Adipic acid, HOOC-(CH2)4-COOH, is a dicarboxylic molecular solid. A small research team compares its dissolution in water (polar, hydrogen-bonding solvent) with its dissolution in liquid heptane (non-polar solvent) over the temperature range 273–373 K. The following qualitative observations are made:
• Breaking the adipic-acid crystal lattice requires a moderate endothermic input.
• Strong hydrogen bonds form between adipic-acid molecules and water; only weak dispersion interactions form with heptane.
• Water molecules must reorganize extensively to accommodate dissolved adipic-acid species, whereas heptane molecules require minimal reorganization.
• The positional entropy of the solute increases substantially upon dissolution in either solvent.
(a) For dissolution in water at 298 K, discuss whether the overall enthalpy change is expected to be endothermic, exothermic, or approximately thermoneutral. Support your answer by considering competing contributions.
(b) For dissolution in heptane at 298 K, predict the sign (positive or negative) of the overall enthalpy change and justify your prediction using the same set of contributions.
(c) Compare the entropy changes associated with dissolution of adipic acid in the two solvents and explain which solvent provides the larger favorable entropic term.
(d) Using your answers to parts (a)–(c), predict in which solvent the dissolution of adipic acid is spontaneous at 298 K. Explain how temperature variation could reverse the spontaneity, specifying which solvent might favor dissolution at higher temperatures.
(e) Design an experiment, including the type of measurements you would collect, to determine the temperature at which ΔG for dissolution of adipic acid in water becomes zero. Explain how the collected data would allow you to locate this temperature.