The enthalpy of vaporization (symbol ), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (
enthalpy
Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
) that must be added to a liquid substance to transform a quantity of that substance into a gas. The enthalpy of vaporization is a function of the
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
at which that transformation takes place.
The enthalpy of vaporization is often quoted for the
normal boiling temperature of the substance. Although tabulated values are usually corrected to 298
K, that correction is often smaller than the
uncertainty
Uncertainty refers to epistemic situations involving imperfect or unknown information. It applies to predictions of future events, to physical measurements that are already made, or to the unknown. Uncertainty arises in partially observable or ...
in the measured value.
The heat of vaporization is temperature-dependent, though a constant heat of vaporization can be assumed for small temperature ranges and for
reduced temperature . The heat of vaporization diminishes with increasing temperature and it vanishes completely at a certain point called the critical temperature (
). Above the
critical temperature, the liquid and
vapor
In physics, a vapor (American English) or vapour (British English and Canadian English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R. H. Petrucci, W. S. Harwood, and F. G. Her ...
phases are indistinguishable, and the substance is called a
supercritical fluid
A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid. It can effuse through porous so ...
.
Units
Values are usually quoted in
J/
mol, or kJ/mol (molar enthalpy of vaporization), although kJ/kg, or J/g (specific heat of vaporization), and older units like
kcal
The calorie is a unit of energy. For historical reasons, two main definitions of "calorie" are in wide use. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the temperature of o ...
/mol, cal/g and
Btu
The British thermal unit (BTU or Btu) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. The modern SI u ...
/lb are sometimes still used among others.
Enthalpy of condensation
The enthalpy of condensation (or heat of condensation) is by definition equal to the enthalpy of vaporization with the opposite sign: enthalpy changes of vaporization are always positive (heat is absorbed by the substance), whereas enthalpy changes of condensation are always negative (heat is released by the substance).
Thermodynamic background
The enthalpy of vaporization can be written as
:
It is equal to the increased
internal energy
The internal energy of a thermodynamic system is the total energy contained within it. It is the energy necessary to create or prepare the system in its given internal state, and includes the contributions of potential energy and internal kinet ...
of the vapor phase compared with the liquid phase, plus the work done against ambient pressure. The increase in the internal energy can be viewed as the energy required to overcome the
intermolecular interactions
An intermolecular force (IMF) (or secondary force) is the force that mediates interaction between molecules, including the electromagnetic forces of attraction
or repulsion which act between atoms and other types of neighbouring particles, e.g. a ...
in the liquid (or solid, in the case of
sublimation). Hence
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
has a particularly low enthalpy of vaporization, 0.0845 kJ/mol, as the
van der Waals force
In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and th ...
s between helium
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, and ...
s are particularly weak. On the other hand, the
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
s in liquid
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
are held together by relatively strong
hydrogen bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
s, and its enthalpy of vaporization, 40.65 kJ/mol, is more than five times the energy required to heat the same quantity of water from 0 °C to 100 °C (
''c''p = 75.3 J/K·mol). Care must be taken, however, when using enthalpies of vaporization to ''measure'' the strength of intermolecular forces, as these forces may persist to an extent in the gas phase (as is the case with
hydrogen fluoride
Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...
), and so the calculated value of the
bond strength
In chemistry, bond energy (''BE''), also called the mean bond enthalpy or average bond enthalpy is the measure of bond strength in a chemical bond. IUPAC defines bond energy as the average value of the gas-phase bond-dissociation energy (usually at ...
will be too low. This is particularly true of metals, which often form
covalently bonded
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
molecules in the gas phase: in these cases, the
enthalpy of atomization
In chemistry, the enthalpy of atomization (also atomisation in British English) is the enthalpy change that accompanies the total separation of all atoms in a chemical substance (either an element or a compound). This is often represented by th ...
must be used to obtain a true value of the
bond energy
In chemistry, bond energy (''BE''), also called the mean bond enthalpy or average bond enthalpy is the measure of bond strength in a chemical bond. IUPAC defines bond energy as the average value of the gas-phase bond-dissociation energy (usually ...
.
An alternative description is to view the enthalpy of condensation as the heat which must be released to the surroundings to compensate for the drop in
entropy
Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
when a gas condenses to a liquid. As the liquid and gas are in
equilibrium at the boiling point (''T''
b),
Δv''G'' = 0, which leads to:
:
As neither entropy nor
enthalpy
Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
vary greatly with temperature, it is normal to use the tabulated standard values without any correction for the difference in temperature from 298 K. A correction must be made if the
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
is different from 100
kPa
KPA may refer to:
* Keele Postgraduate Association, Keele University, UK, formerly Keele Research Association (KRA)
* Kensington (Olympia) station, London, England, National Rail station code
* Kenya Ports Authority
* ''Kiln phosphoric acid'', a ...
, as the entropy of a gas is proportional to its pressure (or, more precisely, to its
fugacity
In chemical thermodynamics, the fugacity of a real gas is an effective partial pressure which replaces the mechanical partial pressure in an accurate computation of the chemical equilibrium constant. It is equal to the pressure of an ideal gas whic ...
): the entropies of liquids vary little with pressure, as the
compressibility
In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...
of a liquid is small.
These two definitions are equivalent: the boiling point is the temperature at which the increased entropy of the gas phase overcomes the intermolecular forces. As a given quantity of matter always has a higher entropy in the gas phase than in a condensed phase (
is always positive), and from
:
,
the
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and pr ...
change falls with increasing temperature: gases are favored at higher temperatures, as is observed in practice.
Vaporization enthalpy of electrolyte solutions
Estimation of the enthalpy of vaporization of electrolyte solutions can be simply carried out using equations based on the chemical thermodynamic models, such as Pitzer model
or TCPC model.
Selected values
Elements
The vaporization of metals is a key step in
metal vapor synthesis In chemistry, metal vapor synthesis (MVS) is a method for preparing metal complexes by combining freshly produced metal atoms or small particles with ligands. In contrast to the high reactivity of such freshly produced metal atoms, bulk metals typ ...
, which exploits the increased reactivity of metal atoms or small particles relative to the bulk elements.
Other common substances
Enthalpies of vaporization of common substances, measured at their respective standard boiling points:
See also
*
Clausius–Clapeyron relation
*
Shimansky equation
In molecular physics, Shimansky equation describes the temperature dependence of the heat of vaporization (also known as the enthalpy of vaporization or the heat of evaporation):Shimansky Yu. I. ''The temperature dependence of the heat of vaporiza ...
, describes the temperature dependence of the heat of vaporization
*
Enthalpy of fusion
In thermodynamics, the enthalpy of fusion of a substance, also known as (latent) heat of fusion, is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a s ...
, specific heat of melting
*
Enthalpy of sublimation
In thermodynamics, the enthalpy of sublimation, or heat of sublimation, is the heat required to sublimate (change from solid to gas) one mole of a substance at a given combination of temperature and pressure, usually standard temperature and p ...
*
Joback method The Joback method (often named Joback/Reid method) predicts eleven important and commonly used pure component thermodynamic properties from molecular structure only.
Basic principles
Group-contribution method
The Joback method is a group- ...
, estimation of the heat of vaporization at the normal boiling point from molecular structures
*
Latent heat
Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition.
Latent heat can be understo ...
References
CODATA Key Values for Thermodynamics*
NIST Chemistry WebBook*
{{DEFAULTSORT:Enthalpy Of Vaporization
Enthalpy