• Dissociation implies the breaking of bonds. Energy is needed to break bonds so the process is endothermic. Hydration implies absorption of moisture. This is normally caused by the formation of bonds between the substance absorbing and the water. Formation of bonds releases energy therefore exothermic. Not quite so simple of course because to bond water to something else you have to unbond it from itself (endothermic). Water should really be a gas at earth temperatures, but the molecules are bonded together by "hydrogen bonds" and these have to be undone if you bond it to something else. In addition, the absorber may also have bonds that need to be broken (endothermic) in order for the water to be absorbed. So there are bith exothermic and endothermic processes going on during hydration. So you have to take all into account. For example, the hydration of sodium hydroxide needs to break ionic bonds between Na+ and OH- in order to form bonds between these ions and water.In balance the latter are stronger than the former so the overall process is exothermic. For sodium chloride, Na+ and Cl-, the latter are weaker than the former so the overall process is endothermic.

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