Note: This isn't an actual homework question, and I tried asking others (including teachers), but no one could answer it. Now, the heat of formation for sodium hydroxide is -425.6 kJ/mol (so it's exothermic). When you dissolve NaOH in water, the reaction is exothermic. But this is dissociation, the opposite of formation. So logically, you would flip the sign around for the heat of formation to get the enthalpy change of the dissociation, right? But if you do, the enthalpy change becomes +425.6 kJ/mol, which means that it is endothermic. However, the dissolving of NaOH in water releases heat to the environment, so it's exothermic! What's going on? This has been baffling me for a long time now.