= Lattice energy
{title2=$\Delta H_{\rm latt,diss}$}
{wiki}
A <lattice energy> needs a reaction, a sign convention, and a stated amount. Using the lattice-dissociation <enthalpy> convention for <sodium chloride>,
$$
\mathrm{NaCl}(s)\longrightarrow\mathrm{Na}^+(g)+\mathrm{Cl}^-(g),\qquad
\Delta H_{\rm latt,diss}>0.
$$
The reverse lattice-formation <enthalpy> is negative with the same magnitude when initial and final states match. <Chemistry 2e> gives $769\,\mathrm{kJ\,mol^{-1}}$ for this separation, per <mole> of <sodium chloride> <formula units>. This is about $7.97$ <electronvolts> per <formula unit>, not per individual <ion>. It concerns separated gaseous <ions>, rather than neutral <chemical atoms>, molten <salt>, or hydrated <ions> in <water>.<Chemistry 2e>[§7.5, “Ionic Bond Strength and Lattice Energy”, positive convention, reverse convention and NaCl value]{ref}
Also distinguish an <internal energy> change from an <enthalpy> change: $H=U+pV$. A static point-charge calculation omits motion and thermal contributions. Reversing a reaction reverses its <energy> change; replacing <internal energy> with <enthalpy> additionally changes the defined quantity. The <Born-Haber cycle> infers a lattice-formation <enthalpy> by combining other <enthalpies>, <ionization energies>, and <electron affinities>. These imagined stages are a thermodynamic accounting path, not a claim about the actual sequence of a <chemical reaction>.<Chemistry 2e>[§7.5, “The Born-Haber Cycle”, Figure 7.13 and Table 7.4]{ref}
Back to article page
