Funny, I asked my advanced inorganic professor this yesterday, and couldn't find the enthalpy of solvation of ammonium nitrate.
We studied NaCl as the example, and that had an enthalpy of solvation of 4kJ/mol, yet water doesn't get "cold" when you dissolve salt in it, I am surprised to hear ammonium nitrate's is so low??? 6.6kj/mol? That's hardly that much more than NaCl, yet it's used in cold packs?
Is there something else going on here?