Enthalpy change of hydration and lattice energy
- The standard enthalpy of solution (ΔHsolꝋ) is the energy absorbed or released when 1 mole of ionic solid dissolves in enough water to form a dilute solution (under standard conditions)
- The ΔHsolꝋ can be either exothermic or endothermic
- For example, the ΔHsolꝋ of sodium chloride (NaCl) is +3.9 kJ mol-1
NaCl (s) + aq → NaCl (aq)
OR
NaCl (s) + aq → Na+ (aq) + Cl- (aq)
- This means, that 3.9 kJ mol-1 of energy is absorbed when one mole of NaCl is dissolved in enough water to form a dilute solution
ΔHsolꝋ = ΔHhydꝋ - ΔHlattꝋ
- The lattice (formation) energy is the energy released when gaseous ions combine to form one mole of an ionic compound under (standard conditions)
- Since energy is released when an ionic compound is formed, the ΔHlattꝋ is always exothermic
- For example, the ΔHlattꝋ of NaCl is -787 kJ mol-1
Na+ (g) + Cl- (g) → NaCl (s)
- This means, that 787 kJ mol-1 of energy is released when NaCl is formed from its gaseous ions
- The standard enthalpy of hydration is the energy released when gaseous ions dissolve in enough water to form a dilute solution (under standard conditions)
- Since energy is released when gaseous ions become hydrated, the ΔHhydꝋ is always exothermic
- For example, the ΔHhydꝋ of the sodium (Na+) ion is -406 kJ mol-1
Na+ (g) → Na+ (aq)
- This means, that 406 kJ mol-1 of energy is released when Na+ ions become hydrated