When an ionic solid dissolves, the following must take place:
STEP 1: The ionic lattice breaks; ions are separated.
For example: NaCl (s) → Na+ (g) + Cl- (g) ΔH positive
Note that the reaction above is the reverse reaction of: Na+ (g) + Cl- (g) → NaCl (s) where ΔH negative = ΔH lattice
(see link for definition of lattice energy)
Therefore, ΔH positive of the reaction at the top equals to - ΔH lattice
STEP 2: The positive cation forms dipole-ion forces with δ-O of the water molecules and negative anions with δ+H atoms of the water molecules. This process is called Hydration of ions, which is exothermic. (see link for definition of enthalpy of hydration)
Hydration of ions
For example: Na+ (g) + Cl- (g) → Na+ (aq) + Cl- (aq) The above reaction is divided into two parts:
Na+ (g) + aq → Na+ (aq) ΔHhyd (Na+)
and
Cl- (g) + aq → Cl- (aq) ΔHhyd (Cl-)
On addition, the result reaction is:
NaCl (s) → Na+ (aq) + Cl- (aq) OR NaCl (s) + aq → NaCl (aq) ΔH solution
Therefore
ΔH solution = -ΔH lattice + ΔHhyd (Na+) + ΔHhyd (Cl-)
From this equation, it can be seen that:
· The more exothermic the lattice energy, the more endothermic (or less exothermic) the enthalpy of solution
· The more exothermic either of the hydration enthalpies, the more exothermic the enthalpy of solution
Endothermic dissolving occurs when: the magnitude of the lattice enthalpy is greater than the sum of the hydration enthalpies of the two ions.
See the following links for more information on:
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