CHEMISTRY · Single correct · A Level · Chemistry · Thermodynamics · Thermodynamics
Problem
A salt has a standard enthalpy of dissolution of −15kJmol−1, yet the process is non-spontaneous at 25∘C. Which of the following best explains this observation?
(a) The exothermic heat released upon dissolving cools the solution, and the resulting temperature drop prevents the ions from remaining separated, so the process is non-spontaneous.
(b) The ions are kinetically trapped in the crystal; the free energy change would be favorable, but the activation energy required to break the lattice is too large at 25∘C, so dissolution does not occur.
(c) The decrease in entropy caused by extensive ordering of water molecules outweighs the entropy gain from disrupting the crystal lattice, giving an overall negative ΔS∘ that renders the process non-spontaneous.
(d) A negative ΔH∘ leads to spontaneity only at high temperatures; at 25∘C the −TΔS∘ term is not large enough to overcome ΔH∘, so ΔG∘ remains positive and dissolution is non-spontaneous.