Enthalpy Changes in Solution
Ionic Compounds
Enthalpy of Solution
• Definition
– The enthalpy change that takes place when
1 mole of a solute dissolves in a solvent to form
an ‘infinitely’ dilute solution.
NaCl(s) + aq  NaCl(aq)
Hsol = +5 kJ mol-1
• Hsol - Enthalpy of Solution.
• Hsol can be exothermic or endothermic.
• Hsol can be measured experimentally.
Enthalpy Changes in Solution
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Enthalpy of Solution
• The sum of two imaginary steps
– reverse of the Lattice Enthalpy
(as we define it).
– the Hydration Enthalpy
(of the cations and anions).
• HLE
• Hhyd
- Lattice Enthalpy
- Hydration Enthalpy
Enthalpy Changes in Solution
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Lattice Enthalpy
• Definition
– The enthalpy change that takes place when
1 mole of a solid ionic lattice forms from its
gaseous ions.
Na+(g) + Cl-(g)  Na+Cl-(s) HLE = -776 kJ mol-1
• HLE is defined exothermically.
• The more closely ions pack together in the solid lattice
the more exothermic is HLE. The smaller and more
highly charged the ions (the greater their charge density)
the closer they pack.
Enthalpy Changes in Solution
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Enthalpy of Hydration
• Definition
– the enthalpy change that takes place when 1 mole
of gaseous ions become hydrated (surrounded by
water molecules).
Na+(g) + aq  Na+(aq)
Hhyd = - 406 kJ mol-1
• The smaller and more highly charged the ions (the
greater their charge density), the more exothermic is
Hhyd.
Enthalpy Changes in Solution
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Enthalpy of Solution
• Calculation
– the sum of two imaginary steps
Hsol = -HLE + (Hhyd(cation) + Hhyd(anion))
• standard conditions of temperature and pressure (298K,
1atm.) are implied above.
– Thermochemical definitions often refer to Standard Molar
Enthalpy Changes (changes measured under standard
conditions).
• an enthalpy diagram can be drawn to represent the
calculation.
– This is illustrated on the next slide.
Enthalpy Changes in Solution
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Enthalpy of Solution
Click to
complete the
diagram.
• Enthalpy Diagram
Na+(g) + Cl-(g)
Hhyd =
-771 kJ mol-1
-HLE =
+776 kJ mol-1
Na+(aq) + Cl-(aq)
NaCl(s) + aq
Hsol = +5 kJ mol-1
Hsol = -HLE + (Hhyd(cation) + Hhyd(anion))
Hsol = - -776 + -771
Enthalpy Changes in Solution
=
+ 5 kJ mol-1
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