Partial pressure concept

The heart of the question of non-ideality deals with the determination of the distribution of the respective system components between the liquid and gaseous phases. The concepts of fugacity and activity are fundamental to the interpretation of the non-ideal systems. For a pure ideal gas the fugacity is equal to the pressure, and for a component, i, in a mixture of ideal gases it is equal to its partial pressure yjP, where P is the system pressure. As the system pressure approaches zero, the fugacity approaches ideal. For many systems the deviations from unity are minor at system pressures less than 25 psig. 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

The results so far available, and the models derived from them, indicate the following a reducing atmosphere is more favourable for amino acid synthesis. If, however, the partial pressure of methane on the primeval Earth was either zero or very low, a relatively high H2/CO or H2/CO2 ratio still allowed good rates of amino acid synthesis. It is, however, still an open question as to whether these concepts are realistic, because of the possibility that hydrogen could have escaped into space. It is arguable that in certain areas on the young Earth (and under unknown conditions),... 

Equation 2.63 is valid for any homogeneous or heterogeneous reaction. The only difference is in the definition of activities. For a species in a perfect gas-phase mixture a = pi/p°, where pi is the partial pressure of species i andp° is the standard pressure (1 bar). For a real gas-phase mixture a =f/p°, where is the fugacity of i. The fugacity concept was developed for the same reason as the activity to extend to real gases the formalism used to describe perfect gas mixtures. In the low total pressure limit (p -> 0), fi = pi. 

Inherent in Henry s law is the concept that when a liquid is exposed to a gas, a partial pressure equilibrium will be achieved between the gas and liquid phases. Thus, molecules of the gas that are physically dissolved in the liquid will exert tension that is equal to the partial pressure of the gas above the liquid. It is not neces-... 

Practically speaking, this concept explains the basis for the establishment of partial pressure equilibrium of anesthetic gas between the lung alveoli and the arterial blood. Gas molecules will move across the alveolar membrane until those in the blood, through random molecular motion, exert pressure equal to their counterparts in the lung. Similar gas tension equilibria also will be established between the blood and other tissues. For example, gas molecules in the blood will diffuse down a tension gradient into the brain until equal random molecular motion (equal pressure) occurs in both tissues. 

A Concept of Anesthetic Dose Based on Partial Pressure-Minimum Alveolar Concentration... 

An elegant new reactor concept for the growth of SiC epitaxial layers was introduced by Ellison et al. [34, 53] and is called the chimney reactor. This technique is similar to the HTCVD technique in that it is a vertical system and the material transport is achieved by allowing large clusters of Si to form. This is achieved by reducing the carrier flow, which thereby increases the partial pressure of the precursors. Hydrogen is used as the carrier gas but, again, only very small flow rates are required. 

At the same time that Dalton proposed his ideas on partial pressure, he developed the concept of vapor pressure. A vapor is the gaseous form of a substance that normally exists as a solid or liquid. A gas is a substance that exists in the gaseous states under normal conditions of temperature and pressure. The vapor pressure of a liquid is the partial pressure of the liquid s vapor at equilibrium. Liquids with strong inter-molecular forces exert lower vapor pressures than those with weak intermolecular forces. In liquids with strong intermolecular forces, it is more difficult for the molecules to leave the liquid state and enter the gaseous state. 

So far we have discussed the basic concept of non-stoichiometry without showing real examples. Here we shall consider case studies in order to understand the non-stoichiometry appearing in various kinds of substances. To construct phase diagrams which contain non-stoichiometric compounds, it is indispensable to know the relationship between the deviation from stoichiometry <5, the partial pressure of coexisted gas, (for diatomic gases),... 

We first define the concept of an ideal solution. This is the ultrasimplified limiting case in which the partial pressure of each component is simply proportional to its concentration in the liquid,... 

Note that the variable Y must be able to affect the position of equilibrium change of the partial pressure of an reactive species (e.g., inert He gas) cannot affect , and therefore cannot result in any re-equilibration change in the system. In all cases, the rigorous thermodynamic formulation (8.43) may be used to guide qualitative applications of the Le Chatelier concept. 

The turnover frequency, N, (commonly called the turnover number) defined, as in enzyme catalysis, as molecules reacting per active site in unit time, can be a useful concept if employed with care. In view of the problems in measuring the number of active sites discussed in 1.2.4, it is important to specify exactly the means used to express Q in terms of active sites. A realistic measure of such sites may be the number of surface metal atoms on a supported catalyst but in other cases estimation on the basis of a BET surface area may be the only readily available method. Of course, turnover numbers (like rates) must be reported at specified conditions of temperature, initial concentration or initial partial pressures, and extent of reaction. 

KEY CONCEPT PROBLEM 9.17 What is the partial pressure of each gas—red, yellow, and green—if the total pressure inside the following container is 600 mm Hg ... 

When the only effects that have to be taken into account are those of cavity formation in the solvent and the dispersion interactions, i.e., when both the solvent and the solute are non-polar, then Hildebrand s solubility parameter concept (Hildebrand and Scott 1950) provides good estimates of the solubility. The mole fraction of a gaseous solute, x2, in a solution in equilibrium at a partial pressure p2 of this gas, can be estimated from the following expression ... 

The concept of partial pressures can also be understood in terms of the mole fraction of component i, defined as the ratio of the number of moles of component i to the total moles of gas,... 

For an ideal gas, V is proportional to n when P and T are fixed. This means that the awkward concept of volume percentage (or fraction) discussed in Problem 5.12 can now be replaced with mole percentage or mole fraction. An assumption is that each gas occupies the entire volume of the mixture, but is at its own partial pressure. 

The above equation determines the relationship between the partial pressures of CO, C02 and 02 at equilibrium. The analysis given above is the basis of the concept of equilibrium constant discussed in the prior section. 

Approaches for calculating activity coefficients will be discussed later in this chapter. Three important concepts are introduced here. The first is that in dilute solutions the activity coefficient approaches 1 as the concentration of all electrolytes approaches zero. The second is that the activity coefficient must be calculated on the same scale (e.g., molality, molarity, etc.) as that used to express concentration. The third is that activity in the gas phase is expressed as fugacity. Because the fugacity coefficient for CO2 is greater than 0.999 under all but the most extreme conditions for sediment geochemistry (e.g., deep subsurface), the partial pressure of CO2 (Pc02 may reasonably used in the place of fugacity. 

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