sir,

can u plz explain me the born haebers cycle and how it is used to calculate lattice enthalpies??? i am not able to apply it??

Dear ocean,

Based on Hess's law Born and Haber in 1919 developed a simplified and cyclic method to correlate lattice energies of ionic crystals to other thermodynamic data. For example, the net energy changes during the formation of sodium chloride from metallic sodium and chlorine gas can be represented by DH_f·

The overall process is thought to take place in following intermediate steps:

Intermediate steps

(i) Metallic sodium into gaseous sodium atom

The energy required per mole of sodium is 'enthalpy of sublimation' which is represented by (Hs). This step is energy consuming process.

(ii) Dissociation of chlorine molecule into chlorine atoms

The energy required per mole of chlorine is 'enthalpy of dissociation' represented by (_d)

(iii) Gaseous sodium atom into gaseous cation

The energy required in this process is called Ionization energy (IE).

(iv) Gaseous chlorine atom into gaseous anion

This step involves the release of energy referred as Electron Affinity (EA).

(v) Combination of oppositely charged gaseous ions to form solid crystal

This involves the release of energy referred as lattice energy (U).

The various energy changes in different steps are as shown:

Born Haber Cycle for NaCl

The sum of the energy changes taking place during various steps is equal to DH_f i.e., heat of formation of NaCl_(s) according to Hess' Law.

Please feel free to ask your queries here. We are all IITians and here to help you in your IIT JEE preparation.

All the best.

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Sagar Singh

B.Tech, IIT Delhi