CHEMISTRY · Single correct · A Level · Chemistry · Thermodynamics · Thermodynamics
Problem
A molecular solid composed of small, polar molecules held together primarily by hydrogen bonding is sparingly soluble in hexane but dissolves readily in ethanol at 25∘C. Which statement best explains the difference in solubility between the two solvents?
(a) In ethanol, the lattice enthalpy of the solid becomes smaller, so the crystal breaks apart more readily; in non-polar hexane the lattice enthalpy remains large, preventing dissolution.
(b) In ethanol, favorable solvent–solute hydrogen bonding interactions provide a sufficiently exothermic ΔH and, together with the entropy increase that accompanies dissolution, give a negative ΔG∘; hexane offers only weak London dispersion attractions and cannot offset the lattice enthalpy.
(c) Hexane has a higher dielectric constant than ethanol, so it should better stabilize separated polar molecules of the solid; because ethanol’s dielectric constant is lower, the solid dissolves more readily in ethanol.
(d) The dissolution process in ethanol is favored because the entropy change is negative (the system becomes more ordered), whereas in hexane the entropy change is positive, which disfavors dissolution.