System, thermodynamic, definition

A system is the region in space that is the subject of the thermodynamic study. It can be as large or small, or as simple or complex, as we want it to be, but it must be carefully and consistently defined. Sometimes the system has definite and precise physical boundaries, such as a gas enclosed in a cylinder so that it can be compressed or expanded by a piston. However, it may be also something as diffuse as the gaseous atmosphere surrounding the earth. 

The building of additive models begins with the portrayal of a system in an equilibrium state. This is true whether we are using a kinematic or a thermodynamic definition of equilibrium states. Each system is assumed to wind down to its lowest level of variable behaviour, a general statement that embraces a number of phenomena depending upon the level of complexity considered. This leads to a static model in which there is a sharp delineation between an event and its absence. There is a clear illumination of discrete events modelled to be cause and effect. 

The Buckingham statement fares no better in this regard, for the concept of a true equilibrium state is no less tautological than that of a perfect crystal. Moreover, the implied restriction to true equilibrium states (presumably, those for which no kinetic conversion is possible on any timescale) is even more strongly at odds with fundamental thermodynamic definitions, as outlined in Sections 2.10 and 2.11. Indeed, such a restriction, if enforced zealously, would preclude application of thermodynamics to any chemical system—past, present, or future—except for the final universal Warmetod state.]... 

In this chapter we will review some of the principles of thermochemistry, with particular attention to the air-water vapor system. Basic definitions in thermodynamics are reviewed along with important physical properties and definitions for gaseous mixtures. It is important that these definitions be learned early on. Note, however, that this chapter is only meant as a review. The references listed at the end of this chapter should be consulted for a detailed treatment of these subjects. Further, example problems are included at the end of the chapter to stress principles discussed. 

Entropy is a measure of the degree of randomness in a system. The change in entropy occurring with a phase transition is defined as the change in the system s enthalpy divided by its temperature. This thermodynamic definition, however, does not correlate entropy with molecular structure. For an interpretation of entropy at the molecular level, a statistical definition is useful. Boltzmann (1896) defined entropy in terms of the number of mechanical states that the atoms (or molecules) in a system can achieve. He combined the thermodynamic expression for a change in entropy with the expression for the distribution of energies in a system (i.e., the Boltzman distribution function). The result for one mole is ... 

In summary, a reference state or standard state must be defined for each component in the system. The definition may be quite arbitrary and may be defined for convenience for any thermodynamic system, but the two states cannot be defined independently. When the reference state is defined, the standard state is determined conversely, when the standard state is defined, the reference state is determined. There are certain conventions that have been developed through experience but, for any particular problem, it is not necessary to hold to these conventions. These conventions are discussed in the following sections. The general practice is to define the reference state. This state is then a physically realizable state and is the one to which experimental measurements are referred. The standard state may or may not be physically realizable, and in some cases it is convenient to speak of the standard state for the chemical potential, for the enthalpy, for the entropy,... 

In order for an object to be regarded as a motor, several basic requirements have to be fulfilled. Even without trying to apply a strict thermodynamic definition, the system will have to convert a certain type of energy into another form of energy, while undergoing some kind of continuous motion. 

Related Calculations. Many systems deviate from the ideal solution behavior in either or both phases, so the K values given by K = // jP are not adequate. The rigorous thermodynamic definition of K is... 

Set out the relevant form of the thermodynamic definition of K value. At this low system pressure, the vapor-phase nonideahty is negligible. Since neither component has a very high vapor pressure at the system temperature, and since the differences between the vapor pressures and the system pressure are relatively small, the pure-liquid fugacities can be taken to be essentially the same as the vapor pressures. 

This is a crude model, but hopefully you now see how the calculus of probabilities, as Maxwell put it, explains why heat flows downhill (from hot to cold), why a gas expands to occupy its container and why the world is. . . getting more disordered and generally going to hell in a hand-basket We also hope that you now have a feel for entropy that cannot be obtained from the purely thermodynamic definition of heat divided by temperature, hi principle, calculating the entropy of a system would now seem to be easy. Just count the num-... 

Equilibrium (thermodynamic definition) the position where the free energy of a reaction system has its lowest possible value. (10.11)... 

When we add an electron to the system at a given temperature and pressure, the electron is necessarily positioned in a level close to The increase in free energy of the electron system due to the addition of one electron is hence (Figure 5). Hence, the Fermi-Dirac occupation function is in accordance with the thermodynamic definition of the electrochemical potential. 

The successful development of the thermodynamics of irreversible phenomena depends on the possibility of an explicit evaluation of the production of entropy, and for this it is necessary to assume that the thermodynamic definition of entropy can be applied equally to systems which are not in equilibrium, that is to states whose mean lifetime is limited. We are thus confronted immediately with the problem of the domain of validity of the thermodynamic treatment of irreversible phenomena, which can be determined only by a comparison of the results of the thermodynamic treatment with those obtained by the use of statistical mechanics. This problem wall be dealt with in more detail in the third volume of this work meanwhile the main conclusions can be summarized as follows. 

Give precise definitions for the terms thermodynamic system, open system, closed system, thermodynamic state, and reversible and irreversible process (Section 12.1). 

Polymorphism can be detected by the differences in physical properties due to individual characteristics. Based on the fugacity, which relates to the thermodynamic term, entropy of the solid molecule, polymorphism may be defined as monotropic or enantiotropic. Furthermore, combination of these two systems, monotropic and enantiotropic, may yield a third system. The definition of these categories may best be illustrated by the solubility-temperature plots, based on the van t Hoff equation. In a monotropic category as shown in Figure 9, the solubility of form I (the stable form) and that of form II (the metastable form) will not intersect each other at the transition temperature calculated only from the extrapolation of the two curves. In the enantiotropic category (Fig. 10), the solubility of form I (the stable form) and that of form II (the metastable form) will intersect each other at the transition temperature. In the combined category (Fig. 11), for which there are two transition temperatures, the solubility of form III will not intersect any other curves. 

From the scientific definition point of view, there is a slight difference between our continuum thermodynamics definition of the Second Law and its statistical mechanical version so that the continuum thermodynamics definition of the Second Law states that an observation of decreased universal entropy is impossible in isolated systems however the statistical mechanical definition says that an observation of universal increased entropy is not probable. 

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