How is the enthalpy of vaporisation different from the enthalpy of fusion?

The enthalpy of vaporisation refers to the energy required to convert a liquid to a gas, while the enthalpy of fusion refers to the energy needed to convert a solid to a liquid.

The enthalpy of vaporisation and the enthalpy of fusion are both thermodynamic quantities that describe the amount of energy required to change the state of a substance. However, they refer to different phase transitions. The enthalpy of vaporisation, often denoted as ΔHvap, is the amount of energy required to convert one mole of a substance from the liquid phase to the gas phase at a constant temperature and pressure. This process is also known as boiling or evaporation.

On the other hand, the enthalpy of fusion, often denoted as ΔHfus, is the amount of energy required to convert one mole of a substance from the solid phase to the liquid phase at a constant temperature and pressure. This process is also known as melting.

The values of these enthalpies depend on the nature of the substance and its intermolecular forces. Substances with stronger intermolecular forces require more energy to overcome these forces during phase transitions, resulting in higher enthalpy values. For example, water has a high enthalpy of vaporisation due to its strong hydrogen bonding.

It's also worth noting that the enthalpy of vaporisation is typically much larger than the enthalpy of fusion for a given substance. This is because the transition from liquid to gas involves breaking more intermolecular forces than the transition from solid to liquid. In the solid phase, particles are closely packed and only vibrate about fixed positions, while in the liquid phase, particles can move more freely but are still relatively close together. In the gas phase, however, particles are far apart and move independently, so more energy is required to achieve this state.

In summary, while both the enthalpy of vaporisation and the enthalpy of fusion relate to the energy required for phase transitions, they refer to different transitions and typically have different values for a given substance.