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The lattice energy of a solid ionic compound is the energy released when one mole of the solid compound is formed from its constituent gaseous ions at $\ce {298 K}$ and $\ce {1 bar}$. However, gaseous ions (e.g. $\ce {K^+ (g), Cl^- (g)}$) do not exist at such temperatures. Thus, how is it possible that the lattice energy of an ionic compound can be measured? A more reasonable definition would be to define the state of the compound formed as the standard state, rather than defining the state of the ions as the standard state. Wouldn't that make much more sense?

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  • $\begingroup$ No, that wouldn't be as useful. Going from the gaseous ions to the solid is one step in the Born–Haber cycle. $\endgroup$ – MaxW Oct 26 at 7:30
  • $\begingroup$ @MaxW Yes I understand that it is precisely the fact that we can't measure the lattice energy that we invented the Born-Haber cycle. However, I am just puzzled at the condition for the standard state in the definition of the lattice energy. $\endgroup$ – Tan Yong Boon Oct 26 at 7:42
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    $\begingroup$ What do you understand as standard state? What standard state are you referring to? $\endgroup$ – Buck Thorn Oct 26 at 15:21
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    $\begingroup$ @BuckThorn 298 K, partial pressure of all gaseous species at 1 bar, concentration of all aqueous species at 1 M $\endgroup$ – Tan Yong Boon Oct 26 at 16:33
  • $\begingroup$ I'd just say that the "standard lattice energy" refers to both reagents and products being in their standard states. I may misunderstand your question, but I'm guessing you are wondering about a reference state for computation of the chemical potential? $\endgroup$ – Buck Thorn Oct 26 at 18:53

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