# Endothermic Reactions

**URL:** <https://boards.straightdope.com/t/endothermic-reactions/231029>\
**Category:** Factual Questions\
**Created:** [February 25, 2004, 3:30am UTC](https://boards.straightdope.com/t/endothermic-reactions/231029 "2004-02-25T03:30:03Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\
**Post date:** [February 25, 2004, 3:30am UTC](https://boards.straightdope.com/t/endothermic-reactions/231029/1 "2004-02-25T03:30:03Z")

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I am looking for an endothermic chemical reaction like would be used in a chemical cold pack. I know KCl and water is endothermic but IIRC it dosent get real cold and not for very long.

Are there any other endothermic reactions that use relatively shelf stable materials, and can be drawn out pretty long, on say 8oz of reagents that can be contained in a plastic wrapper like a typical chemical cold pack.

In a nutshell whats the next step up from garden variety chemical cold packs.

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**Author:** ![scm1001](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@scm1001](https://boards.straightdope.com/u/scm1001)\
**Post date:** [February 25, 2004, 5:14pm UTC](https://boards.straightdope.com/t/endothermic-reactions/231029/2 "2004-02-25T17:14:52Z")

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fraid you may be out of luck. Endothermic reactions are rather rare, as the heat given off in exothermic reactions helps drive most reactions on. Only when there is a large entropic contribution can endothermic reactions prevail.

Ice/salt mixtures are a good way of cooling, but not practical in a two pack system. Expansion of gases is another

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [February 25, 2004, 6:50pm UTC](https://boards.straightdope.com/t/endothermic-reactions/231029/3 "2004-02-25T18:50:21Z")

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Urea and ammonium chloride are what’s used in commercial instant-cold packs.  
([here](http://www.google.com/search?q=cache:KxBe2_dMSakJ:www.visionlearning.com/library/science/chemistry-1/CHE1.8-equations.htm+%22cold+pack%22+%22ammonium+chloride%22&hl=en&ie=UTF-8)). There are other mixtures that’ll work, but I don’t recall what they are at the moment.  
Check to see if your library has the “Chemical Formulary” series. IIRC, there were several good recipes for endothermic mixes listed there.

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**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\
**Post date:** [February 26, 2004, 1:21am UTC](https://boards.straightdope.com/t/endothermic-reactions/231029/4 "2004-02-26T01:21:20Z")

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> [@scm1001](#):
>
> fraid you may be out of luck. Endothermic reactions are rather rare, as the heat given off in exothermic reactions helps drive most reactions on. Only when there is a large entropic contribution can endothermic reactions prevail.
> 
> Ice/salt mixtures are a good way of cooling, but not practical in a two pack system. Expansion of gases is another

I am familiar with adiabatic cooling and it would be inappropriate for the application I had in mind since it would require rigid containment for high pressure gas…although it may work to flash chill it before use…I will have to ponder this further.

I am trying something involving a chemical cold pack that would need to be no more than 4" Wide, 10" long, and about an inch thick. I was just curious if there was something out there but possibly financially infeasable for a typical cold pack.

Thank you for the link \*\*Squink[b/]