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Clausius-Clapeyron Equation for Some Common Liquids

​
compound
H
2
O
plot
near normal BP
extended range
log scale
The Clausius–Clapeyron equation determines the vapor pressure
p
of a liquid as a function of temperature
T
(in degrees K). In differential form, the CC equation can be written
dlnp
dT
=
Δ
o
H
vap
R
2
T
, where
Δ
o
H
vap
is the standard molar enthalpy of vaporization. If the enthalpy of vaporization is assumed constant over a temperature range, the equation can be integrated to give
ln
p
p
0
=
Δ
o
H
vap
R
1
T
0
-
1
T
. For
p
0
=1
atm,
T
0
is the normal boiling point. Data for 20 common gases is tabulated. The first set of plots, in the vicinity of the normal boiling point (marked by a red circle), is likely to be quite accurate. Vapor pressure data for each compound is also tabulated over an extended range of temperatures, from the triple point (TP) to the critical point (CP). Results farther removed from the normal BP are likely to be less dependable. Improved accuracy can be obtained using the Antoine equation
log
10
p=A-
B
C+T
, where
A
,
B
,
C
are empirical constants for each substance.
A plot of
logp
versus
1/T
represents the Clausius–Clapeyron equation as a straight line.
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