Pure substances properties

When a solid is in liquid solution it behaves according to its liquid state properties because it is in a liquid mixture. When applying Raoult s Law or similar expressions, the pure substance property is that of the liquid. Liquids such as crude oils and PCB mixtures consist largely of solid substances, but they are in the liquid state and generally unable to precipitate as solid crystals because of their low individual concentrations. 

Despite its age, Lyman s Handbook is still a valuable source of basic information on chemical properties, and many of the estimation methods contained therein remain valid. It also contains a suite of chapters on pure substance properties that normally are not of direct interest to environmental scientists e.g., flash point and heat capacity. 

Measuring devices for the determination of pure substance property data, such as the lower and upper explosion limits of liquids or gases, are quite complex and are usually not part of the repertoire found in safety laboratories. One explanation for this, is the existence of numerous data bases and printed publications containing such data for the majority of basic chemicals used. For the not so fiiequently occurring case that such data are required but can neither be retrieved nor approximated with the help of theoretical methods with sufficient accuracy, national institutes like those mentioned in section 3.4 and special laboratories can be consulted. 

It was made clear in Chapter II that the surface tension is a definite and accurately measurable property of the interface between two liquid phases. Moreover, its value is very rapidly established in pure substances of ordinary viscosity dynamic methods indicate that a normal surface tension is established within a millisecond and probably sooner [1], In this chapter it is thus appropriate to discuss the thermodynamic basis for surface tension and to develop equations for the surface tension of single- and multiple-component systems. We begin with thermodynamics and structure of single-component interfaces and expand our discussion to solutions in Sections III-4 and III-5. 

At the outset it will be profitable to deal with an ideal solution possessing the following properties (i) there is no heat effect when the components are mixed (ii) there is no change in volume when the solution is formed from its components (iii) the vapour pressure of each component is equal to the vapour pressure of the pure substances multiplied by its mol fraction in the solution. The last-named property is merely an expression of Raoult s law, the vapour pressure of a substance is pro-... 

The values of the thermodynamic properties of the pure substances given in these tables are, for the substances in their standard states, defined as follows For a pure solid or liquid, the standard state is the substance in the condensed phase under a pressure of 1 atm (101 325 Pa). For a gas, the standard state is the hypothetical ideal gas at unit fugacity, in which state the enthalpy is that of the real gas at the same temperature and at zero pressure. 

Physical Properties. The physical properties of the provitamins and vitamins D2 and are Hsted ia Table 6. The values are Hsted for the pure substances. The D vitamins are fat-soluble and, as such, are hydrophobic. 

Hctivity Coefficients. Most activity coefficient property estimation methods are generally appHcable only to pure substances. Methods for properties of multicomponent systems are more complex and parameter fits usually rely on less experimental data. The primary group contribution methods of activity coefficient estimation are ASOG and UNIEAC. Of the two, UNIEAC has been fit to more combinations of groups and therefore can be appHed to a wider variety of compounds. Both methods are restricted to organic compounds and water. 

AU substances are listed in alphabetical order in Table 2-6(3. Compiled from Daubert, T E., R. R Danner, H. M. Sibiil, and C. C. Stebbins, DIPPR Data Compilation of Pure Compound Properties, Project 801 Sponsor Release, July, 1993, Design Institute for Physical Property Data, AlChE, New York, NY and from Thermodynamics Research Center, Selected Values of Properties of Hydrocarbons and Related Compounds, Thermodynamics Research Center Hydrocarbon Project, Texas A M University, College Station, Texas (extant 1994). 

The term "pliase" for a pure substance indicates a state of matter - that is, solid, liquid, or gas. For mi. tures, however, a more stringent connotation must be used, since a totally liquid or solid system may contain more dian one phase. A phase is characterized by uniformity or homogeneity die same composition and properties must c. ist tliroughout the pliase region. At most temperatures and pressures, a pure substance normally exists as a single phase. At certain temperatures mid pressures, two or perhaps even dmee phases can coe.xist in equilibrium. 

For practical applications of the numerous thermodynamic relationships, it is necessary to have available the properties of the system. In general, a given property of a pure substance can be expressed in terms of any other two properties to completely define the state of the substance. Thus one can represent an equation of state by the functional relationship ... 

Tables 2-20-2-25 present thermodynamic properties for several pure substances commonly encountered in petroleum engineering practice.

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. 

The energy of a system can be changed by means of thermal energy or work energy, but a further possibility is to add or subtract moles of various substances to or from the system. The free energy of a pure substance depends upon its chemical nature, its quantity (AG is an extensive property), its state (solid, liquid or gas), and temperature and pressure. Gibbs called the partial molar free heat content (free energy) of the component of a system its chemical potential... 

Every pure substance has its own unique set of properties that serve to distinguish it from all other substances. A chemist most often identifies an unknown substance by measuring its properties and comparing them with the properties recorded in the chemical literature for known substances. 

A pure substance X has the following properties mp 90°C, increasing slightly as pressure increases normal bp = 120°C liquid vp = 65 mmHg at 100°C, 20 mmHg at the triple point... 

Notice the pattern here. First, we established the characteristic properties of water that cause us to identify it as a pure substance. Second, we found a change in which two other substances were formed in definite amounts from water alone. This second piece of information shows that water contains more than one kind of atom and that, hence, water is a compound. 

Suppose we compare two liquid samples, one of distilled water, and one of salt water. Each sample is a homogeneous system consisting of a single phase. However, one of the liquids is a pure substance whereas the other is a solution. We cannot tell, merely by visual observation, which of these clear liquids is the pure substance and which is the solution. True, there are differences—for example, the salt water has a greater density than the pure water—but even this property does not indicate which is the pure substance. 

Though many solutions are colorless and closely resemble pure water in appearance, the differences among solutions are great. This can be demonstrated with the five pure substances, sodium chloride (salt), iodine, sugar, ethyl alcohol, and water. Two of these substances, ethyl alcohol and water, are liquids at room temperature. Let s investigate the properties of the solutions these two substances form. 

Functional groups, 330,335 Fundamental property, 78 unit of electricity, 241 Furnace, electric arc, 404 Fusion, heat of, 68 pure substances, table, 69 Fusion, nuclear, 121, 419... 

If the solubility of either component in the other is unlimited ( free miscibility, as with alcohol and water), there may be an infinite number of solutions, lying between the two pure substances as limiting cases. The solubility may be limited in one or both directions. Thus, water and salt form a series of solutions extending indefinitely towards pure water as one limit, but bounded by saturated salt solution as the other limit water and ether form a continuous series of solutions bounded on one side by a saturated solution of ether in water, and on the other side by a saturated solution of water in ether. In the region of continuous miscibility all the properties of the solution vary... 

For pure substances, n is usually held constant. We will usually be working with molar quantities so that n = 1. The number of moles n will become a variable when we work with solutions. Then, the number of moles will be used to express the effect of concentration (usually mole fraction, molality, or molarity) on the other thermodynamic properties. 

Hence, for a pure substance, the chemical potential is a measure of its molar Gibbs free energy. We next want to describe the chemical potential for a component in a mixture, but to do so, we first need to define and describe a quantity known as a partial molar property. 

Most often, we will choose the independent variables to be those quantities we control in the laboratory. The usual thermodynamic choices are (p and T) or (Vand T), Then, we measure changes in the thermodynamic properties of the system as these variables are altered. Thus, for a pure substance, writing... 

Chapters 7 to 9 apply the thermodynamic relationships to mixtures, to phase equilibria, and to chemical equilibrium. In Chapter 7, both nonelectrolyte and electrolyte solutions are described, including the properties of ideal mixtures. The Debye-Hiickel theory is developed and applied to the electrolyte solutions. Thermal properties and osmotic pressure are also described. In Chapter 8, the principles of phase equilibria of pure substances and of mixtures are presented. The phase rule, Clapeyron equation, and phase diagrams are used extensively in the description of representative systems. Chapter 9 uses thermodynamics to describe chemical equilibrium. The equilibrium constant and its relationship to pressure, temperature, and activity is developed, as are the basic equations that apply to electrochemical cells. Examples are given that demonstrate the use of thermodynamics in predicting equilibrium conditions and cell voltages. 

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. 

---