However we are still obtaining answers within about 12% error even for AgI. Looking at the table, we see that the error is small for AgF and becomes progressively larger for the heavier silver halides. It is interesting to repeat this exercise for the silver halides, which have either the NaCl structure (AgF, AgCl, AgBr) or zincblende structure (AgI). The errors in this case are only about 1% of E L. The table below shows results of more detailed lattice energy calculations for ionic fluorides in which the van der Waals term is explicitly included. We can do better by explicitly including the short-range van der Waals attractive energy between ions. The two errors partially compensate, so the overall error in the calculation is small. If we underestimate the attractive energy of the crystal lattice, the energy minimization criterion ensures that the repulsion energy is underestimated as well. This is because we used energy minimization to obtain the repulsion energy in the Born-Mayer equation. The result is promising because we neglected the van der Waals term.īut.how did we get away with neglecting the van der Waals term? Here we have to subtract 2RT to convert our cycle of energies to a cycle of enthalpies, because we are compressing two moles of gas in making NaCl(s) and PΔV = ΔnRT, where Δn = -2.Įxperimentally ΔH f for NaCl is -411 kJ/molīecause all the other numbers in the cycle are known accurately, the error in our calculation is only about 15 kJ (about 2% of E L).
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