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The standard enthalpy of mutarotation of glucose in aqueous solution is given as: alpha-D-glucose(aq) -> beta-D-glucose(aq), delta-H = -1.16 kJ/mol. The enthalpies of dissolution are: alpha-D-glucose(s) -> alpha-D-glucose(aq), delta-H = +10.72 kJ/mol; and beta-D-glucose(s) -> beta-D-glucose(aq), delta-H = +4.68 kJ/mol. Calculate the standard enthalpy change (in kJ/mol) for the solid-phase mutarotation: alpha-D-glucose(s) -> beta-D-glucose(s).

1. +4.88 kJ/mol
2. -4.88 kJ/mol
3. -2.44 kJ/mol
4. +2.44 kJ/mol

Correct answer: +4.88 kJ/mol

Solution

Using Hess's Law: dissolve alpha solid (+10.72), mutarotate in solution (-1.16), then crystallize beta solid (-4.68, reverse of given). Total delta-H = 10.72 - 1.16 - 4.68 = +4.88 kJ/mol.

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