Enthalpy of Vaporization

The enthalpy of vaporization (vapH) is the amount of heat energy required to convert a substance from its liquid state to its gaseous state at a constant temperature and pressure. It is also known as the heat of vaporization.

Definition of enthalpy

Enthalpy is a thermodynamic property that represents the total heat content of a system. It is denoted by the symbol H and is defined as the sum of the internal energy (U) of the system and the product of pressure (P) and volume (V) of the system.

Factors affecting the enthalpy of vaporization

The enthalpy of vaporization depends on several factors, including:
1. Nature of the substance: Different substances have different intermolecular forces, which affect the strength of the bonds between the molecules. Substances with stronger intermolecular forces require more energy to break these bonds and vaporize, resulting in higher enthalpy of vaporization.
2. Temperature: The enthalpy of vaporization generally increases with increasing temperature, as higher temperatures provide more energy to overcome intermolecular forces.
3. Pressure: The enthalpy of vaporization is typically measured at a constant pressure. However, changes in pressure can affect the enthalpy of vaporization by altering the boiling point of the substance.

The critical temperature

The critical temperature is the temperature above which a substance cannot exist in the liquid state, regardless of the pressure applied. At the critical temperature, the liquid and gas phases become indistinguishable, and the substance is said to be in a supercritical state. The critical temperature is a characteristic property of a substance.

Enthalpy of vaporization at the critical temperature

At the critical temperature, the enthalpy of vaporization becomes zero. This is because the distinction between the liquid and gas phases disappears, and the substance undergoes a continuous transition from the liquid to the gas phase without any change in enthalpy. In other words, the enthalpy of vaporization is no longer applicable at the critical temperature.

Therefore, the correct option is 'B' - critical temperature. At the critical temperature, the enthalpy of vaporization is zero.