Heat of Vaporization

heat of vaporization [′hēt əv ‚vā·pə·rə′zā·shən]

(thermodynamics)

The quantity of energy required to evaporate 1 mole, or a unit mass, of a liquid, at constant pressure and temperature. Also known as enthalpy of vaporization; heat of evaporation; latent heat of vaporization.

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Heat of Vaporization 

(or heat of evaporation, latent heat of vaporization), the amount of heat that must be supplied to a substance in an equilibrium constant-pressure and constant-temperature process to convert the substance from the liquid state to the gaseous state. The same amount of heat is liberated when the vapor condenses into a liquid.

Table 1. Heat of vaporization of several substances
Substance	tb(°C)	Lv(kcal/kg)	Lv(j/kg)
Hydrogen ...............	–252.6	107	4.48 × 105
Nitrogen ...............	–195.8	47.6	1.99 × 105
Ethyl alcohol ...............	78.4	216	9.05 × 105
Water ...............	100	539	22.6 × 105
Mercury ...............	357	69.7	2.82 × 105
Lead ...............	1740	204	8.55 × 105
Copper ...............	2600	1,150	48.2 × 105
Iron	3200	1,460	61.2 × 105

The heat of vaporization is a special case of the heat of a firstorder transition. For a given substance, the heat of vaporization may be determined per unit mass or per mole. In the former case, the heat of vaporization is measured in, for example, joules per kg (J/kg) or kilocalories per kg (kcal/kg). In the latter case, the heat of vaporization may be expressed in joules per mole. The term “molar heat of vaporization” is sometimes applied to the heat of vaporization per mole. Table 1 gives the values of the heat of vaporization per kg L_v for several substances at normal external pressure (760 mm Hg, or 101,325 newtons per m²) and at the boiling point t_b.