COMMUNICATIONS TO THE EDITOR

2238

Vol. 67

COMMUNICATIONS TO THE EDITOR TEMPERATURE DEPEKDEKCE OF LATENT HEAT OF VAPORIZATION

Sir: The temperature dependence of latent heat of vaporization is usually represented on the basis of the following empirical equation proposed several years ago.1-6 X, = (.(To - T)“

(4)

Associated liquids

A,

350.8K”8v05/3(T, - T)”/” [PI”/”

=

loooTI

(1)

where X, is the molal latent heat, in cal./niole, T , is the critical temperature, OK., and a and n are constants. Of the two constants appearing in the equation, n has a value in the range 0.35 to 0.40 and a is characteristic of the liquid and is essentially independent of temperature. In this note justification will be cited for the form of latent heat correlation indicated which would establish the numerical value of the constant 72 and relate the other constant to temperature independent properties. Starting from the Eotvos equation5band making use of the equation for Parachor, the critical temperature function ( T , - T ) ’ / 8may be related as

220.3Ka/8~v,5~3(Tc - T)”/” [P]’/z

X, =

I

I

I

(44

T - r T q

L

-

-

500 400 -

300 -

200-

* ‘0

x looa , -

-

5040-

30

-

20 -

where K is the Eotvos constant. It has been further shown that the product of molal latent heat of vaporization and liquid molal volume raised to the 5/4thpower is practically independent of temperature. This empirical observation bears close resemblance to the equation of Bowden and Jonese7 The latent heat function : XvV{/4 has been satisfactorily correlated against the critical molal volume (Fig. 1) for two classes of compounds : normal liquids and associated liquids. The following two expressions have been obtained on the basis of the best fit of experimental data presented in Fig. 1. Normal liquids X v ~ 1 5 / 4=

4 = 220.3~,‘/~

(3)

I

O 20

U 30

I I 1 I l l 50 I00

I 200

*

NORMAL

0

ASSOCIATED

1

300

I

500

d IO00

Vc iml/grn mole)-

Fig. 1.-Latent Q =

v:/4xv

heat correlation for some liquids: ,/120.00:/3 normal =

(350.80,”/3associated

Comparison of eq. 4 or 4a with eq. 1 clearly indicates that the constant n is 0.375 and that the other constant a is essentially independent of temperature. X more detailed investigation is in progress and the results thereof will appear elsewhere. G . SARSIRIHAY DlvIsIos O F CHEMIC.4L ENGINEEHIXG S-4TIoNAL CHERfICAL LABOR.4TORY

Associated liquids X v ~ 1 5 / 4=

Poos.4-8, ISOIA

4

=

350.8~,”/”

RECEIVED MAY20, 1963

(3a)

The preceding equations have been established on the basis of data accumulated for about’ 15 liquids, both normal and associated, in a temperature range of 0-50’. At temperatures far removed from critical, the assumption that (v, - u1) is approximately equal to zlg is valid and this leads to t’he equation for the temperature dependence of latent heat. Normal liquids Roy. Belge, 12, 296 (1926). (2) J. C. DeWijs, J . Phgs. Chem., 54, 599 (1950). (3) J. C. De Wijs, Rec. trar. ch