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
Measurement and units[edit]
log P = A − B C + T displaystyle log P=A- frac B C+T and it can be transformed into this temperature-explicit form: T = B A − log P − C displaystyle T= frac B A-log P -C where: P displaystyle P is the absolute vapor pressure of a substance
T displaystyle T is the temperature of the substance A displaystyle A , B displaystyle B and C displaystyle C are substance-specific coefficients (i.e., constants or parameters) log displaystyle log is typically either log 10 displaystyle log _ 10 or log e displaystyle log _ e [3] A simpler form of the equation with only two coefficients is sometimes used: log P = A − B T displaystyle log P=A- frac B T which can be transformed to: T = B A − log P displaystyle T= frac B A-log P 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
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
A log-lin vapor pressure chart for various liquids 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] At the
normal boiling point of a liquid, the vapor pressure is equal to the
standard atmospheric pressure defined as 1 atmosphere[7] (760
P t o t = ∑ i P y i = ∑ i P i s a t x i displaystyle P_ tot =sum _ i Py_ i =sum _ i P_ i ^ sat x_ i , where p t o t displaystyle p_ tot is the mixture's vapor pressure, x i displaystyle x_ i is the mole fraction of component i displaystyle i in the liquid phase and y i displaystyle y_ i is the mole fraction of component i displaystyle i in the vapor phase respectively. P i s a t displaystyle P_ i ^ sat is the vapor pressure of component i displaystyle i .
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. 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:[9] ln P s o l i d S = ln P l i q u i d S − Δ H m R ( 1 T − 1 T m ) displaystyle ln ,P_ solid ^ S =ln ,P_ liquid ^ S - frac Delta H_ m R left( frac 1 T - frac 1 T_ m right) with: P s o l i d S displaystyle P_ solid ^ S = Sublimation pressure of the solid component at the temperature T < T m displaystyle T<T_ m P l i q u i d S displaystyle P_ liquid ^ S = Extrapolated vapor pressure of the liquid component at the temperature T < T m displaystyle T<T_ m Δ H m displaystyle Delta H_ m = Heat of fusion R displaystyle R = Gas constant T displaystyle T = Sublimation temperature T m displaystyle T_ m = Melting point temperature 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.
Graph of water vapor pressure versus temperature. At the normal
boiling point of 100 °C, it equals the standard atmospheric pressure
of 760
Main article:
log 10 P = 8.07131 − 1730.63 233.426 + T b displaystyle log _ 10 P=8.07131- frac 1730.63 233.426+T_ b or transformed into this temperature-explicit form: T b = 1730.63 8.07131 − log 10 P − 233.426 displaystyle T_ b = frac 1730.63 8.07131-log _ 10 P -233.426 where the temperature T b displaystyle T_ b is the boiling point in degrees
P displaystyle P_ is in Torr.
Dühring's rule[edit]
Main article: Dühring's rule
Substance
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
Propanol 2.4 kPa 0.024 18.0 20 °C Methyl isobutyl ketone 2.66 kPa 0.0266 19.95 25 °C Ethanol 5.83 kPa 0.0583 43.7 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[10] 997.8 kPa 9.978 7584 26.85 °C Carbonyl sulfide 1.255 MPa 12.55 9412 25 °C Nitrous oxide[11] 5.660 MPa 56.60 42453 25 °C Carbon dioxide 5.7 MPa 57 42753 20 °C Estimating vapor pressure from molecular structure[edit] Several empirical methods exist to estimate liquid vapor pressure from molecular structure for organic molecules. Some examples are SIMPOL,[12] the method of Moller et al.,[9] and EVAPORATION.[13][14] Meaning in meteorology[edit] 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,[15] 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.[16] See also[edit] Vapour pressure of water
Absolute humidity
Lee–Kesler method
Reid vapor pressure
Relative humidity
Relative volatility
Saturation vapor density
Triple point
True vapor pressure
Vapor–liquid equilibrium
References[edit] ^ Růžička, K.; Fulem, M. & Růžička, V. "
External links[edit] Fluid Characteristics Chart
Hyperphysics
MSDS
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