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Encyclopedia > Partial pressure

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume.


In chemistry, the partial pressure of a gas in a mixture of gases or of a gas dissolved in a liquid is the pressure which that gas would have if all other components of the gas mixture or liquid suddenly vanished without its temperature changing.[1] The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases will always flow from a region of higher partial pressure to one of lower pressure; the larger this difference, the faster the flow. Chemistry (from the Greek word χημεία (chemeia) meaning cast together or pour together) is the science of matter at the atomic to molecular scale, dealing primarily with collections of atoms (such as molecules, crystals, and metals). ... A gas is one of the four main phases of matter (after solid and liquid, and followed by plasma), that subsequently appear as a solid material is subjected to increasingly higher temperatures. ... Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface. ... In thermodynamics, temperature is the physical property of a system that underlies the common notions of hot and cold —something that is hotter has the greater temperature. ... In chemistry, a molecule is an aggregate of at least two atoms in a definite arrangement held together by special forces. ...


Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases (i.e., liquid or solid). Most often the term is used to describe a liquid's tendency to evaporate. It is a measure of the tendency of molecules and atoms to escape from a liquid or a solid. A liquid's boiling point corresponds to the point where its vapor pressure is equal to the surrounding atmospheric pressure. Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases. ... Vapor (US English) or vapour (British English) is the gaseous state of matter. ... A liquid will assume the shape of its container. ... Evaporation is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state. ... In chemistry, a molecule is an aggregate of at least two atoms in a definite arrangement held together by special forces. ... Properties For other uses, see Atom (disambiguation). ... In jewelry, a solid gold piece is the alternative to gold-filled or gold-plated jewelry. ...


Gases dissolve, diffuse, and react according to their partial pressures, and not necessarily according to their concentrations in a gas mixture.

In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. ...

Contents


Dalton's law of partial pressures

The pressure of an ideal gas in a mixture is equal to the pressure it would exert if it occupied the same volume alone at the same temperature. This is because ideal gas molecules are so far apart that they don't interfere with each other at all. Actual real-world gases come very close to this ideal. In chemistry and physics, Daltons law (also called Daltons law of partial pressure) is related to the ideal gas laws. ... An ideal gas or perfect gas is a hypothetical gas consisting of identical particles of negligible volume, with no intermolecular forces. ...


A consequence of this is that the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the individual gases in the mixture as stated by Dalton's law.[2] For example, given an ideal gas mixture of nitrogen (N2), hydrogen (H2) and ammonia (NH3): In chemistry and physics, Daltons law (also called Daltons law of partial pressure) is related to the ideal gas laws. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Ammonia is a compound of nitrogen and hydrogen with the formula NH3. ...

P = P_{{mathrm{N}}_2} + P_{{mathrm{H}}_2} + P_{{mathrm{NH}}_3}
where:  
P = total pressure of the gas mixture
P_{{mathrm{N}}_2} = partial pressure of nitrogen (N2)
P_{{mathrm{H}}_2} = partial pressure of hydrogen (H2)
P_{{mathrm{NH}}_3} = partial pressure of ammonia (NH3)

Ideal gas mixtures

The mole fraction of an individual gas component in an ideal gas mixture can be expressed in terms of the component's partial pressure or the moles of the component: The mole fraction is one way of expressing the relative concentration of a given species. ... The mole and its simple conversions into different units of measurements. ...

x_{mathrm{i}} = frac{P_{mathrm{i}}}{P} = frac{n_{mathrm{i}}}{n}

and the partial pressure of an individual gas component in an ideal gas can be obtained using this expression:

P_{mathrm{i}} = x_{mathrm{i}} cdot P
where:  
xi = mole fraction of any individual gas component in a gas mixture
Pi = partial pressure of any individual gas component in a gas mixture
ni = moles of any individual gas component in a gas mixture
n = total moles of the gas mixture

The mole fraction of a gas component in a gas mixture is equal to the volumetric fraction of that component in a gas mixture.[3]


Equilibrium constants of reactions involving gas mixtures

It is possible to work out the equilibrium constant for a chemical reaction involving a mixture of gases given the partial pressure of each gas and the overall reaction formula. For a reversible reaction involving gas reactants and gas products, such as:

In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction which is a useful tool to determine the concentration of various reactants or products in a system where chemical equilibrium occurs. ...

a,A + b,B leftrightarrow c,C + d,D

the equilibrium constant of the reaction would be:

K_P = frac{P_C^c, P_D^d} {P_A^a, P_B^b}
where:  
KP =  the equilibrium constant of the reaction
a =  coefficient of reactant A
b =  coefficient of reactant B
c =  coefficient of product C
d =  coefficient of product D
P_C^c =  the partial pressure of C raised to the power of c
P_D^d =  the partial pressure of D raised to the power of d
P_A^a =  the partial pressure of A raised to the power of a
P_B^b =  the partial pressure of B raised to the power of b

For reversible reactions, changes in the total pressure, temperature or reactant concentrations will shift the equilibrium so as to favor either the right or left side of the reaction in accordance with Le Chatelier's Principle. However, the reaction kinetics may either oppose or enhance the equilibrium shift. In some cases, the reaction kinetics may be the over-riding factor to consider. Chemical equilibrium is the state in which a chemical reaction proceeds at the same rate as its reverse reaction; the rates of the forward and reverse reactions are equal, and the concentration of the reactants and products stop changing. ... In chemistry, Le Chateliers principle can be used to predict the effect of a change in conditions on a chemical equilibrium. ... In physical chemistry, chemical kinetics or reaction kinetics study reaction rates in a chemical reaction. ...


Henry's Law and the solubility of gases

Gases will dissolve in liquids to an extent that is determined by the equilibrium between the undissolved gas and the gas that has dissolved in the liquid (called the solvent).[4] The equilibrium constant for that equilibrium is: For the connotation of the term relating to chemistry, see Solvation. ... A liquid will assume the shape of its container. ... A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. ...

(1)     k = frac {P_X}{C_X}
where:  
k =  the equilibrium constant for the solvation process
PX =  partial pressure of gas X in equilibrium with a solution containing some of the gas
CX =  the concentration of gas X in the liquid solution

The form of the equilibrium constant shows that the concentration of a solute gas in a solution is directly proportional to the partial pressure of that gas above the solution. This statement is known as Henry's Law and the equilibrium constant k is quite often referred to as the Henry's Law constant.[4][5][6] Solvation is the attraction and association of molecules of a solvent with molecules or ions of a solute. ... Dissolving table salt in water In chemistry, a solution is a homogeneous mixture composed of one or more substances, known as solutes, dissolved in another substance, known as a solvent. ... A substance is soluble in a fluid if it dissolves in the fluid. ... In chemistry, Henrys law is one of the gas laws. ...


Henry's Law is sometimes written as:[7]

(2)     k' = frac {C_X}{P_X}

where k' is also referred to as the Henry's Law constant.[7] As can be seen by comparing equations (1) and (2) above, k' is the reciprocal of k. Since both may be referred to as the Henry's Law constant, readers of the technical literature must be quite careful to note which version of the Henry's Law equation is being used.


Henry's Law is an approximation that only applies for dilute, ideal solutions and for solutions where the liquid solvent does not react chemically with the gas being dissolved. A chemical reaction is a process that results in the interconversion of chemical substances [1]. The substance or substances initially involved in a chemical reaction are called reactants. ...


Partial pressure in diving breathing gases

In recreational diving and professional diving the richness of individual component gases of breathing gases is expressed by partial pressure. Recreational diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment. ... Professional diving is diving for payment. ... Air is the most common and only natural breathing gas. ...


Using diving terms, partial pressure is calculated as:

partial pressure = total absolute pressure x volume fraction of gas component

For the component gas "i":

ppi = P x Fi

For example, at 50 metres (165 feet), the total absolute pressure is 6 bar (i.e., 1 bar of atmospheric pressure + 5 bar of water pressure) and the partial pressures of the main components of air, oxygen 21% by volume and nitrogen 79% by volume are: diurnal (daily) rhythm of air pressure in northern Germany (black curve is air pressure) Atmospheric pressure is the pressure above any area in the Earths atmosphere caused by the weight of air. ... Impact of a drop of water. ... Layers of Atmosphere (NOAA) Air redirects here. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance transparent (gas) very pale blue (liquid) Atomic mass 15. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ...

ppN2 = 6 bar x 0.79 = 4.74 bar absolute
ppO2 = 6 bar x 0.21 = 1.26 bar absolute
where:  
ppi = partial pressure of gas component i  = Pi in the terms used in this article
P = total pressure = P in the terms used in this article
Fi = volume fraction of gas component i  =  mole fraction, xi, in the terms used in this article
ppN2 = partial pressure of nitrogen  = P_{{mathrm{N}}_2} in the terms used in this article
ppO2 = partial pressure of oxygen  = P_{{mathrm{O}}_2} in the terms used in this article

The minimum safe lower limit for the partial pressures of oxygen in a gas mixture is 0.16 bar absolute. Hypoxia and sudden unconsciousness becomes a problem with an oxygen partial pressure of less than 0.16 bar absolute. The NOAA Diving Manual recommends a maximum single exposure of 45 minutes at 1.6 bar absolute, of 120 minutes at 1.5 bar absolute, of 150 minutes at 1.4 bar absolute, of 180 minutes at 1.3 bar absolute and of 210 minutes at 1.2 bar absolute. Oxygen toxicity, involving convulsions, becomes a risk when these oxygen partial pressures and exposures are exceeded. The partial pressure of oxygen determines the maximum operating depth of a gas mixture. Hypoxia is a pathological condition in which the body as a whole (generalised hypoxia) or region of the body (tissue hypoxia) is deprived of adequate oxygen supply. ... The National Oceanic and Atmospheric Administration (NOAA) is a scientific agency of the United States Department of Commerce focused on the conditions of the oceans and the atmosphere. ... Oxygen toxicity or oxygen toxicity syndrome is severe hyperoxia caused by breathing oxygen at elevated partial pressures. ... In technical diving, the maximum operating depth (MOD) of a breathing gas is the depth at which the partial pressure of oxygen (ppO2) of the gas mix exceeds a safe limit. ...


Nitrogen narcosis is a problem with gas mixes containing nitrogen. A typical planned maximum partial pressure of nitrogen for technical diving is 3.5 bar absolute, based on an equivalent air depth of 35 metres (115 feet). Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in SCUBA divers at depth. ... Technical diving is a form of SCUBA diving that exceeds the scope of recreational diving. ... In technical diving, the equivalent air depth (EAD) is a way of expressing the narcotic effect of breathing gases such as trimix and heliox. ...


References

  1. ^ University of Illinois class notes on thermodynamics
  2. ^ Chemistry notes at Ohio Sate University
  3. ^ Pittsburgh University chemical engineering class notes
  4. ^ a b PSIgate University Introductory Chemistry
  5. ^ University of Delware physical chemistry lecture
  6. ^ Rice University chemistry class notes
  7. ^ a b University of Arizona chemistry class notes

See also


  Results from FactBites:
 
Dalton's Law of Partial Pressures | World of Chemistry (1068 words)
The pressure exerted by one gas in a mixture of gases is the partial pressure of that individual gas.
Atmospheric pressure is the sum of the partial pressures of all of the gases present in the atmosphere.
The total pressure of the air is the sum of the partial pressures of all of the gases present.
Partial pressure - Wikipedia, the free encyclopedia (1119 words)
The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules.
Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases (i.e., liquid or solid).
A consequence of this is that the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the individual gases in the mixture as stated by Dalton's law.
  More results at FactBites »


 

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