Pure substances behavior

After a careful examination of the phase behavior of a pure substance, we will discuss the behavior of systems which contain two or more components and point out the differences between multicomponent behavior and pure substance behavior. 

The material in this section is divided into three parts. The first subsection deals with the general characteristics of chemical substances. The second subsection is concerned with the chemistry of petroleum it contains a brief review of the nature, composition, and chemical constituents of crude oil and natural gases. The final subsection touches upon selected topics in physical chemistry, including ideal gas behavior, the phase rule and its applications, physical properties of pure substances, ideal solution behavior in binary and multicomponent systems, standard heats of reaction, and combustion of fuels. Examples are provided to illustrate fundamental ideas and principles. Nevertheless, the reader is urged to refer to the recommended bibliography [47-52] or other standard textbooks to obtain a clearer understanding of the subject material. Topics not covered here owing to limitations of space may be readily found in appropriate technical literature. 

It is impossible to have liquid carbon dioxide at temperatures above 31°C, no matter how much pressure is applied. Even at pressures as high as 1000 atm, carbon dioxide gas does not liquefy at 35 or 40°C. This behavior is typical of all substances. There is a temperature, called the critical temperature, above which the liquid phase of a pure substance cannot exist The pressure that must be applied to cause condensation at that temperature is called the critical pressure. Quite simply, the critical pressure is the vapor pressure of the liquid at the critical temperature. 

In a similar way, if we boil a sample of water until half of it has changed to steam, condense the steam to water in a different vessel, and then compare the separate samples, we find that the fractions of the original sample are indistinguishable. Such behavior on boiling (condensing) or freezing (melting) characterizes pure substances. Solutions behave differently. 

The most common states of a pure substance are solid, liquid, or gas (vapor), state property See state function. state symbol A symbol (abbreviation) denoting the state of a species. Examples s (solid) I (liquid) g (gas) aq (aqueous solution), statistical entropy The entropy calculated from statistical thermodynamics S = k In W. statistical thermodynamics The interpretation of the laws of thermodynamics in terms of the behavior of large numbers of atoms and molecules, steady-state approximation The assumption that the net rate of formation of reaction intermediates is 0. Stefan-Boltzmann law The total intensity of radiation emitted by a heated black body is proportional to the fourth power of the absolute temperature, stereoisomers Isomers in which atoms have the same partners arranged differently in space, stereoregular polymer A polymer in which each unit or pair of repeating units has the same relative orientation, steric factor (P) An empirical factor that takes into account the steric requirement of a reaction, steric requirement A constraint on an elementary reaction in which the successful collision of two molecules depends on their relative orientation. 

Thus the free energy of solvation may be calculated from the Henry s law constant or from the vapor pressure of the pure substance and the limiting activity coefficient. Thus, if the deviation of the solution from Raoult s law behavior is known, calculation of the standard state free energy of solvation requires only the vapor pressure of the pure substance (in the standard state... 

Pure substance, phase behavior of, 24 663 Pure supercritical fluids, physical properties of, 24 4... 

Liquid crystals (LC) are phase structures that are intermediate between liquid and crystal phases. They have also been mentioned as mesophases (Greek mesos = middle). Liquid crystals have an intermediate range of order between liquid and crystal phases (Soltis et al., 2004 Friberg, 1976). LC may be described as follows. If a pure substance, such as stearic acid, is heated, it melts at a very specific temperature. Heating a pure solid shows the following behavior ... 

The early application of volumetric data for hydrocarbons made use of the perfect gas laws. They were not sufficiently descriptive of the actual behavior to permit their widespread use at pressures in excess of several hundred pounds per square inch. The need for accurate metering aroused interest in the volumetric behavior of petroleum and its products at elevated pressures. Table II reviews references relating to the volumetric behavior of a number of components of petroleum and their mixtures. For many purposes the ratio of the actual volume to the volume of a perfect gas at the same pressure and temperature has been considered to be a single-valued function of the reduced pressure and temperature or of the pseudo-reduced (38) pressure and temperature. The proposals of Dodge (15), Lewis (12), and Brown (8) with their coworkers serve as examples of the nature of these correlations. The Beattie-Bridgeman (2) and Benedict (4) equations of state describe the volumetric behavior of many pure substances and their mixtures with an accuracy adequate (31) for most purposes. However, at pressures above 3000 pounds per square inch the accuracy of representation with existing constants leaves something to be desired. 

The primary tool for representing the phase behavior of a chemical system is the phase diagram, a graphical roadmap of phase stability domains. For a pure substance, with... 

Surprising behavior of liquid and ice phases is found if we follow various 7, P paths in this extended phase diagram. Sidebar 7.4 illustrates how to determine the expected phase transitions and properties of H20 for various temperatures and pressures far outside the realm of ordinary experience. It is remarkable that such multiplicity of forms and properties can result from a pure substance composed of only a single type of molecule. 

We will first consider systems which consist of a single, pure substance. These systems behave differently from systems made up of two or more components. In particular, we will be interested in phase behavior, that is, the conditions of temperature and pressure for which different phases can exist. 

Next we will consider the phase behavior of mixtures of two components. The petroleum engineer does not normally work with two-component systems usually mixtures consisting of many components are encountered. However, it is instructive to observe the differences in phase behavior between two-component mixtures and pure substances. These differences are amplified in multicomponent mixtures. 

The similarity of this graph to the graph showing the density of a pure substance indicates that the law of corresponding states should hold for viscosity as well as for volumetric behavior. 

In many solutions strong interactions may occur between like molecules to form polymeric species, or between unlike molecules to form new compounds or complexes. Such new species are formed in solution or are present in the pure substance and usually cannot be separated from the solution. Basically, thermodynamics is not concerned with detailed knowledge of the species present in a system indeed, it is sufficient as well as necessary to define the state of a system in terms of the mole numbers of the components and the two other required variables. We can make use of the expressions for the chemical potentials in terms of the components. In so doing all deviations from ideal behavior, whether the deviations are caused by the formation of new species or by the intermolecular forces operating between the molecules, are included in the excess chemical potentials. However, additional information concerning the formation of new species and the equilibrium constants involved may be obtained on the basis of certain assumptions when the experimental data are treated in terms of species. The fact that the data may be explained thermodynamically in terms of species is no proof of their existence. Extra-thermodynamic studies are required for the proof. 

Figure 3.2 indicates the complexity of the PVT behavior of a pure substance and suggests the difficulty of its description by an equation. However, for the gas region alone relatively simple equations often suffice. For an isotherm such as Tj we note from Fig. 3.2 that as P increases V decreases. Thus the PV product for a gas or vapor should be much more nearly constant than either of its members. This suggests the representation of PV along an isotherm by a power series expansion in P ... 

The urine (20 ml) of anadromous, ovulated females was collected by catheterization. Repeated fractionation, guided by male behavioral response, over complementary chromatographic supports led to the isolation of a sufficient quantity of pure substance for full spectroscopic characterization. The active pheromonal compound was shown to be L-kynurenine (6). It was estimated to be present at a concentration of 1.1 mg 100 ml-1 of urine. Interestingly, that is a hundred or more times higher than the concentration in the ovulated female urine of rainbow or brown trout. The absolute configuration of 6 was determined by Marfey s analysis,40 which involves... 

It will he shown that the behavior of heterogeneous systems is influenced by the number of components it contains. A system which consists of a single, pure substance will behave differently from one which is made up of two or more components when the pressure and temperature are such that both a liquid phase and a gas phase are present. Consequently, the discussion of phase behavior will begin with a description of single-component systems. This will be followed by a description of two-component systems. Finally, multicomponent... 

These results demonstrate the different reaction velocity of the two primary hydroxyl groups and the markedly lower reaction velocity of the secondary hydroxyl group. In another paper, " it was shown that the same relative difference in reaction velocity is observed on treating compounds VII and VIII with p-toluenesulfonyl chloride, and hence that the nature of the hydroxyl group and not that of the chloride is responsible for these differences. Without precise knowledge of the difference in reaction rates, this behavior had been used previously for the selective tosylation, " mesylation""- and qualitative estimation of primary hydroxyl groups. However, the preparation of pure substances is easier when tritylation is employed. 

---