Enthalpy of vaporization

Temperature-dependency of the heats of vaporization for water, methanol, benzene, and acetone.

The enthalpy of vaporization, (symbol ∆H_vap) also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to the liquid substance, to transform a quantity of that substance into a gas. The enthalpy of vaporization is a function of the pressure 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 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 $T_r$ $\ll 1$ . The heat of vaporization diminishes with increasing temperature and it vanishes completely at a certain point called the critical temperature ( $T_{r}=1$ ). Above the critical temperature, the liquid and vapor phases are indistinguishable, and the substance is called a supercritical fluid.

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/mol, cal/g and Btu/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

\Delta{}H_{\mathrm{vap}} = \Delta{}U_{\mathrm{vap}} + p\Delta\,V

It is equal to the increased internal energy 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 in the liquid (or solid, in the case of sublimation). Hence helium has a particularly low enthalpy of vaporization, 0.0845 kJ/mol, as the van der Waals forces between helium atoms are particularly weak. On the other hand, the molecules in liquid water are held together by relatively strong hydrogen bonds, 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⁻¹mol⁻¹). 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), and so the calculated value of the bond strength will be too low. This is particularly true of metals, which often form covalently bonded molecules in the gas phase: in these cases, the enthalpy of atomization must be used to obtain a true value of the bond energy.

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 when a gas condenses to a liquid. As the liquid and gas are in equilibrium at the boiling point (T_b), Δ_vG = 0, which leads to:

\Delta\,_v S = S_{gas} - S_{liquid} = \Delta\,_v H/T_b

As neither entropy nor enthalpy 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 is different from 100 kPa, as the entropy of a gas is proportional to its pressure (or, more precisely, to its fugacity): the entropies of liquids vary little with pressure, as the compressibility 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 ( $\Delta\,_v S$ is always positive), and from

\Delta\,G = \Delta\,H - T\Delta\,S

the Gibbs free energy 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^[1] or TCPC model.^[2]

Selected values

Elements

Enthalpies of vaporization of the elements
	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Group →
↓ Period
1	H0.44936																	He0.0845
2	Li145.92	Be292.40											B489.7	C355.8	N2.7928	O3.4099	F3.2698	Ne1.7326
3	Na96.96	Mg127.4											Al293.4	Si300	P12.129	S1.7175	Cl10.2	Ar6.447
4	K79.87	Ca153.6	Sc314.2	Ti421	V452	Cr344.3	Mn226	Fe349.6	Co376.5	Ni370.4	Cu300.3	Zn115.3	Ga258.7	Ge330.9	As34.76	Se26.3	Br15.438	Kr9.029
5	Rb72.216	Sr144	Y363	Zr581.6	Nb696.6	Mo598	Tc660	Ru595	Rh493	Pd357	Ag250.58	Cd100	In231.5	Sn295.8	Sb77.14	Te52.55	I20.752	Xe12.636
6	Cs67.74	Ba142		Hf575	Ta743	W824	Re715	Os627.6	Ir604	Pt510	Au334.4	Hg59.229	Tl164.1	Pb177.7	Bi104.8	Po60.1	At27.2	Rn16.4
7	Frn/a	Ra37		Rfn/a	Dbn/a	Sgn/a	Bhn/a	Hsn/a	Mtn/a	Dsn/a	Rgn/a	Cnn/a	Nhn/a	Fln/a	Mcn/a	Lvn/a	Tsn/a	Ogn/a

			La414	Ce414	Prn/a	Ndn/a	Pmn/a	Smn/a	Eun/a	Gdn/a	Tbn/a	Dyn/a	Hon/a	Ern/a	Tmn/a	Ybn/a	Lun/a
			Acn/a	Th514.4	Pan/a	Un/a	Npn/a	Pun/a	Amn/a	Cmn/a	Bkn/a	Cfn/a	Esn/a	Fmn/a	Mdn/a	Non/a	Lrn/a

	Enthalpy in kJ/mol, measured at their respective normal boiling points
	0–10 kJ/mol			10–100 kJ/mol			100–300 kJ/mol			>300 kJ/mol

The vaporization of metals is a key step in metal vapor synthesis, 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:

Compound	Boiling Point at normal pressure	Heat of vaporization (J mol⁻¹)	Heat of vaporization (J g⁻¹)
Acetone	329 K, 56 °C, 133 °F	31300	538.9
Aluminium	2792 K, 2519 °C, 4566 °F	294000	10500
Ammonia	240 K, −33.34 °C, −28 °F	23350	1371
Butane	272–274 K, −1 °C, 30–34 °F	21000	320
Diethyl ether	307.8 K, 34.6 °C, 94.3 °F	26170	353.1
Ethanol	352 K, 78.37 °C, 173 °F	38600	841
Hydrogen	20.271 K, -252.879 °C, -423.182 °F	460	451.9
Iron	3134 K, 2862 °C, 5182 °F	340000	6090
Isopropyl alcohol	356 K, 82.6 °C, 181 °F	44000	732.2
Methane	112 K, −161 °C, −259 °F	8170	480.6
Methanol	338 K, 64.7 °C, 148 °F	35200^[3]	1104
Propane	231 K,-42 °C, −44 °F	15700	356
Phosphine	185 K, −87.7 °C, −126 °F	14600	429.4
Water	373.15 K, 100 °C, 212 °F	40660	2257

References

↑ Ge, Xinlei; Wang, Xidong (20 May 2009). "Estimation of Freezing Point Depression, Boiling Point Elevation, and Vaporization Enthalpies of Electrolyte Solutions". Industrial & Engineering Chemistry Research. 48 (10): 5123–5123. doi:10.1021/ie900434h.
↑ Ge, Xinlei; Wang, Xidong (2009). "Calculations of Freezing Point Depression, Boiling Point Elevation, Vapor Pressure and Enthalpies of Vaporization of Electrolyte Solutions by a Modified Three-Characteristic Parameter Correlation Model". Journal of Solution Chemistry. 38 (9): 1097–1117. ISSN 0095-9782. doi:10.1007/s10953-009-9433-0.
↑ NIST

CODATA Key Values for Thermodynamics
Gmelin, Leopold (1985). Gmelin-Handbuch der anorganischen Chemie / 08 a (8., völlig neu bearb. Aufl. ed.). Berlin [u.a.]: Springer. pp. 116–117. ISBN 3-540-93516-9.
NIST Chemistry WebBook
Young, Francis W. Sears, Mark W. Zemansky, Hugh D. (1982). University physics. (6th ed.). Reading, Mass.: Addison-Wesley. ISBN 0-201-07199-1.

States of matter (list)
State	Solid Liquid Gas / Vapor Plasma
Low energy	Bose–Einstein condensate Fermionic condensate Degenerate matter Quantum Hall Rydberg matter Strange matter Superfluid Supersolid Photonic matter
High energy	QCD matter Lattice QCD Quark–gluon plasma Supercritical fluid
Other states	Colloid Glass Liquid crystal Quantum spin liquid Magnetically ordered Antiferromagnet Ferrimagnet Ferromagnet String-net liquid Superglass
Transitions	Boiling Boiling point Condensation Critical line Critical point Crystallization Deposition Evaporation Flash evaporation Freezing Chemical ionization Ionization Lambda point Melting Melting point Recombination Regelation Saturated fluid Sublimation Supercooling Triple point Vaporization Vitrification
Quantities	Enthalpy of fusion Enthalpy of sublimation Enthalpy of vaporization Latent heat Latent internal energy Trouton's ratio Volatility
Concepts	Binodal Compressed fluid Cooling curve Equation of state Leidenfrost effect Macroscopic quantum phenomena Mpemba effect Order and disorder (physics) Spinodal Superconductivity Superheated vapor Superheating Thermo-dielectric effect

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