At 100 °C, the molar enthalpy of vaporization of water is , whereas the molar…
CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics
Problem
At 100 °C, the molar enthalpy of vaporization of water is +40.7kJ mol−1, whereas the molar enthalpy of condensation is −40.7kJ mol−1. Which statement best accounts for the equal magnitudes but opposite signs of these two quantities?
(a) Because the system is at its boiling-point equilibrium, its overall enthalpy cannot change; therefore the forward (vaporization) and reverse (condensation) processes must have enthalpy changes that are equal in magnitude but opposite in sign.
(b) The molar mass of water is the same in both phases, so the energy per mole remains constant and the enthalpy differs only by sign depending on direction.
(c) Reversing the direction of heat flow means that the energy released when intermolecular forces form equals the energy absorbed when they are broken, giving opposite signs but equal magnitudes.
(d) Because both processes occur at constant temperature, no net heat is transferred to the system, so their enthalpy changes must cancel, resulting in equal values with opposite signs.