Thermodynamic properties of real substances

Chapter 6 The Thermodynamic Properties of Real Substances with... 

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. 

The standard state (and thus any standard thermodynamic property) of a pure solid refers to the pure substance in the solid phase under the pressure p of 1 bar (0.1 MPa). The standard state of a pure liquid refers to the pure substance in the liquid phase at p = 1 bar. When the substance is a pure gas, its standard state is that of an ideal gas at p = 1 bar (or, which is equivalent, that of a real gas at P = o). 

Fortunately, there is a way out of this predicament. Using the second law of thermodynamics it is possible to establish a temperature scale that is independent of the particular properties of any substance, real or hypothetical. This scale is the absolute, or the thermodynamic, temperature scale, also called the Kelvin scale after Lord Kelvin, who first demonstrated the possibility of establishing such a scale. By choosing the same size degree, and with the usual definition of the mole of substance, the Kelvin scale and the ideal gas scale become numerically identical. The fact of this identity does not destroy the more fundamental character of the Kelvin scale. We establish this identity because of the convenience of the ideal gas scale compared with other possible scales of temperature. 

Time elapsed between the state of interaction of the primary radiation, whatever it is, and the state of full acquisition of the energy by the bulk of the material is long enough to affect the thermodynamic (TD) properties of the substance but too short to be detected easily. Some scientists may look at the interaction of radiation with matter as a single event interaction as in the atomic level. That is not a real situation, for example, if the radiation interaction with matter is considered as a single event, bond breakage of organic material should be random. What happened in practice is that specific portions of the molecule are affected by radiation, which is related to the TD of the interaction. 

Thermodynamics deals with its subject matter (energy levels, energy changes) in an abstract way. The states and processes it describes are idealized it does not describe or deal with any objects or processes in the real world, except to the extent that the variables in its equations are properties (e.g., volumes, energies) of real substances. Some processes in the real world are very similar to these idealized processes, and some are not. Where they are similar, thermodynamics is directly useful. Where they are not, we invent correction factors (e.g., activity coefficients ) to account for the differences. 

The application of thermodynamics to any real problem starts with the identification of a particular body of matter as the focus of attention. This quantity of matter is called the system, and its thermodynamic state is defined by a few measurable macroscopic properties. These depend on the fundamental dimensions of science, of which length, time, mass, temperature, and amount of substance are of interest here. 

The standard state of a substance is a reference state that allows us to obtain relative values of such thermodynamic quantities as free energy, activity, enthalpy, and entropy. All substances are assigned unit activity in their standard state. For gases, the standard state has the properties of an ideal gas, but at one atmosphere pressure. It is thus said to be a hypothetical state. For pure liquids and solvents, the standard states are real states and are the pure substances at a specified temperature and pressure. For solutes In dilute solution, the standard state is a hypothetical state that has the properties of an infinitely dilute solute, but at unit concentration (molarity, molality, or mole fraction). The standard state of a solid is a real state and is the pure solid in its most stable crystalline form. 

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