  Electronic structure calculations are carried out to estimate the heats of
formation of rare-earth orthophosphates from their oxides. The calculated heats
of formation are systematically about 40 kJ/mol less exothermic than the
measured values. Based on estimated corrections for zero-point energies and
H(298.15)-H(0), the discrepancy is almost entirely electronic in origin. The
decreasingly exothermic {\Delta}Hfox with decreasing ionic radius (i.e. LaPO4
more exothermic than ScPO4) results from the higher charge localization on the
oxide anion (O2-) relative to the phosphate anion (PO43-). The lattice energy,
of course, becomes more negative with decreasing ionic radius for both oxide
and phosphate phases, but does so more rapidly for the oxide, making the
reaction less exothermic as ionic radius becomes smaller. This effect should
carry over to {\Delta}Hfox other oxyacids, such as silicates and sulfates.
