Substances in the gas state

Gas chromatography is a similar method to that of HPLC. It provides a quick and easy way of determining alkaloids in a mixture. The only requirement is some degree of stability at the temperature necessary to maintain the substance in the gas state . 

From Eq. 3.42 we obtain the molar entropy of a pure substance in the gas state at constant pressure as shown in Eq. 3.43 ... 

Considering a molecule as equivalent from the electrostatical point of view to a dipole, which could be polarized under the effect created by its neighbors, Mosotti and Clausius, and independently from each other Lorentz and Lorenz i ), obtained the general connection between the dielectric constant of a substance in the gas state and its dipole moment ... 

Our arguments for water-air mixtures also apply to pure samples of substances in the gas state. Higher-energy particles prevent lower-energy particles from clustering. Where the holding power is very weak, only the very lowest-energy particles would cluster and these are far outnumbered by the rest. 

The simplest way to demonstrate diffusion in the gas state is to use substances in the gas state. Traditionally, samples are positioned in alternate vertical arrangements (Figure 2.10a). However, this adds the complication of weight and different densities. Arranging the tubes horizontally avoids the issue (Figure 2.10b). 

Why is the standard entropy of a substance in the gas state greater than its standard entropy in the liquid state ... 

The enthalpy of formation of the substance that is obtained is for the gaseous state. The enthalpy that corresponds to the gas -> liquid change of state therefore needs to be added. The enthalpy corresponding to the liquid solid change is to be ignored if the enthalpy of formation of a substance in the solid state is required, since there is not much difference between the two. For the enthalpy of formation in the liquid state the enthalpy of condensation is calculated from the equation that bonds it to the entropic factor discussed in Chapter 1, ie ... 

Measurements of gas chromatographic retention time are often used as a fast and easy method of estimating vapor pressure. These estimated pressures are related to the gas/substrate partition coefficient, which can be regarded as a ratio of solubility of the substance in the gas to that in the substrate, both solubilities being of the substance in the liquid state. As a result the estimated vapor pressures are of the liquid state. To obtain the solid vapor pressure requires multiplication by the fugacity ratio. It is important to establish if the estimated and reported property is of the vapor or liquid. 

For most of the situations encountered in solvent extraction the gas phase above the two liquid phases is mainly air and the partial (vapor) pressures of the liquids present are low, so that the system is at atmospheric pressure. Under such conditions, the gas phase is practically ideal, and the vapor pressures represent the activities of the corresponding substances in the gas phase (also called their fugacities). Equilibrium between two or more phases means that there is no net transfer of material between them, although there still is a dynamic exchange (cf. Chapter 3). This state is achieved when the chemical potential x as... 

Let us consider the catalytic isomerization reaction whose steady-state kinetic model has already been considered in the previous section. Its detailed mechanism is of the form (1) A + Z AZ (2) AZ = BZ (3) BZ B + Z. Under the assumption of constant concentrations of substances in the gas phase, it will be written as... 

Here Nlt, Ngt and N are the number of moles of substance in the gas phase for the first, second, and equilibrium states, respectively. 

The Standard-State chemical potentials of substances in the gas, liquid, and .olul phases, as well as of solutes in aqueous solution, can be determined by a v.uiely of experimental methods, among them spectroscopic, colorimetric, mi 11 ib i lily, colligative-property, and electrochemical techniques.817 The accepted values of these fundamental thermodynamic properties are and should be undergoing constant revision under the critical eyes of specialists. It is not the puipose of this book lo discuss the practice of determining values of /i° for all < (impounds of interest in soils. This is best left lo. specialized works on... 

Substances can change phase— from solid to liquid or from gas to hquid, for example. It takes energy to change a substance from solid to liquid, solid to gas, or liquid to gas, but the same quantity of energy is released in the reverse process. On a submicroscopic level, energy is needed to overcome the high attractive forces between particles in the solid state to get to the somewhat lower attractions in the hquid state or the negligible attractions in the gas state. 

The same substance in the three states solid liquidi gaseous. Triple point.—great number of substances of definite composition may be observed, according to circumstances, in the three states of solid, liquid, and gas may these three forms coexist in equilibrium The system formed of a single independent component, divided into three phases, would be an invariant system as we know, there can be but one temperature d and one pressure for which the system is in equilibrium. If, as in the preceding chapter, we lay off on a set of coordinates axes the temperatures T as abscissse and the pressures it as ordinates... 

Dubinin, M.M., Bering, B.P., Serpinsky, V.V., and Vasil ev, B.N. (1958). The properties of substances in the adsorbed state studies of gas adsorption over a wide temperature and pressure range. In Surface Phenomena in Chemistry and Biology (J.F. DanieUi, K.G.A. Pankhunt and A.C. Riddiford, eds). Pergamon Press, pp. 172-88. 

The transformation of a pure compound from a liquid to a gaseous state and vice versa corresponds to a phase change that can be induced over a limited domain by pressure or temperature. For example, a pure substance in the gaseous state cannot be liquefied above a given temperature, called the critical temperature Tq, irrespective of the pressure applied to it. The minimum pressure required to liquefy a gas at its critical temperature is called the critical pressure Pq (Figure 6.1). These points are the defining boundaries on a phase diagram for a pure substance. The curve, which limits the gas and liquid domains, stops at the critical point... 

TRC. Thermodyrramic Propertie s of the Substances in Ideal Gas State l.OM ed. Thermodyrrarrric Research Center of the Texas Engineering Experiment Station Texas A M University, 1994. 

Liquids are substances in the amorphous state of matter between gases and solids in which the molecules exist in no set relationship to one another and are able to move and flow within the limits set by their intermolecular forces. Liquids are more concentrated than gases but are usually less dense than solids. Water is an exception it is denser than ice. See Terminology, Gas, p.239 Terminology, Solid, p.241. 

Thus far we have considered only the behavior of pure gases— those that consist of only one substance in the gaseous state. How do we deal with gases composed of a mixture of two or more different substances While studying the properties of air, John Dalton (Section 2.1) observed that the total procure of a mixture of gases equals the sum of the pressures that each would exert if it were present alone. The pressure exerted by a particular component of a mixture of gases is called the partial pressure of that gas, and Dalton s observation is known as Dalton s law of partial pressures. 

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