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preface to fifteenth edition

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THERMODYNAMICPROPERTIES 6.3<br />

6.1.1.2 Enthalpy of Vaporization (or Sublimation) When the pressure of the vapor in equilibrium<br />

with a liquid reaches 1 atm, the liquid boils and is completely converted <strong>to</strong> vapor on absorption<br />

of the enthalpy of vaporization Hv at the normal boiling point T b . A rough empirical relationship<br />

between the normal boiling point and the enthalpy of vaporization (Trou<strong>to</strong>n’s rule) is:<br />

Hv<br />

1 1<br />

88 J · mol · K (6.6)<br />

T b<br />

It is best applied <strong>to</strong> nonpolar liquids which form unassociated vapors.<br />

To a first approximation, the enthalpy of sublimation Hs at constant temperature is:<br />

Hs Hm Hv (6.7)<br />

where Hm is the enthalpy of melting.<br />

The Clapeyron equation expresses the dynamic equilibrium existing between the vapor and the<br />

condensed phase of a pure substance:<br />

dP Hv<br />

(6.8)<br />

dT TV<br />

where V is the volume increment between the vapor phase and the condensed phase. If the condensed<br />

phase is solid, the enthalpy increment is that of sublimation.<br />

Substitution of V RT/P in<strong>to</strong> the foregoing equation and rearranging gives the Clausius-Clapeyron<br />

equation,<br />

or<br />

dP Hv<br />

(6.9)<br />

PdT RT 2<br />

d(ln P)<br />

Hv R (6.10)<br />

1/T<br />

which may be used for calculating the enthalpy of vaporization of any compound provided its boiling<br />

point at any pressure is known. If an An<strong>to</strong>ine equation is available (such as Eq. (5.1), page 5.30),<br />

differentiation and insertion in<strong>to</strong> the foregoing equation gives:<br />

4.5757T2B<br />

Hv (6.11)<br />

(T C 273.15) 2<br />

Inclusion of a compressibility fac<strong>to</strong>r in<strong>to</strong> the foregoing equation, as suggested by the Haggenmacher<br />

equation improves the estimate of Hv:<br />

<br />

<br />

RT2 dP T3P<br />

1/2<br />

c<br />

Hv 1 <br />

3<br />

(6.12)<br />

P dT T P c<br />

where T c and P c are critical constants (Table 6.5). Although critical constants may be unknown, the<br />

compressibility fac<strong>to</strong>r is very nearly constant for all compounds belonging <strong>to</strong> the same family, and<br />

an estimate can be deduced from a related compound whose critical constants are available.<br />

6.1.1.3 Heat Capacity (or Specific Heat) The temperature dependence of the heat capacity is<br />

complex. If the temperature range is restricted, the heat capacity of any phase may be represented<br />

adequately by an expression such as:<br />

2<br />

C a bT cT (6.13)<br />

p

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