Environmental Engineering Reference
In-Depth Information
3.3.2 V
APOR
P
RESSURE OF
O
RGANIC
C
OMPOUNDS
,C
LAUSIUS
-C
LAPEYRON
E
QUATION
The definition of vapor pressure
P
i
is based on the equilibrium between a pure
component and its vapor. It is the equilibrium pressure of the vapor in contact
with its condensed phase (that is, a liquid or solid). If a pure liquid is in equilib-
rium with its vapor, one intuitively pictures a static system. From a macroscopic
point of view this is indeed correct. But from a molecular point of view the sit-
uation is far from serene. In fact, there is continuous interchange of molecules at
the surface, which is in a state of
dynamic equilibrium
. Temperature will greatly
influence the dynamic equilibrium and hence
P
i
is sensitive to temperature. More-
over, it should be obvious that since the intermolecular forces are vastly different
for different compounds, the range of
P
i
should be large. For typical compounds of
environmental significance, the range is between 10
−
12
and 1 bar at room temperature
(see Appendix 1).
In order to formulate the thermodynamic relationships involving
P
i
for solids
and liquids, we shall first study how a pure condensed phase (e.g., water) behaves
as the pressure and temperature are varied. This variation is usually represented on
a
P
-
V
-
T
plot called a
phase diagram
. Figure 3.2 is the phase diagram for water.
Each line in the diagram is a representation of the equilibrium between the adja-
cent phases. For example, line AC is the equilibrium curve between the vapor and
liquid phases. Point A is called the
triple point
, which is the co-existence point
of all three phases (ice, liquid water, and water vapor) in equilibrium. By defini-
tion the triple point of water is at 273.16 K. The pressure at this point for water is
Critical point
C
Ice
Water
1
Sub-cooled liquid
A
B
'
Vap or
B
T
m
=
273.0004
K
T
b
=
372.98
K
T
t
=
273.01
K
Temperature/K
FIGURE 3.2
Schematic of the phase diagram for water.
Search WWH ::
Custom Search