Enthalpy of vaporization

Enthalpy of vaporization
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Molar heat content of zinc above 298.15 K and at 1 atm pressure, showing discontinuities at the melting and boiling points. The enthalpy of melting (ΔH°m) of zinc is 7323 J/mol, and the enthalpy of vaporization (ΔH°v) is 115 330 J/mol.The enthalpy of vaporization, (symbol ΔvH), also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas. It is measured at the boiling point of the substance, although tabulated values are usually corrected to 298 K: the correction is small, and is often smaller than the uncertainty in the measured value. Values are usually quoted in kJ/mol, although kJ/kg, kcal/mol, cal/g and Btu/lb are also possible, among others.

The enthalpy of condensation (or heat of condensation) is numerically exactly equal to the enthalpy of vaporization, but has 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).

The enthalpy of vaporization 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, 40.8 kJ/mol, is more than five times the energy required 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 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 (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 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 ( is always positive), and from

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

Contents [hide]
1 Selected values
1.1 Elements
1.2 Other common substances
2 See also
3 References



[edit] Selected values

[edit] Elements
Enthalpies of vaporization of the elements in kJ/mol

Group → 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
↓ Period

1 H
0.44936
He
0.0845
2 Li
145.92 Be
292.40
B
489.7 C
355.8 N
2.7928 O
3.4099 F
3.2698 Ne
1.7326
3 Na
96.96 Mg
127.4
Al
293.4 Si
384.22 P
12.129 S
1.7175 Cl
10.2 Ar
6.447
4 K
79.87 Ca
153.6 Sc
314.2 Ti
421 V
452 Cr
344.3 Mn
226 Fe
349.6 Co
376.5 Ni
370.4 Cu
300.3 Zn
115.3 Ga
258.7 Ge
330.9 As
34.76 Se
26.3 Br
15.438 Kr
9.029
5 Rb
72.216 Sr
144 Y
363 Zr
581.6 Nb
696.6 Mo
598 Tc
660 Ru
595 Rh
493 Pd
357 Ag
250.58 Cd
100 In
231.5 Sn
295.8 Sb
77.14 Te
52.55 I
20.752 Xe
12.636
6 Cs
67.74 Ba
142 *
Hf
575 Ta
743 W
824 Re
715 Os
627.6 Ir
604 Pt
510 Au
334.4 Hg
59.229 Tl
164.1 Pb
177.7 Bi
104.8 Po
60.1 At
114 Rn
16.4
7 Fr
n/a Ra
37 **
Rf
n/a Db
n/a Sg
n/a Bh
n/a Hs
n/a Mt
n/a Ds
n/a Rg
n/a Uub
n/a Uut
n/a Uuq
n/a Uup
n/a Uuh
n/a Uus
n/a Uuo
n/a


* Lanthanides La
414 Ce
414 Pr
n/a Nd
n/a Pm
n/a Sm
n/a Eu
n/a Gd
n/a Tb
n/a Dy
n/a Ho
n/a Er
n/a Tm
n/a Yb
n/a Lu
n/a
** Actinides Ac
n/a Th
514.4 Pa
n/a U
n/a Np
n/a Pu
n/a Am
n/a Cm
n/a Bk
n/a Cf
n/a Es
n/a Fm
n/a Md
n/a No
n/a Lr
n/a
0–10 kJ/mol 10–100 kJ/mol 100–300 kJ/mol >300 kJ/mol


[edit] Other common substances
Common substances sorted by heat of vaporization:

Compound Heat of vaporization (kJ/mol)
Water 40.65 (540 calories per gram)
Ethanol 38.6
Methanol 37.4
Ammonia 23.35
Butane 21.0 (362 kJ/kg)
Propane 15.7 (356 kJ/kg)
Phosphine 14.6
Methane 8.19
Compound Heat of vaporization (kJ/mol)


[edit] See also
Enthalpy of fusion

[edit] References
Sears, Zemansky et al., University Physics, Addison-Wessley Publishing Company, Sixth ed., 1982, ISBN 0-201-07199-1

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Categories: Chemical properties | Thermodynamics | Heat