jgourlay - 18-12-2008 at 08:06
Quick question: is the crystallization of a salt from a supersaturated solution (saturated at boiling, cooled to room temp) an endothermic or
exothermic event?
DJF90 - 18-12-2008 at 08:25
I think it may depend on the salt? Although if all salts behaves the same I would say exothermic as enthalpy of lattice formation is always negative,
and although this applies to forming a lattice from gaseous ions, I beleive it could in principle be applied to a supersaturated solution as there is
more solute than there should be and so not all of it can be properly solvated? Also the crystallisation of NaOAc is exothermic ("hot ice") and
although this does not mean crystallisation of other salts from supersaturated solutions are also exothermic, it is an example of one that is.
12AX7 - 18-12-2008 at 13:03
Depends on heat of solvation. Salts like sodium chlorate and ammonium nitrate not only dissolve readily but absorb a lot of heat in so doing. The
statistical mechanics description of this would be, there are more states available (I suppose states of water molecules moving around ions and ions
moving through water), therefore the heat energy has spread out into those available states, with the result that the apparent temperature has
dropped. If you were to seperate them again, let's say by evaporating the water (being careful to subtract the heat of vaporization carried away in
so doing) and crystallizing the salt, it should heat up, because you are removing those extra states and constricting the heat energy, resulting in an
apparent excess.
Ironically, this suggests if there were a material that, when dissolved in water, it absorbed more energy than it takes to boil the same amount of
water (notice it would get *extremely* cold, including freezing the water, so such a material probably cannot exist), a concentrated solution of it
could be heated to the boiling point and evaporated, the heat source removed, and all the water would exothermically evaporate away as the steam
leaves crystalline material behind. In a less extreme case, this in general suggests that such materials aren't all that fond of water; but we know
not only that ammonium nitrate takes some heat to crystallize, but that it's difficult to remove all the water, and it is, in fact, so fond of water
that it absorbs it from the atmosphere!
I hate thermodynamics...
Tim