Overview

The enthalpy of vaporization, often called the latent heat of vaporization, is the amount of heat energy required to transform a quantity of liquid into its vapor without a change in temperature. In thermodynamic terms it is the difference in enthalpy between the vapor and the liquid phase at a specified pressure. Because phase change occurs at equilibrium pressure and temperature, the value is reported for a particular pressure (commonly atmospheric pressure) and is an intrinsic property of the substance.

Properties and units

Enthalpy of vaporization is usually given as a molar quantity (joules per mole) but may also be expressed on a mass basis (joules per kilogram). Standard tabulations often list values at the normal boiling point of the substance. Typical SI units are J per mol, while engineering tables sometimes give kJ/kg. The magnitude varies widely: light volatile organic liquids have small molar enthalpies, whereas water and other hydrogen-bonded liquids require considerably more energy to vaporize.

Thermodynamic context and temperature dependence

At the equilibrium boiling temperature for a given pressure, the Gibbs free energy change for vaporization is zero and the entropy change equals the enthalpy change divided by temperature (ΔS = ΔH/T). The temperature dependence of the enthalpy of vaporization is related to the variation of vapor pressure with temperature and can be described approximately by the Clausius–Clapeyron relation for many substances. As the critical point of a substance is approached, the distinction between liquid and vapor disappears and the enthalpy of vaporization falls to zero.

Rules of thumb and notable exceptions

Many nonpolar molecular liquids share similar molar entropy changes on vaporization, an empirical observation known as Trouton's rule: the molar entropy of vaporization at the normal boiling point is often near 85–88 J·mol⁻¹·K⁻¹. Strongly hydrogen-bonded or associating liquids (for example water or alcohols) deviate significantly from this rule and typically have larger enthalpies of vaporization because extra energy is needed to break intermolecular attractions.

Examples and everyday relevance

Water is a widely cited example: its molar enthalpy of vaporization at 100 °C and 1 atm is about 40.7 kJ·mol⁻¹, which underpins many natural and technological phenomena. For instance, evaporative cooling of sweat helps regulate human body temperature: when warm weather heat or vigorous exercise raises body temperature, sweat (perspiration) evaporates from the skin and removes thermal energy. As ambient humidity rises, the rate of evaporation falls and cooling is less effective. This everyday example illustrates how the enthalpy of vaporization affects comfort and heat balance.

Applications, measurement, and practical considerations

Enthalpy of vaporization is important in distillation, chemical process design, meteorology, refrigeration, and heat-engine calculations. Common ways to determine it include calorimetric measurements and indirect methods using vapor-pressure temperature data fitted to the Clausius–Clapeyron equation. Engineers and scientists distinguish between molar and specific values, and between latent heat (energy per unit mass) and molar enthalpy differences. When designing equipment such as condensers, boilers, or air-conditioning systems, correct values and their temperature dependence are essential for energy balances and safety calculations.

  • Typical uses: distillation, drying, HVAC, meteorology, phase-change materials.
  • Measurement methods: calorimetry, vapor pressure fitting, differential scanning calorimetry.
  • Important distinctions: latent heat vs enthalpy change, molar vs specific basis, deviations from Trouton's rule.

Understanding enthalpy of vaporization links microscopic intermolecular forces to macroscopic energy flows and is central to describing phase behavior in both natural and engineered systems.

For more detailed data, property tables and further reading, consult thermodynamic reference sources and substance-specific literature (enthalpy references, boiling point tables).

Example resources: unit definitions, molar quantities, practical examples.