How does the vapor pressure affect the atmospheric pressure?



The Vapor pressure is a substance and temperature dependent gas pressure.

In clear terms, the vapor pressure is the ambient pressure below which a liquid begins to change into a gaseous state at a constant temperature. In technology, the steam pressure in the water-air system is of particular importance.

Definitions

Substances occur in three aggregate states, namely: solid, liquid and gaseous.

If there is also a liquid phase in addition to the gas, the gas is called vapor. The vapor pressure is therefore the gas pressure in a multi-phase system.

If the temperature of a closed system is kept constant, an equilibrium is established between the liquid and the gaseous phase. The gas phase is saturated and the vapor pressure is measured. When the equilibrium has been fully established, one also speaks of saturation vapor pressure.

When the liquid phase disappears and only a gaseous phase exists, we no longer measure the vapor pressure, but the gas pressure.

If there are different substances in the system under consideration, the measured pressure of the gas phase is made up of the partial pressures of the different substances. The condition is that these behave like ideal gases (Dalton's law).

Definition in terms of chemistry

In chemistry, vapor pressure is understood to be the partial pressure of a gas (one-component system) that is in thermodynamic equilibrium with its liquid or solid phase.

Definition in terms of meteorology

In meteorology, the vapor pressure is only understood to be the partial pressure of the gas itself and, as a rule, that of the water vapor (i.e. not nitrogen, carbon dioxide, ...). The maximum vapor pressure that prevails during saturation is referred to here as the saturation vapor pressure, which is identical to the definition of vapor pressure in chemistry.

Water vapor pressure

 

 

If water and water vapor coexist in thermodynamic equilibrium, then the pressure is a pure function of the temperature:

p = ps(T)

This temperature-dependent and substance-specific pressure is called the vapor pressure and the graph of this function is called the vapor pressure curve. The vapor pressure curve ends at the critical point.

When the temperature rises, the vapor pressure and vapor density rise sharply, while the density of the liquid decreases. The properties of water and steam become more and more similar with increasing temperature, up to the critical point at T = Tc = 374.12 ° C and p = pc = 221.2 bar the difference has completely disappeared and only a single phase still exists. When the critical point is approached, the heat of vaporization disappears and strong density fluctuations occur, recognizable as critical opalescence.

Practical meaning

In an open pot, heated water boils when its vapor pressure exceeds the ambient air pressure. Like the air pressure, the boiling temperature of the (tea) water varies with the weather and decreases with increasing altitude above sea level (meters above sea level). At an altitude of 2000 m, the water boils at 93 ° C, at an altitude of 8000 m at 74 ° C (mean values ​​based on weather-related fluctuations).

The physical laws of vapor pressure and evaporation (vapor pressure curve, Clausius-Clapeyron equation, etc.) were first investigated and formulated in connection with the steam engine. Here, too, there is a coexistence of liquid and gas. The steam engine made use of the fact that the steam pressure is independent of the volume as long as one moves in the two-phase system "liquid-gas". The only thing that changes at constant temperature is the "liquid-gas" ratio. The pressure in the boiler, which moves the piston, does not change as a result of the piston movement (piston movement -> volume change in the cylinder).

calculation

The saturation vapor pressure can be calculated, for example, using the Lee-Kesler and Ambrose-Walton methods. Both methods are based on the correspondence principle, in which critical data and the acentric factor are used.

The vapor pressure equations, which use substance-specific parameters adapted to experimental data, such as the Wagner and Antoine equations, meet higher accuracy requirements.

Water vapor pressure in meteorology

In meteorology, the term vapor pressure is usually understood to mean the vapor pressure of the water vapor (water vapor pressure) and thus its partial pressure. The vapor density corresponds to the absolute humidity.

The vapor pressure according to the definition of meteorology, i.e. the partial pressure of a gas within a gas mixture, can be approximately calculated by converting the general gas equation with the following formula:

The individual symbols stand for the following quantities:

The individual gas constant for water vapor is 461.5 J / (kgK).

Since the water vapor partial pressure makes up only a small part of the air pressure, a thermodynamic equilibrium only arises after a very long period of time, which means that substantial undersaturation in the atmosphere is possible without the existing liquid water boiling immediately. For this reason and the high dynamics in the atmosphere, thermodynamically stable states are usually rarely or only briefly encountered, especially in very weather-active zones of the earth's atmosphere. Due to the locally relatively higher vapor pressure above the liquid phase (see above), if water and ice are present in a cloud at the same time, the ice particles grow at the expense of the water droplets.

The vapor pressure over a non-superheated solid phase is lower than over a liquid phase at the same temperature. If both phases are in contact with one another via the gases surrounding them, the proportion of solid matter increases at the expense of the liquid. This is due to the fact that the stronger binding of the particles in the solid and the resulting heat of fusion in the case of sublimation, i.e. the solid-gaseous phase transition, must also be overcome or applied. The consequence of this is that evaporation or evaporation of particles in the liquid phase occurs more easily and thus more frequently than sublimation of particles above the solid phase. So there are more particles in the gaseous state above the liquid than above the solid, which increases the vapor pressure locally and leads to the growth of the solid phase.

See also

literature

Tables of values ​​for the water vapor pressure:

  • K. Scheffler, J. Straub and U. Grigull (1981): Water vapor boards. Thermodynamic properties of water and steam up to 800 ° C and 800 bar. Springer publishing house. ISBN 3540109307
  • U. Grigull, J. Straub and P. Schiebener (1990): Steam tables in SI units. Water vapor boards. Springer publishing house. ISBN 3540518886
  • B.I.Lee, M.G. Kesler: AIChE J. 21, 510 (1975)
  • K.S. Pitzer, D.Z. Lippmann, R.F. Curl, Jr., C.M. Huggins, D.E. Petersen: J. Am. Chem. Soc. 77, 3433 (1955)
  • W. Wagner, J. Ewers, W. Petermann: J. Chem. Thermodyn. 8, 1049 (1976)
  • D. Ambrose, J. Walton: Pure Appl. Chem. 61, 1395 (1989)

Categories: Thermodynamics | Substance property