Enthalpy of vaporization

File:Heat of Vaporization (Benzene+Acetone+Methanol+Water).png

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

In thermodynamics, the enthalpy of vaporization (symbol Template:Math), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) 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 and temperature at which the transformation (vaporization or evaporation) 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 Template:Math. The heat of vaporization diminishes with increasing temperature and it vanishes completely at a certain point called the critical temperature (Template:Math). 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

File:Enthalpy of Zn(c,l,g).PNG

Molar enthalpy of zinc above 298.15Template:NbspK and at 1Template:Nbspatm pressure, showing discontinuities at the melting and boiling points. The enthalpy of melting (ΔH°m) of zinc is 7323Template:NbspJ/mol, and the enthalpy of vaporization (ΔH°v) is Template:Val.

The enthalpy of vaporization can be written as

$\mathrm{\Delta }{H}_{\text{vap}}=\mathrm{\Delta }{U}_{\text{vap}}+p\mathrm{\Delta }V$

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·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), 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:

${\mathrm{\Delta }}_{\text{v}}S=S_{\text{gas}}-S_{\text{liquid}}=\frac{{\mathrm{\Delta }}_{\text{v}}H}{T_{\text{b}}}$

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 an ideal gas is proportional to the logarithm of its pressure. The entropies of liquids vary little with pressure, as the coefficient of thermal expansion 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 (${\mathrm{\Delta }}_{\text{v}}S$ is always positive), and from

$\mathrm{\Delta }G=\mathrm{\Delta }H-T\mathrm{\Delta }S$,

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 or TCPC model.

Selected values

Elements

colspan=20 style="background:Template:Element color; padding:2px 4px;" | 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	title="H, hydrogen" style="text-align:center; padding:0.15em; background:#ffccff; border:Template:Element color;" | H0.90																		title="He, helium" style="text-align:center; padding:0.15em; background:#ffccff; border:Template:Element color;" | He0.08
2	title="Li, lithium" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Li136	title="Be, beryllium" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Be292												title="B, boron" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | B508	title="C, carbon" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | C715	title="N, nitrogen" style="text-align:center; padding:0.15em; background:#ffccff; border:Template:Element color;" | N5.57	title="O, oxygen" style="text-align:center; padding:0.15em; background:#ffccff; border:Template:Element color;" | O6.82	title="F, fluorine" style="text-align:center; padding:0.15em; background:#ffccff; border:Template:Element color;" | F6.62	title="Ne, neon" style="text-align:center; padding:0.15em; background:#ffccff; border:Template:Element color;" | Ne1.71
3	title="Na, sodium" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | Na97.4	title="Mg, magnesium" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Mg128												title="Al, aluminium" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Al284	title="Si, silicon" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Si359	title="P, phosphorus" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | P12.4	title="S, sulfur" style="text-align:center; padding:0.15em; background:#cfc; border:Template:Element color;" | S45	title="Cl, chlorine" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | Cl20.4	title="Ar, argon" style="text-align:center; padding:0.15em; background:#ffccff; border:Template:Element color;" | Ar6.53
4	title="K, potassium" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | K76.9	title="Ca, calcium" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Ca155		title="Sc, scandium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Sc333	title="Ti, titanium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Ti425	title="V, vanadium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | V444	title="Cr, chromium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Cr339	title="Mn, manganese" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Mn221	title="Fe, iron" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Fe340	title="Co, cobalt" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Co377	title="Ni, nickel" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Ni379	title="Cu, copper" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Cu300	title="Zn, zinc" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Zn115	title="Ga, gallium" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Ga256	title="Ge, germanium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Ge334	title="As, arsenic" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | As32.4	title="Se, selenium" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | Se95.5	title="Br, bromine" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | Br30.0	title="Kr, krypton" style="text-align:center; padding:0.15em; background:#ffccff; border:Template:Element color;" | Kr9.08
5	title="Rb, rubidium" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | Rb75.8	title="Sr, strontium" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Sr141		title="Y, yttrium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Y390	title="Zr, zirconium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Zr573	title="Nb, niobium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Nb690	title="Mo, molybdenum" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Mo617	title="Tc, technetium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Tc585	title="Ru, ruthenium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Ru619	title="Rh, rhodium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Rh494	title="Pd, palladium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Pd358	title="Ag, silver" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Ag254	title="Cd, cadmium" style="text-align:center; padding:0.15em; background:#cfc; border:Template:Element color;" | Cd99.9	title="In, indium" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | In232	title="Sn, tin" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Sn296	title="Sb, antimony" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Sb193	title="Te, tellurium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Te114	title="I, iodine" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | I41.6	title="Xe, xenon" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | Xe12.6
6	title="Cs, caesium" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | Cs63.9	title="Ba, barium" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Ba140	Template:Element cell-asterisk	title="Lu, lutetium" style="text-align:center; padding:0.15em; background:#ffc; border:Template:Element color;" | Lu414	title="Hf, hafnium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Hf648	title="Ta, tantalum" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Ta733	title="W, tungsten" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | W807	title="Re, rhenium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Re704	title="Os, osmium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Os678	title="Ir, iridium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Ir564	title="Pt, platinum" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Pt510	title="Au, gold" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Au342	title="Hg, mercury (element)" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | Hg59.1	title="Tl, thallium" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Tl165	title="Pb, lead" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Pb179	title="Bi, bismuth" style="text-align:center; padding:0.15em; background:#99ccff; border:Template:Element color;" | Bi179	title="Po, polonium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Po103	title="At, astatine" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | At54.4	title="Rn, radon" style="text-align:center; padding:0.15em; background:#ccffcc; border:Template:Element color;" | Rn18.1
7	title="Fr, francium" style="text-align:center; padding:0.15em; background:#cfc; border:Template:Element color;" | Fr65	title="Ra, radium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Ra113	Template:Element cell-asterisk	title="Lr, lawrencium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Lrn/a	title="Rf, rutherfordium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Rfn/a	title="Db, dubnium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Dbn/a	title="Sg, seaborgium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Sgn/a	title="Bh, bohrium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Bhn/a	title="Hs, hassium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Hsn/a	title="Mt, meitnerium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Mtn/a	title="Ds, darmstadtium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Dsn/a	title="Rg, roentgenium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Rgn/a	title="Cn, copernicium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Cnn/a	title="Nh, nihonium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Nhn/a	title="Fl, flerovium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Fln/a	title="Mc, moscovium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Mcn/a	title="Lv, livermorium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Lvn/a	title="Ts, tennessine" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Tsn/a	title="Og, oganesson" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Ogn/a
colspan="4" Template:Element cell-asterisk	title="La, lanthanum" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | La400	title="Ce, cerium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Ce398	title="Pr, praseodymium" style="text-align:center; padding:0.15em; background:#ffc; border:Template:Element color;" | Pr331	title="Nd, neodymium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Nd289	title="Pm, promethium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Pm289	title="Sm, samarium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Sm172	title="Eu, europium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Eu176	title="Gd, gadolinium" style="text-align:center; padding:0.15em; background:#ffc; border:Template:Element color;" | Gd301	title="Tb, terbium" style="text-align:center; padding:0.15em; background:#ffc; border:Template:Element color;" | Tb391	title="Dy, dysprosium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Dy280	title="Ho, holmium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Ho251	title="Er, erbium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Er280	title="Tm, thulium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Tm191	title="Yb, ytterbium" style="text-align:center; padding:0.15em; background:#9cf; border:Template:Element color;" | Yb129
colspan="4" Template:Element cell-asterisk	title="Ac, actinium" style="text-align:center; padding:0.15em; background:#ffc; border:Template:Element color;" | Ac400	title="Th, thorium" style="text-align:center; padding:0.15em; background:#ffffcc; border:Template:Element color;" | Th514	title="Pa, protactinium" style="text-align:center; padding:0.15em; background:#ffc; border:Template:Element color;" | Pa481	title="U, uranium" style="text-align:center; padding:0.15em; background:#ffc; border:Template:Element color;" | U417	title="Np, neptunium" style="text-align:center; padding:0.15em; background:#ffc; border:Template:Element color;" | Np336	title="Pu, plutonium" style="text-align:center; padding:0.15em; background:#ffc; border:Template:Element color;" | Pu333	title="Am, americium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Amn/a	title="Cm, curium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Cmn/a	title="Bk, berkelium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Bkn/a	title="Cf, californium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Cfn/a	title="Es, einsteinium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Esn/a	title="Fm, fermium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Fmn/a	title="Md, mendelevium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Mdn/a	title="No, nobelium" style="text-align:center; padding:0.15em; background:#ffffff; border:Template:Element color;" | Non/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
	(K)	(°C)	(°F)	(kJ/mol)	(J/g)
Acetone	329	56	133	31.300	538.9
Aluminium	2792	2519	4566	294.0	10500
Ammonia	240	−33.34	−28	23.35	1371
Butane	272–274	−1	30–34	21.0	320
Diethyl ether	307.8	34.6	94.3	26.17	353.1
Ethanol	352	78.37	173	38.6	841
Hydrogen (parahydrogen)	20.271	−252.879	−423.182	0.8992	446.1
Iron	3134	2862	5182	340	6090
Isopropyl alcohol	356	82.6	181	44	732.2
Methane	112	−161	−259	8.170	480.6
Methanol	338	64.7	148	35.2	1104
Propane	231	−42	−44	15.7	356
Phosphine	185	−87.7	−126	14.6	429.4
Water	373.15	100	212	40.66	2257

See also

• Clausius–Clapeyron relation
• Shimansky equation, describes the temperature dependence of the heat of vaporization
• Enthalpy of fusion, specific heat of melting
• Enthalpy of sublimation
• Joback method, estimation of the heat of vaporization at the normal boiling point from molecular structures
• Latent heat

References

• CODATA Key Values for Thermodynamics
• NIST Chemistry WebBook