Editing Standard enthalpy of formation (section)

== Ionic compounds: Born–Haber cycle ==
[[File:Born-haber cycle LiF.svg|class=skin-invert-image|450px|thumb|Standard enthalpy change of formation in [[Born–Haber cycle|Born–Haber]] diagram for [[lithium fluoride]]. {{math|Δ{{sub|latt}}''H''}} corresponds to {{math|U{{sub|L}}}} in the text. The downward arrow "electron affinity" shows the negative quantity {{math|–EA{{sub|F}}}}, since {{math|EA{{sub|F}}}} is usually [[Electron affinity#Sign convention|defined as positive.]]]]
For ionic compounds, the standard enthalpy of formation is equivalent to the sum of several terms included in the [[Born–Haber cycle]]. For example, the formation of [[lithium fluoride]],

:<chem>Li(s) + 1/2 F2(g) -> LiF(s)</chem>

may be considered as the sum of several steps, each with its own enthalpy (or energy, approximately): 
# {{math|''H''{{sub|sub}}}}, the [[standard enthalpy of atomization]] (or [[Sublimation (phase transition)|sublimation]]) of solid lithium.
# {{math|IE{{sub|Li}}}}, the [[first ionization energy]] of gaseous lithium.
# {{math|B(F–F)}}, the standard [[enthalpy of atomization]] (or bond energy) of fluorine gas.
# {{math|EA{{sub|F}}}}, the [[electron affinity]] of a fluorine atom.
# {{math|U{{sub|L}}}}, the [[lattice energy]] of lithium fluoride.

The sum of these enthalpies give the standard enthalpy of formation ({{math|&Delta;''H''{{sub|f}}}}) of lithium fluoride:

:<math>\Delta H_\text{f} = \Delta H_\text{sub} + \text{IE}_\text{Li} + \frac{1}{2}\text{B(F–F)} - \text{EA}_\text{F} + \text{U}_\text{L}.</math>

In practice, the enthalpy of formation of lithium fluoride can be determined experimentally, but the lattice energy cannot be measured directly. The equation is therefore rearranged to evaluate the lattice energy:<ref>Moore, Stanitski, and Jurs. ''Chemistry: The Molecular Science''. 3rd edition. 2008.  {{ISBN|0-495-10521-X}}. pages 320–321.</ref>

:<math>-U_\text{L} = \Delta H_\text{sub} + \text{IE}_\text{Li} + \frac{1}{2}\text{B(F–F)} - \text{EA}_\text{F} - \Delta H_\text{f}.</math>