Suggest why ammonium nitrate is soluble in water at room temperature despite the fact the enthalpy change of solution of ammonium nitrate is endothermic. Suggest also why it becomes very soluble at 75ºC.
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Answer:
∆H(sol) = ∆H(hyd) - ∆H(latt)
∆H(sol) is endothermic => L.E of ammonium nitrate is more exothermic than ∆H(hyd).
[note that both L.E and ∆H(hyd) are always negative. so when L.E is more exothermic, i.e higher magnitude than ∆H(hyd), ∆H(soln) becomes more positive]
When ammonium nitrate is added to water, the disorder of breaking lattice is greater than the ordering of water molecules about in aqueous ions. ∆S is therfore positive as the system becomes less orderly and so ∆G is negative (since ∆G = ∆H - T∆S).
The reaction is feasible and takes place spontaneously. Hence ammonium nitrate dissolves in water at room temperature.
As temperature T increases, the term (-T∆S)becomes more negative. Hence, ∆G becomes more negative and so, the reaction is more feasible and spontaneous. The solubility of ammonium nitrate increase with temp.
