Phase changes involve intermolecular forces. For example, vaporization of water involves snapping hydrogen bonds between water molecules.
Chemical changes involve chemical bonding forces. For example, atomization of water involves snapping two oxygen-hydrogen bonds. Since chemical bonds are much stronger than intermolecular forces, chemical changes require (or release) far more energy than phase changes.
Nuclear changes involve even stronger forces that bind the nucleus together, and so will usually require or release even more energy than chemical changes.
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/thermo/faq/print-phase-chemical-nuclear-energy.shtml