Which of the following best explains why the molar enthalpy of vaporization for water…
CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics
Problem
Which of the following best explains why the molar enthalpy of vaporization for water (about 40.7kJ mol−1) is much larger than its molar enthalpy of fusion (about 6.01kJ mol−1)?
(a) Fusion occurs at 0∘C, a much lower temperature than vaporization at 100∘C, so less kinetic energy is available to break intermolecular attractions during melting.
(b) More energy is needed for vaporization mainly because the system must perform work to expand against atmospheric pressure when a gas forms, whereas melting involves almost no volume change.
(c) Vaporization requires complete separation of molecules, overcoming essentially all intermolecular attractions, whereas fusion only partially disrupts these attractions.
(d) Hydrogen bonding is significant in liquid water but largely absent in the crystal structure of ice, so the extra energy needed to break hydrogen bonds is required only for vaporization.