What is the enthalpy of fusion?

The enthalpy of fusion refers specifically to the amount of heat energy required to change a substance from its solid phase to its liquid phase at constant pressure, without a change in temperature. This process involves overcoming the intermolecular forces holding the particles in a solid state.

When considering the difference in enthalpy per mole between the liquid and solid states, this definition aligns perfectly with the enthalpy of fusion. This value is typically expressed in joules or calories per mole and is characteristic of each substance, reflecting how much energy is needed for the transition at the melting point.

In contrast, the other options discuss different thermodynamic processes. For example, the heat absorbed during the transition from solid to gas refers to sublimation, not fusion. The heat released during gas condensation pertains to the transition from gas to liquid, while the total energy of an ideal gas is related to the kinetic energy of particles within the gas phase and does not apply to the phase change described for fusion.

Thus, the definition associated with the difference in enthalpy per mole between the liquid and solid states accurately captures the essence of the enthalpy of fusion.