Born - Cl (g) 1/2 Cl 2(g) H = -120.5 kJ/mole Na (g) + 1/2...

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Born-Haber Cycle Lattice Energies can also be measured experimentally using Hess's law in what is called  the  Born-Haber Cycle . Recall that Hess's law tells us that the change in energy when going  from reactant to products is the same whether the reaction takes place in one step or in a  series of steps. For example, given the enthalpy changes for the following reactions we can  obtain the enthalpy change for the formation of the crystalline NaCl lattice. Cl - (g) Cl (g) + e - H = -E.A.(Cl) = 349 kJ/mole Δ Na + (g) + e - Na (g) H = -E.A.(Na) = -496 kJ/mole Δ Na (g) Na (s) H = -104.8 kJ/mole
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Unformatted text preview: Cl (g) 1/2 Cl 2(g) H = -120.5 kJ/mole Na (g) + 1/2 Cl 2(g) NaCl (s) H = -411.2 kJ/mole --------------------------------------------------------------------------------------------------------Na + (g) + Cl-(g) NaCl (s) H = -783.5 kJ/mole That is pretty close to our calculated number of -861 kJ/mole (< 10% error). But, of course, our first calculation was based on infinitely seperated ions at 0 K, and the Born-Haber cycle is based on reaction enthalpy changes at room temperature and pressure, so we expect some differences....
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