Q. 60

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).

Answer :

Na(s) + 1/2 Cl(g) NaCl(s) fH


BORN-HABER CYCLE is an enthalpy diagram which is an indirect method of calculating lattice enthalpy.


One mole of NaCl(s) is prepared by the following steps:


FIRST STEP: Sublimation of Na (sodium) (subH)


• Na(s) Na(g)


SECOND STEP: Ionization of sodium (iH)


• Na(g) Na+ + 1e-


THIRD STEP: Dissociation of chlorine molecule to a chlorine atom (aH)


• 1/2 Cl2(g) Cl(g)


FOURTH STEP: Chlorine atom gains 1e- (egH)


• Cl(g) + 1e- Cl-


Na+(g) + Cl- (s) NaCl(s) latticeH


The sequence of the steps is known as BORN-HABER CYCLE. The importance of this cycle is, the sum of enthalpy changes round a cycle is zero.


Applying Hess’sLaw


Hess’s law state that if the reaction takes place in many steps, then the standard enthalpy of the reaction is the sum of the standard enthalpies of intermediate reaction, into which overall is divided at the same temperature.


fH = subH + iH + aH + egH + latticeH


latticeH = subH + iH + aH + egH - fH


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