The lattice enthalpy of an ionic compound is the enthalpy when one mole of an ionic compound present in its gaseous state, dissociates into its ions. It is impossible to determine it directly by experiment. Suggest and explain an indirect method to measure lattice enthalpy of NaCl(s).

Answers (1)

When one mole of an ionic compound dissociates into its ions, the enthalpy change related to this process is called lattice enthalpy.

Na^{+} Cl^{-}\rightarrow Na^{+} + Cl^{-}

\Delta H_{lattice}= 788 kJ/mol

Lattice enthalpy for NaCl is calculated as follows:

1) Sublimation of sodium metal

Na(s)\rightarrow Na(g)

2) Ionization enthalpy: ionization of sodium atoms

Na(g)\rightarrow Na^{+}(g)+e^{-1}(g) 

3) Bond dissociation enthalpy: dissociation of chlorine

\frac{1}{2}Cl_{2}(g)\rightarrow Cl^{-}(g)

4) Electron gain enthalpy: electron gain by chlorine

Cl^{-}(g)+e^{-1}(g)\rightarrow Cl(g) 

5)Na^{+}(g)+Cl^{-}(g)\rightarrow Na^{+}Cl^{-}(s)

These steps collectively form the BORN-HABER Cycle, wherein the sum total of enthalpy changes round a cycle is equal to 0.

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