10/20/13 Vapor pressure - Wikipedia, the free encyclopedia

Vapor pressure
From Wikipedia, the free encyclopedia

Vapor pressure or equilibrium vapor pressure is the pressure

exerted by a vapor in thermodynamic equilibrium with its condensed
phases (solid or liquid) at a given temperature in a closed system. The
equilibrium vapor pressure is an indication of a liquid's evaporation rate.
It relates to the tendency of particles to escape from the liquid (or a
solid). A substance with a high vapor pressure at normal temperatures is
often referred to as volatile.

The vapor pressure of any substance increases non-linearly with

temperature according to the Clausius–Clapeyron relation. The
atmospheric pressure boiling point of a liquid (also known as the normal
boiling point) is the temperature at which the vapor pressure equals the
ambient atmospheric pressure. With any incremental increase in that
temperature, the vapor pressure becomes sufficient to overcome
atmospheric pressure and lift the liquid to form vapor bubbles inside the
bulk of the substance. Bubble formation deeper in the liquid requires a
higher pressure, and therefore higher temperature, because the fluid
pressure increases above the atmospheric pressure as the depth The picture shows the particle
increases. transition, as a result of their vapor
pressure, from the liquid phase to the
The vapor pressure that a single component in a mixture contributes to
gas phase and converse.
the total pressure in the system is called partial pressure. For example, air
at sea level, and saturated with water vapor at 20 °C, has partial
pressures of about 23 mbar of water, 780 mbar of nitrogen, 210 mbar of oxygen and 9 mbar of argon.

Contents
1 Measurement and units
2 Estimating vapor pressures with Antoine equation
3 Relation to boiling point of liquids
4 Liquid mixtures
5 Solids
6 Boiling point of water
7 Dühring's rule
8 Examples
9 Estimating vapor pressure from molecular structure
10 Meaning in meteorology
11 See also
12 References
13 External links

en.wikipedia.org/wiki/Vapor_pressure 1/8
10/20/13 Vapor pressure - Wikipedia, the free encyclopedia

Measurement and units

Vapor pressure is measured in the standard units of pressure. The International System of Units (SI) recognizes
pressure as a derived unit with the dimension of force per area and designates the pascal (Pa) as its standard unit.
One pascal is one newton per square meter (N·m−2 or kg·m−1·s−2).

Experimental measurement of vapor pressure is a simple procedure for common pressures between 1 and 200
kPa.[1] Most accurate results are obtained near the boiling point of substances and large errors result for
measurements smaller than 1 kPa. Procedures often consist of purifying the test substance, isolating it in a container,
evacuating any foreign gas, then measuring the equilibrium pressure of the gaseous phase of the substance in the
container at different temperatures. Better accuracy is achieved when care is taken to ensure that the entire
substance and its vapor are at the prescribed temperature. This is often done, as with the use of an isoteniscope, by
submerging the containment area in a liquid bath.

Estimating vapor pressures with Antoine equation

The Antoine equation [2][3] is a mathematical expression of the relation between the vapor pressure and the
temperature of pure liquid or solid substances. The basic form of the equation is:

and it can be transformed into this temperature-explicit form:

where: is the absolute vapor pressure of a substance

is the temperature of the substance

, and are substance-specific coefficients (i.e., constants or parameters)
[3]
is typically either or

A simpler form of the equation with only two coefficients is sometimes used:

which can be transformed to:

Sublimations and vaporizations of the same substance have separate sets of Antoine coefficients, as do components
in mixtures.[2] Each parameter set for a specific compound is only applicable over a specified temperature range.
Generally, temperature ranges are chosen to maintain the equation's accuracy of a few up to 8-10 percent. For
en.wikipedia.org/wiki/Vapor_pressure 2/8
10/20/13 Vapor pressure - Wikipedia, the free encyclopedia

many volatile substances, several different sets of parameters are available and used for different temperature
ranges. The Antoine equation has poor accuracy with any single parameter set when used from a compound's
melting point to its critical temperature. Accuracy is also usually poor when vapor pressure is under 10 Torr
because of the limitations of the apparatus used to establish the Antoine parameter values.

The Wagner Equation[4] gives "one of the best"[5] fits to experimental data but is quite complex. It expresses
reduced vapor pressure as a function of reduced temperature.

Relation to boiling point of liquids

Further information: Boiling point

As a general trend, vapor pressures of liquids at ambient

temperatures increase with decreasing boiling points. This is
illustrated in the vapor pressure chart (see right) that shows
graphs of the vapor pressures versus temperatures for
a variety of liquids.[6]

For example, at any given temperature, methyl chloride has

the highest vapor pressure of any of the liquids in the chart.
It also has the lowest normal boiling point (−24.2 °C),
which is where the vapor pressure curve of methyl chloride
(the blue line) intersects the horizontal pressure line of one
atmosphere (atm) of absolute vapor pressure.

Although the relation between vapor pressure and

temperature is non-linear, the chart uses a logarithmic
vertical axis to produce slightly curved lines, so one chart
can graph many liquids. A nearly straight line is obtained
when the logarithm of the vapor pressure is plotted against
1/(T+230)[7] where T is the temperature in degrees Celsius.
The vapor pressure of a liquid at its boiling point equals the
pressure of its surrounding environment.
A typical vapor pressure chart for various liquids
Liquid mixtures
Raoult's law gives an approximation to the vapor pressure of mixtures of liquids. It states that the activity (pressure
or fugacity) of a single-phase mixture is equal to the mole-fraction-weighted sum of the components' vapor
pressures:

where p tot is the mixture's vapor pressure, i is one of the components of the mixture and Χi is the mole fraction of
that component in the liquid mixture. The term piΧi is the partial pressure of component i in the mixture. Raoult's
Law is applicable only to non-electrolytes (uncharged species); it is most appropriate for non-polar molecules with
only weak intermolecular attractions (such as London forces).
en.wikipedia.org/wiki/Vapor_pressure 3/8
10/20/13 Vapor pressure - Wikipedia, the free encyclopedia

Systems that have vapor pressures higher than indicated by the above formula are said to have positive deviations.
Such a deviation suggests weaker intermolecular attraction than in the pure components, so that the molecules can
be thought of as being "held in" the liquid phase less strongly than in the pure liquid. An example is the azeotrope of
approximately 95% ethanol and water. Because the azeotrope's vapor pressure is higher than predicted by Raoult's
law, it boils at a temperature below that of either pure component.

There are also systems with negative deviations that have vapor pressures that are lower than expected. Such a
deviation is evidence for stronger intermolecular attraction between the constituents of the mixture than exists in the
pure components. Thus, the molecules are "held in" the liquid more strongly when a second molecule is present. An
example is a mixture of trichloromethane (chloroform) and 2-propanone (acetone), which boils above the boiling
point of either pure component.

Solids
Equilibrium vapor pressure can be defined as the pressure
reached when a condensed phase is in equilibrium with its
own vapor. In the case of an equilibrium solid, such as a
crystal, this can be defined as the pressure when the rate of
sublimation of a solid matches the rate of deposition of its
vapor phase. For most solids this pressure is very low, but
some notable exceptions are naphthalene, dry ice (the
vapor pressure of dry ice is 5.73 MPa (831 psi, 56.5 atm)
at 20 degrees Celsius, which causes most sealed containers
to rupture), and ice. All solid materials have a vapor
pressure. However, due to their often extremely low values,
measurement can be rather difficult. Typical techniques
include the use of thermogravimetry and gas transpiration.

There are a number of methods for calculating the

sublimation pressure (i.e., the vapor pressure) of a solid. Vapor pressure of liquid and solid benzene
One method is to estimate the sublimation pressure from
extrapolated liquid vapor pressures (of the supercooled
liquid), if the heat of fusion is known, by using this particular form of the Clausius–Clapeyron relation:[8]

with:

= Sublimation pressure of the solid component at the temperature

= Extrapolated vapor pressure of the liquid component at the temperature
= Heat of fusion
= Gas constant
= Sublimation temperature
= Melting point temperature

en.wikipedia.org/wiki/Vapor_pressure 4/8
10/20/13 Vapor pressure - Wikipedia, the free encyclopedia

This method assumes that the heat of fusion is temperature-independent, ignores additional transition temperatures
between different solid phases, and it gives a fair estimation for temperatures not too far from the melting point. It
also shows that the sublimation pressure is lower than the extrapolated liquid vapor pressure (ΔHm is positive) and
the difference grows with increased distance from the melting point.

Boiling point of water

Main article: Vapor pressure of water

Like all liquids, water boils when its vapor pressure reaches its
surrounding pressure. In nature, the atmospheric pressure is lower at
higher elevations and water boils at a lower temperature. The boiling
temperature of water for atmospheric pressures can be approximated by
the Antoine equation:

Graph of water vapor pressure versus

temperature. At the normal boiling
or transformed into this temperature-explicit form: point of 100°C, it equals the standard
atmospheric pressure of 760 Torr or
101.325 kPa.

where the temperature is the boiling point in degrees Celsius and the pressure is in Torr.

Dühring's rule
Main article: Dühring's rule

Dühring's rule states that a linear relationship exists between the temperatures at which two solutions exert the same
vapor pressure.

Examples
The following table is a list of a variety of substances ordered by increasing vapor pressure.

en.wikipedia.org/wiki/Vapor_pressure 5/8
10/20/13 Vapor pressure - Wikipedia, the free encyclopedia

Vapor Pressure Vapor Pressure Vapor Pressure

Substance Temperature
(SI units) (Bar); (mmHg);
Tungsten 100 Pa 0.001 0.75 3203 °C
Ethylene glycol 500 Pa 0.005 3.75 20 °C
Xenon difluoride 600 Pa 0.006 4.50 25 °C
Water (H2O) 2.3 kPa 0.023 17.5 20 °C
Propanol 2.4 kPa 0.024 18.0 20 °C
Ethanol 5.83 kPa 0.0583 43.7 20 °C
Methyl isobutyl ketone 2.66 kPa 0.0266 19.95 25 °C
Freon 113 37.9 kPa 0.379 284 20 °C
Acetaldehyde 98.7 kPa 0.987 740 20 °C
Butane 220 kPa 2.2 1650 20 °C
Formaldehyde 435.7 kPa 4.357 3268 20 °C
Propane 1.013 MPa 10.133 7600 25.6 °C
Carbonyl sulfide 1.255 MPa 12.55 9412 25 °C
Carbon dioxide 5.7 MPa 57 42753 20 °C

Estimating vapor pressure from molecular structure

Several empirical methods exist to estimate liquid vapor pressure from molecular structure for organic molecules.
Some examples are SIMPOL,[9] the method of Moller et al.,[8] and EVAPORATION.[10][11]

Meaning in meteorology
In meteorology, the term vapor pressure is used to mean the partial pressure of water vapor in the atmosphere,
even if it is not in equilibrium,[12] and the equilibrium vapor pressure is specified otherwise. Meteorologists also
use the term saturation vapor pressure to refer to the equilibrium vapor pressure of water or brine above a flat
surface, to distinguish it from equilibrium vapor pressure, which takes into account the shape and size of water
droplets and particulates in the atmosphere.[13]

See also
Absolute humidity
Clausius-Clapeyron relation
Partial pressure
Relative humidity
Relative volatility
Raoult's law
Saturation vapor density
en.wikipedia.org/wiki/Vapor_pressure 6/8
10/20/13 Vapor pressure - Wikipedia, the free encyclopedia

Triple point
Vapor-liquid equilibrium
Vapor pressure of water
Volatility
Reid vapor pressure
True vapor pressure
Vapor pressures of the elements (data page)

References
1. ^ K. Růžička, M. Fulem, V. Růžička. "Vapor Pressure of Organic Compounds. Measurement and Correlation"
(http://www.capec.kt.dtu.dk/documents/overview/Vapor-pressure-Ruzicka.pdf).
2. ^ a b What is the Antoine Equation? (http://antoine.frostburg.edu/chem/senese/101/liquids/faq/antoine-vapor-
pressure.shtml) (Chemistry Department, Frostburg State University, Maryland)
3. ^ a b R.K.Sinnot (2005). Chemical Engineering Design (http://books.google.ca/books?
id=DJaxUL3numgC&pg=PA331&lpg=PA331&dq=antoine+equation+constants&source=bl&ots=2c0cqzbR0t&si
g=YPuvrW2kWnWP2s4QvY9TTpxGgNM&hl=en&sa=X&ei=widrUcu9Cce0qgHq24CgCA&ved=0CGcQ6AEwBzg
K#v=onepage&q=antoine%20equation%20constants&f=false) (4th ed.). Butterworth-Heinemann. p. 331. ISBN 0-
7506-6538-6.
4. ^ Wagner, W. (1973), "New vapour pressure measurements for argon and nitrogen and an new method for
establishing rational vapour pressure equations", Cryogenics 13 (8): 470–482
5. ^ Perry's Chemical Engineers' Handbook, 7th Ed. pg 4-15
6. ^ Perry, R.H. and Green, D.W. (Editors) (1997). Perry's Chemical Engineers' Handbook (7th ed.). McGraw-Hill.
ISBN 0-07-049841-5.
7. ^ Dreisbach, R. R. and Spencer, R. S. (January 1949). "Infinite Points of Cox Chart Families and dt/dP Values at
any Pressure". Industrial and Engineering Chemistry, 41 (1). p. 176.
8. ^ a b Moller B., Rarey J., Ramjugernath D., "Estimation of the vapour pressure of non-electrolyte organic
compounds via group contributions and group interactions ", J.Mol.Liq., 143(1), 52-63, 2008
9. ^ J. F. Pankow et al. (2008). "SIMPOL.1: a simple group contribution method for predicting vapor pressures and
enthalpies of vaporization of multifunctional organic compounds" (http://www.atmos-chem-
phys.net/8/2773/2008/acp-8-2773-2008.html). Atmos. Chem. Phys. 8: 2773–2796.
10. ^ "Vapour pressure of pure liquid compounds. Estimation by EVAPORATION"
(http://tropo.aeronomie.be/models/evaporation_run.htm)
11. ^ S. Compernolle et al. (2011). "EVAPORATION: a new vapour pressure estimation method for organic molecules
including non-additivity and intramolecular interactions" (http://www.atmos-chem-phys.net/11/9431/2011/acp-11-
9431-2011.html). Atmos. Chem. Phys. 11: 9431–9450.
12. ^ Glossary (http://amsglossary.allenpress.com/glossary/search?id=vapor-pressure1) (Developed by the American
Meteorological Society)
13. ^ A Brief Tutorial (http://fermi.jhuapl.edu/people/babin/vapor/index.html) (An article about the definition of
equilibrium vapor pressure)

External links
Fluid Characteristics Chart (http://www.engineersedge.com/fluid_flow/fluid_data.htm)
Hyperphysics (http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html#c2)
MSDS Vapor Pressure (http://www.ilpi.com/msds/ref/vaporpressure.html)
Online vapor pressure calculation tool (Requires Registration) (http://www.envmodels.com/freetools.php?
menu=pression)

en.wikipedia.org/wiki/Vapor_pressure 7/8
10/20/13 Vapor pressure - Wikipedia, the free encyclopedia

Prediction of Vapor Pressures of Pure Liquid Organic Compounds

(http://www.aim.env.uea.ac.uk/aim/ddbst/pcalc_main.php)

Retrieved from "http://en.wikipedia.org/w/index.php?title=Vapor_pressure&oldid=574147533"

Categories: Thermodynamic properties Chemical engineering Meteorology Gases Pressure

This page was last modified on 23 September 2013 at 07:09.

Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may apply.
By using this site, you agree to the Terms of Use and Privacy Policy.
Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization.

en.wikipedia.org/wiki/Vapor_pressure 8/8