The dissolution of LiMn2O4 spinel in acid is a serious impediment to its use as a cathode material in Li batteries. In this paper, the free energy difference between reactants and products that drives the dissolution reaction is calculated using first principles methods for solid phases, and literature values for the solvation free energies of H, Li, and Mn ions and water molecules. We find that the dissolution reaction is endothermic in the absence of acid and exothermic in acid. Doping with Li on the Mn sublattice lowers the magnitude of the reaction energy Delta G by an amount proportional to the dilution of trivalent Mn in the host, thus confirming the role of disproportionation in the dissolution reaction.