What is the strongest endothermic reaction involving salt and liquid water?

[Alistair1992]

Hi,

I'm looking for the reaction that produces the largest temperature drop/absorbs the most energy from its surroundings. It needs to be some kind of dissolution process of a salt being mixed with liquid water. I've looked around on the internet, and it seems tricky to search for, and I don't know much about chemistry (being an engineer), so am struggling to calculate it from theory.

Thanks,

Alistair

 

[Borek]

I would start with CRC handbook or something similar - I doubt you will find much better choices than those already known and listed.

 

[Alistair1992]

Thanks. I've looked, and the strongest one I can find absorbs 403J/g from its surroundings. Ideally, I was looking for something over 1000 for a refrigeration idea.

 

[Borek]

Hard to find highly endothermic reactions, as for spontaneity they need a huge entropic factor (ΔG=ΔH-TΔS > 0). Perhaps something with a gaseous product could work, but simple dissolution doesn't sound likely to me.

 

[Quantum Defect]

There are commercial products available that do this. Ammonium nitrate salt is encapsulated in an inner plastic bag inside a second bag with water. Breaking the inner bag mixes the salt and water, and cools off. This salt has an enthalpy of solvation that is smaller than the enthalpy of crystallization. Many salts (e.g. NaOH) are the opposite -- i.e. they release heat upon dissolution.

c.f. http://www.pleasanton.k12.ca.us/avhsweb/cutter/Chemistry/Portfolio_files/Hot&ColdPacks.pdf

There is a common chemical demonstration where you freeze a flask to a bit of wood, using a reaction between ammonium nitrate and barium hydroxide.



Other spontaneous endothermic processes (evaporation -- think sweat and evapotranspiration)

 

[Borek]

Ammonium nitrate

325 J/g, three times less than OP asks for.