Temperature-dependency of the heats ofvaporization for water, methanol, benzene, andacetone

Fig. 1 Schematic cross section of the proposed vaporizationmodel for monoatomic liquids with one atomic surface layer.

Enthalpy of vaporizationFrom Wikipedia, the free encyclopedia (Redirected from Heat of vaporization)

The enthalpy of vaporization, (symbol ), also known as the heat ofvaporization or heat of evaporation, is the energy required to transform a givenquantity of a substance from a liquid into a gas at a given pressure (oftenatmospheric pressure).

It is often measured at the normal boiling point of a substance; although tabulatedvalues are usually corrected to 298 K, the correction is often smaller than theuncertainty in the measured value.

The heat of vaporization is temperature-dependent, though a constant heat ofvaporization can be assumed for small temperature ranges and for Tr<<1.0. Theheat of vaporization diminishes with increasing temperature and it vanishescompletely at the critical temperature (Tr=1) because above the critical temperaturethe liquid and vapor phases no longer exist, since the substance is a supercriticalfluid.

Contents1 Units2 Physical model for vaporization3 Enthalpy of condensation4 Thermodynamic background5 Vaporization enthalpy of electrolyte solutions6 Selected values

6.1 Elements6.2 Other common substances

7 See also8 References

UnitsValues are usually quoted in J/mol or kJ/mol (molar enthalpy of vaporization), although kJ/kg or J/g (specific heat of vaporization), andolder units like kcal/mol, cal/g and Btu/lb are sometimes still used, among others.

Physical model for vaporizationA simple physical model for the liquid-gas phase transformation hasbeen proposed recently.[1] It is suggested that the energy required tofree an atom from the liquid is equivalent to the energy needed toovercome the surface resistance of the liquid. The model allowscalculating the latent heat by multiplying the maximum surface areacovering an atom (Fig. 1) with the surface tension and the number ofatoms in the liquid. The calculated latent heat of vaporization values forthe investigated 45 elements agrees well with experiments.

Enthalpy of condensation

V · T · E (//en.wikipedia.org/w/index.php?title=Template:Periodic_table_(enthalpy_of_vaporisation)&action=edit)

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 condensationare always negative (heat is released by the substance).

Thermodynamic backgroundThe enthalpy of vaporization can be viewed as the energy required toovercome the intermolecular interactions in the liquid (or solid, in the caseof sublimation). Hence helium has a particularly low enthalpy ofvaporization, 0.0845 kJ/mol, as the van der Waals forces between heliumatoms are particularly weak. On the other hand, the molecules in liquidwater are held together by relatively strong hydrogen bonds, and itsenthalpy of vaporization, 40.65 kJ/mol, is more than five times the energyrequired to heat the same quantity of water from 0 °C to 100 °C (cp =75.3 J K−1 mol−1). Care must be taken, however, when using enthalpiesof vaporization to measure the strength of intermolecular forces, as theseforces may persist to an extent in the gas phase (as is the case withhydrogen fluoride), and so the calculated value of the bond strength willbe too low. This is particularly true of metals, which often form covalently bonded molecules in the gas phase: in these cases, theenthalpy 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 compensatefor the drop in entropy when a gas condenses to a liquid. As the liquid and gas are in equilibrium at the boiling point (Tb), ΔvG = 0,which leads to:

As neither entropy nor enthalpy vary greatly with temperature, it is normal to use the tabulated standard values without any correctionfor the difference in temperature from 298 K. A correction must be made if the pressure is different from 100 kPa, as the entropy of agas is proportional to its pressure (or, more precisely, to its fugacity): the entropies of liquids vary little with pressure, as thecompressibility 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 theintermolecular forces. As a given quantity of matter always has a higher entropy in the gas phase than in a condensed phase ( isalways positive), and from

,

the Gibbs free energy change falls with increasing temperature: gases are favored at higher temperatures, as is observed in practice.

Vaporization enthalpy of electrolyte solutionsEstimation of the enthalpy of vaporization of electrolyte solutions can be simply carried out using equations based on the chemicalthermodynamic models, such as Pitzer model[2] or TCPC model.[3]

Selected values

Elements

Enthalpies of vaporization of the elements1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Group →↓ Period

H He

1 0.44936 0.0845

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 * Hf

575Ta743

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 ** Rf

n/aDbn/a

Sgn/a

Bhn/a

Hsn/a

Mtn/a

Dsn/a

Rgn/a

Cnn/a

Uutn/a

Fln/a

Uupn/a

Lvn/a

Uusn/a

Uuon/a

* Lanthanides 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

** Actinides 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 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

Other common substances

Enthalpies of vaporization of common substances, measured at their respective standard boiling points:

CompoundHeat of vaporization

(kJ mol-1)Heat of vaporization

(kJ kg−1)Ammonia 23.35 1371

Butane 21.0 320

Ethanol 38.6 841

Hydrogen 0.46 451.9

Methane 8.17 480.6

Methanol 35.3 1104

Propane 15.7 356

Phosphine 14.6 429.4

Water 40.68 2260

Aluminium 294.0 10500

See alsoEnthalpy of fusionEnthalpy of sublimationJoback method (Estimation of the heat of vaporization at the normal boiling point from molecular structures)

References1. ^ Garai, J. (2009). "Physical model for vaporization". Fluid Phase Equilibria 283: 89–77. doi:10.1016/j.fluid.2009.06.005

(http://dx.doi.org/10.1016%2Fj.fluid.2009.06.005).2. ^ X. Ge, X. Wang. Estimation of Freezing Point Depression, Boiling Point Elevation and Vaporization enthalpies of electrolyte solutions.

Ind. Eng. Chem. Res. 48(2009)2229-2235. http://pubs.acs.org/doi/abs/10.1021/ie801348c (Correction: 2009, 48,5123)http://pubs.acs.org/doi/abs/10.1021/ie900434h

3. ^ X. Ge, X. Wang. Calculations of Freezing Point Depression, Boiling Point Elevation, Vapor Pressure and Enthalpies of Vaporization ofElectrolyte Solutions by a Modified Three-Characteristic Parameter Correlation Model. J. Sol. Chem. 38(2009)1097-1117.http://www.springerlink.com/content/21670685448p5145/

CODATA Key Values for Thermodynamics (http://www.codata.org/resources/databases/key1.html)Kugler HK & Keller C (eds) 1985, Gmelin handbook of inorganic and organometallic chemistry, 8th ed., 'At, Astatine',system no. 8a, Springer-Verlag, Berlin, ISBN 3-540-93516-9, pp. 116–117NIST Chemistry WebBook (http://webbook.nist.gov/chemistry/)Sears, Zemansky et al., University Physics, Addison-Wesley Publishing Company, Sixth ed., 1982, ISBN 0-201-07199-1

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