed Golden Delicious (or similar) apples

1/4 cup maple syrup

1/4 cup chopped walnuts

1/4 cup Craisins

11/2 cups grated sharp cheddar cheese

1 egg, beaten (for glaze)

Some salt and sugar

Enough puff pastry to satisfy the 2nd Law of Thermodynamics

Good luck!


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## Katnapper (Oct 16, 2009)

So can you substitute those turnovers for a heat pack?


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## PhilinYuma (Oct 16, 2009)

Katnapper said:


> So can you substitute those turnovers for a heat pack?


I'm not sure that they'd heat as well, but they'd sure taste better!


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## sbugir (Oct 16, 2009)

Darn you Phil, you're my Chem book...it's too far away! BTW, those apple turnovers sound delicious... Perhaps you could explain the thermodynamics involved with turnovers?


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## Rick (Oct 17, 2009)

Some kind of chemical reaction that is triggered by being exposed to air.


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## ddvw123abc (Oct 17, 2009)

Isnt it like two seperate pouches inside like water in one and like some substance in another and when you squeeze it the barrier breaks and they mix together to make it cold or hot


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## Rick (Oct 17, 2009)

ddvw123abc said:


> Isnt it like two seperate pouches inside like water in one and like some substance in another and when you squeeze it the barrier breaks and they mix together to make it cold or hot


Not a heat pack. There is just some sort of dry mixture inside.


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## sbugir (Oct 17, 2009)

Yeah, Phil already confirmed it's a sodium acetate reaction, hence the substance and water... or w/e, I'm curious for the duration, I'm going to guess it's simply the amount of acetate to water?


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## PhilinYuma (Oct 17, 2009)

Rick said:


> Not a heat pack. There is just some sort of dry mixture inside.


Years ago, I got into a Chicago school bus to accompany one of my kids, under ten at the time, on some trip. To my horror, when I sat down, I found that there was a hole in the floor of the bus large enough for a kid to stick his leg through. The chemistry that caused this hole, the corrosive oxidation of iron (rust) accelerated by the presence of sodium chloride, is exactly the same as that used in the "one time only" heat packs. The oxidation of iron to ferrous oxide (rust, again!) is exothermic, ie, it gives off heat, but usually the process is so slow that we don't notice. In theory, at least, finely divided iron powder is brought in contact with a salt (sodium chloride) solution in the presence of oxygen (air) that causes rapid oxidation of the iron, with the release of heat, until it is all converted to a nasty mixture of hydrated iron oxides and ferrous hydroxide, Fe2O3·nH2O FeO(OH).

BUT, I don't think that this makeshift equation accounts for the speed at which this reaction takes place in a hot pack. Perhaps, after all, it is due to magic.

The phase shift in sodium acetate, from anhydrous to crystalline is used in those rechargeable hot packs that Old Folks use (they are streaming into Yuma now that the weather is more suitable for their aged bones). Phase shifts, like that from ice to water, or water to steam, are much more fun, but, alas, a different subject altogether. One reason that there was a hole in my bus was that salt, aside from being highly corrosive, lowers the phase shift of water, so that it becomes ice at a lower temperature. Seriously folks, science is all around us, and how cool that is!


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## sbugir (Oct 17, 2009)

I actually understood the majority of that  . Phil like I've said before, your explanations are beloved and so are your stories as well as apple turnovers...at least by me   ^_^


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