✔ 最佳答案
first, Hung Pan is COMPLETELY wrong.
- ion-dipole interaction cannot be stronger than the ionic bond. instead, hydration of ions is exothermic.
still, dissolution of ionic compounds can be exo- or endothermic, depending on relative enthalpy of hydration AND breaking of lattice of ions.
- only a little water is added. salt is NOT dissolved. instead, this only hydrates the ions inside, giving water of crystallization.
Brian may be correct, but he's not really explaining the situation. he just say "this reaction is exothermic because the reverse reaction is endothermic".
but WHY this reaction is exothermic? he didn't explained.
here's my explanation.
as mentioned above, ions are separated (a little bit) and energy is absorbed.
the breaking of ion lattice is endothermic; at this point, you understand that.
next... what happens after ions are separated?
the naked ions have high charge density. water molecules will surround the ions, forming (dative) bonds.
naked ions become hydrated ions, and the hydration process is exothermic.
finally, remember that copper(II) sulphate is NOT dissolved; it's still solid copper(II) sulphate-5-water, the hydrated salt.
the hydrated ions recombine to give new lattice for this "new" salt. the recombination is exothermic as lattice is formed.
surely these two lattice energy may not be the same, but overall speaking, there's net energy release. thus temperature increases.
lattice of anhydrous ion ---(a little water)---> separated, hydrated ions ---(recombination)---> lattice of hydrated ions