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).
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• Na (s) + Cl (g) → Na+(g) + Cl (g) ; ∆latticeH 2 2 83 Thermodynamics • Bonn - Haber Cycle • Steps to measure lattice enthalpy from Bonn - Haber cycle • Sublimation of sodium metal (1) Na(s) → Na (g) ; ∆sub H (2) Ionisation of sodium atoms Na(g) → Na+(g) + e–(g) ; ∆tH i.e., ionisation enthalpy (3) Dissociation of chlorine molecule 1 1 V Cl (g) → Cl(g) ; ∆ H i.e., One-half of bond dissociation 2 2 2 bond enthalpy. (4) Cl(g) + e–(g) → Cl–(g) ; ∆egH i.e., electron gain enthalpy. + Na (g) + Cl (g) 1 V V ∆ H ∆egH 2 bond + 1 Na (g) + Cl2(g) – Na+(g) + Cl (g) ∆iH Na(g) + Cl (g) 2 2 V ∆latticeH ∆subH Na(s) + Cl (g) 2 2 ∆f H NaCl(s)