Physical Chemistry , 1st ed.

the Gvalue is not zero. This process is not just the phase transition. It in-
cludes a change in temperature as well.
One consequence of equation 6.6 comes from the equation for the isother-
mal G:

GHTS

IfGis zero for an isothermal phase transition, then we have

0 transHTtranstransS

Rewriting, we have

transS



T

tr

t

a

r

n

an

sH

s

(6.7)

Since transHrepresents the vapHand fusHvalues that are commonly tabu-
lated, it is relatively easy to calculate the change in entropy that accompanies a
phase transition. However,vapHand fusHvalues are usually tabulated as
positivenumbers. This implies an endothermic process. Only fusion and va-
porization are endothermic; condensation phase transitions (gas to liquid and
gas to solid) and crystallization or solidification phase transitions are exother-
mic.When using equation 6.7 to calculate the change in entropy, the endo- or
exothermicity of the process must be determined to get the correct sign on
transS. Example 6.3 illustrates this.

Example 6.3

Calculate the change in entropy for the following phase transitions.

a.One mole of mercury liquid, Hg, freezes at its normal melting point of

38.9°C. The heat of fusion of mercury is 2.33 kJ/mol.

b.One mole of carbon tetrachloride, CCl 4 , vaporizes at its normal boiling point

of 77.0°C. The heat of vaporization of carbon tetrachloride is 29.89 kJ/mol.

6.3 Phase Transitions 147

Table 6.2 Values for enthalpy and entropy of phase transitionsa

Substance fusH vapH subH fusS vapS subS

Acetic acid 11.7 23.7 51.6 (15°C) 40.4 61.9 107.6 ( 35  10 °C)

Ammonia 5.652 23.35 28.93 97.4

Argon 1.183 6.469 74.8

Benzene 9.9 30.7 33.6 (1°C) 38.0 87.2 133 ( 30  5 °C)

Carbon dioxide 8.33 15.82 25.23

Dimethyl sulfoxide 13.9 43.1 52.9 (4°)

Ethanol 5.0 38.6 42.3 (1°C) 109.8

Gallium 5.59 270.3 286.2 18.44

Helium 0.0138 0.0817 4.8 19.9

Hydrogen 0.117 0.904 8.3 44.6

Iodine 15.52 41.95 62.42

Mercury 2.2953 51.9 61.38 92.92

Methane 0.94 8.2 73.2 91.3 ( 190 °C)

Naphthalene 19.0 43.3 72.6 (10°C) 82.6 167

Oxygen 0.444 6.820 8.204 8.2 75.6

Water 6.009 40.66 50.92 22.0 109.1

Sources:J. A. Dean, ed.Lange’s Handbook of Chemistry,14th ed., McGraw-Hill, New York, 1992; D. R. Lide, ed.,CRC

Handbook of Chemistry and Physics,a 82nd ed., CRC Press, Boca Raton, Fla., 2001.

All H’s are in kJ/mol and all S’s are in J/(molK). All values are applicable to the normal melting and boiling

points of the substances. Sublimation data are applicable to standard temperature unless otherwise noted.