Direction of natural processes

For a scientist, the primary interest in thermodynamics is in predicting the spontaneous direction of natural processes, chemical or physical, in which by spontaneous we mean those changes that occur irreversibly in the absence of restraining forces—for example, the free expansion of a gas or the vaporization of a hquid above its boiling point. The first law of thermodynamics, which is useful in keeping account of heat and energy balances, makes no distinction between reversible and irreversible processes and makes no statement about the natural direction of a chemical or physical transformation. 

The two observations that (1) a system free from forced flows will evolve to an equilibrium state and (2) once in equilibrium a system will never spontaneously evolve to a nonequiiibrium state, are evidence for a unidirectional character of natural processes. Thus we can take as a general principle th - tt the direction of natural processes is such that systems evolve toward an equilibriurii state, not away from it. 

The Second Law introduces simultaneously the entropy (S) and the thermodynamic temperature (T) as state variables and, furthermore, determines the direction of natural processes. 

Notice that the word spontaneous has a different meaning in thermodynamics than it does in everyday speech. Ordinarily, spontaneous refers to an event that takes place without any effort or premeditation. For example, a crowd cheers spontaneously for an outstanding performance. In thermodynamics, spontaneous refers only to the natural direction of a process, without regard to whether it occurs rapidly and easily. Chemical kinetics, which we introduce in Chapter 15, describes the factors that determine the speeds of chemical reactions. Thermodynamic spontaneity refers to the direction that a process will take if left alone and given enough time. 

It is desirable to find some common measure (preferably a quantitative measure) of the tendency to change and of the direction in which change can occur. In the 1850s, Clausius and Kelvin independently formulated the second law of thermodynamics, and Clausius invented the term entropy S (from the Greek word TpoTT-rj, which means transformation), to provide a measure of the transformational content or the capacity for change. In this chapter, we will develop the properties of this function and its relationship to the direction and extent of natural processes as expressed in the second law of thermodynamics. 

A distinction sometimes needs to be made between the natural background level of a substance, which arises purely as a result of natural processes, and the ambient background level, which is the concentration measurable in the environment at a pristine site (see Section 5.10). In practice, a pristine site is often considered to be one that does not receive any direct inputs from local sources, although it needs to be accepted that for many substances there may be an appreciable input from diffuse atmospheric sources. 

A distinction between inactive and active fillers is at present hardly relevant, since the properties of the final product depend more or less strongly upon all the fillers utilized and their use has for a long time not been primarily determined in terms of cost reduction. The efforts of the fdler producers in the direction of improved processing methods and dedicated manufacturing processes take this development into account. The surface treatment of natural and synthetic fillers has also acquired great importance. For years there have been products which in the classic sense are not fillers at all, but are in fact active substances (e.g. silica aerosols). Apart from any cost reduction considerations, fillers have essentially the following functions ... 

Another defect in our present development of thermodynamics has to do with the unidirectional character of natural processes that was considered in Sec. 1.3. There it was pointed out that all spontaneous or natural processes proceed only in the direction that tends to dissipate the gradients in the system and thus lead to a state of equilibrium, and never in the reverse direction. This characteristic of natural processes has not yet been included in our thermodynamic description. 

When the thermowell is installed perpendicular or at a 45° angle to the pipe wall, the thermowell tip should be at or near the centerline of the pipe. Thermowells and their connections should point counter to the flow in the process line whether installed on pipe elbows or on a 45° angle. The well perpendicular to the pipe will naturally be in the turbulent flow section of the pipe. Such an arrangement will not always be possible because of the piping arrangement and direction of the process flow. 

Land can contain substances that are undesirabie or even hazardous as a result of natural processes, for example, as a result of mineralization. However, most cases of contaminated land are associated with human activity. In particular, in many of the industrialized countries of the world, one of the legacies of the past two centuries is that land has been contaminated. Hence, when such sites are cleared for redevelopment, they can pose problems. Contaminated land is by no means easy to define but it can be regarded as land that contains substances that, when present in sufficient concentrations, are likely to cause harm, directly or indirectly, to man, to the environment or to other targets. 

An imdeistanding of thermodynamics enables us to predict whether or not a reaction will occur when reactants are combined. This is important in the synthesis of new compounds in the laboratory, the manufacturing of chemicals on an industrial scale, and the understanding of natural processes such as cell function. A process that does occur under a specific set of conditions is called a spontaneous process. One that does not occur under a specific set of conditions is called nonspontaneous. Table 18.1 lists examples of familiar spontaneous processes and their nonspon-taneous counterparts. These examples illustrate what we know intuitively Under a given set of conditions, a process that occurs spontaneously in one direction does not also occur spontaneously in the opposite direction. 

Unlike (1.17) and (1.17a), the second terms on the right-hand sides of (1.23) and (1.23a) have the same sign, since we have chosen different directions (cathodic and anodic) for the processes while calculating AS. The choice of the direction of a process is naturally conditional for example, we could choose the anodic direction for all cases and thus replace minus by plus in (1.23) (such a choice was made in[9]). However it seemed more convenient to us to calculate the equilibrium quantities by assuming the same direction of the process as for a nonequilibrium reaction under investigation. It is clear from this form of equations that in principle there is no difference between the cathodic and anodic processes. 

Relaxation kinetics may be monitored in transient studies tlirough a variety of metliods, usually involving some fonn of spectroscopy. Transient teclmiques and spectrophotometry are combined in time resolved spectroscopy to provide botli tire stmctural infonnation from spectral measurements and tire dynamical infonnation from kinetic measurements that are generally needed to characterize tire mechanisms of relaxation processes. The presence and nature of kinetic intennediates, metastable chemical or physical states not present at equilibrium, may be directly examined in tliis way. 

Uranium-235 can be concentrated by gaseous diffusion and other physical processes, if desired, and used directly as a nuclear fuel, instead of natural uranium, or used as an explosive. 

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

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