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Born Haber Cycle

It is known that the elements combine to form a compound only if the overall process results in the lowering of energy. When the elements combine to form one mole of the compound, then the net-amount of energy released is calculated. More the energy released, stronger is the bond. Lattice energy of a compound is defined as the amount of energy released when one mole of the compound is formed due to electrostatic attraction between oppositely charged ions. Consider the formation of sodium chloride. The overall process is called Born-Haber Cycle.

 ΔHo f = ΔHoS + ΔHo D/2 + ΔHo I.E + ΔHo E.A. + ΔHo U

Here +ve sign shows that energy is absorbed and -ve sign shows that energy is released.
Thus, the heat of formation or net amount of energy released (Lattice energy) in the formation of one mole sodium chloride is given below:
ΔHof = 108.70 + 122.0 + +495 -349 + ΔHo U
-411.3 = 108.70 + 122.0 + +495 -349 + ΔHo U
∴ ΔHo U = -788.0 kJ/mol

Substituting the values with proper signs; -ve for energy released and +ve for energy absorbed, we get
ΔHf = 108.5 + (243.0) + 495 + (-348) + (-758.7)
      = 108.5 + 121.5 + 495 - 348 - 758.7 = -318.7 kJ mol-1
More the energy released, greater is the stability of the compound.

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