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In this section we will discuss Lattice Energy, the change in energy when
ions are packed together to form ionic solids. One way to determine
Lattice energy is with Coulomb's law, which states that:
Lattice
energy= k( Q1Q2/r)
where Q is the numerical charge, r is the shortest distance between nuclei
and k is Coulomb's constant.
When dealing with Lattice energy, you are usually asked to find the
overall energy in a reaction which takes a lot more work. In this form,
you calculate the energy for every step of the reaction and find the
overall energy change, as in this example:
Li (s)+ 1/2 F2(g) ----> LiF(g)
1. Sublimination of Li (s) to Li(g) = 161kJ
2. Form Li+ ions, Li (g) to Li+(g) + e- = 520 kJ
3. Breaking F-F bond, 1/2 F2 (g) to F (g) =77 kJ
4. Form F- ions, F(g) = e- to F-(g) = -328 kJ
5. Formation of LiF, Li+(g) + F-(g) to LiF (s) = -1047 kJ
Net overall: -617 kJ/mole, a highly favored
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