CHEMISTRY · Single correct · A Level · Chemistry · Thermodynamics · Thermodynamics
Problem
Tetramethylammonium chloride, (CH₃)₄NCl, has a slightly positive standard enthalpy of dissolution yet is highly soluble in water at room temperature. Which of the following best accounts for this observation?
(a) The chloride anion forms exceptionally strong ion–dipole interactions with water, producing a large exothermic hydration enthalpy that outweighs the positive lattice enthalpy.
(b) Breakage of the crystal is easy because the ionic lattice is stabilized chiefly by weak London dispersion forces, so very little energy is absorbed.
(c) Dissolution greatly increases the number of accessible microstates, especially because the bulky tetramethylammonium ion disrupts water structure, giving a large positive ΔS that overcomes the small positive ΔH.
(d) Self-ionization of water continuously generates additional Cl− ions; by Le Châtelier’s principle this pulls the dissolution equilibrium to the product side, making the process spontaneous.