The System, Surroundings, and Boundary

Chemists are interested in systems containing matter—that which has mass and occupies physical space. Classical thermodynamics looks at macroscopic aspects of matter. It deals with the properties of aggregates of vast numbers of microscopic particles (molecules, atoms, and ions). The macroscopic viewpoint, in fact, treats matter as a continuous material medium rather than as the collection of discrete microscopic particles we know are actually present. Although this book is an exposition of classical thermodynamics, at times it will point out connections between macroscopic properties and molecular structure and behavior. 

A thermodynamic system is any three-dimensional region of physical space on which we wish to focus our attention. Usually we consider only one system at a time and call it simply the system. The rest of the physical universe constitutes the surroundings of the system. 

The boundary is the closed three-dimensional surface that encloses the system and separates it from the surroundings. The boundary may (and usually does) coincide with real physical surfaces the interface between two phases, the inner or outer surface of the wall of a flask or other vessel, and so on. Alternatively, part or all of the boundary may be an imagined intangible surface in space, unrelated to any physical structure. The size and shape of the system, as defined by its boundary, may change in time. In short, our choice of the three-dimensional region that constitutes the system is arbitrary—but it is essential that we know exactly what this choice is. 

We usually think of the system as a part of the physical universe that we are able to influence only indirectly through its interaction with the surroundings, and the surroundings 

For some purposes we may wish to treat the system as being divided into subsystems, or to treat the combination of two or more systems as a supersystem. 

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CHAPTER 2 SYSTEMS AND THEIR PROPERTIES 2.1 The System, Surroundings, and Boundary... 

For example, say we wish to study the piston-cylinder assembly in Figure 1.1. The usual choice of system, surroundings, and boundary are labeled. The boundary is depicted by the dashed line just inside the walls of the cylinder and below the piston. The system contains the gas within the piston-cylinder assembly but not the physical housing. The surroundings are on the other side of the boundary and comprise the rest of the universe. Likewise the system, surroundings, and boundary of an open system are labeled in Figure 1.2. In this case, the inlet and outlet flow streams, labeled in and out, respectively, allow mass to flow into and out of the system, across the system boundary. 

Figure 1.1 Schematic of a piston-cylinder assembly. The system, surroundings, and boundary are delineated.

Consider a pot of boiling water. Sketch the system, surroundings, and boundary. Identify all the heat-transfer mechanisms associated with this system. 

Any industrial system can be represented by a system boundary that encloses aU the operations of interest. The region surrounding this boundary is known as the system environment (fig. 4.1). The inputs to the system are all raw materials taken from the environment, and the outputs are waste materials released back into the environment. 

This term accounts for how the dielectric constant of the surrounding medium influences the potential energy of the system. Often tinfoil boundary conditions, in which the surrounding medium is a conductor (e = 3) and = 0, are used. 

Here V is the system volume and is the surrounding (external) pressure exerted on the boundary to produce the deformation. The pressure Pg t is always positive. The negative sign in (2.1.9) is chosen so that when the system volume decreases (dV < 0), the work is positive, and when the volume increases (dV > 0), the work is negative. For a finite deformation from Vj to V2,... 

In (c) we have a condenser similar to those found in chemistry labs. Usually, hot vapor flows through the center of the condenser while cold water flows on the outside, causing the vapor to condense. This system is open because it allows mass flow through its boundaries. It is composite because of the waU that separates the two fluids. It is adiabatic because it is insulated from the surroundings. Even though heat is transferred between the inner and outer tube, this transfer is internal to the system (it does not cross the system bounds) and does not make the system diathermal. 

To start with a thermodynamic description of an object of interest, it is best to define it as a system as illustrated schematically in Fig. 2.16. The system must be well delineated from its surroundings by real or imaginary boundaries. These surroundings are in the widest sense the rest of the universe. This makes the surroundings ill defined. One simply does not know the size, content, and behavior of the universe. For this reason, it is convenient to work with the system alone and separately assess and account for the effects of the surroundings. 

For example, suppose the system is a fluid in a gravitational field. Let the system boundary be at the inner walls of the container, and let the local frame be fixed with respect to the container and have negligible acceleration in the lab frame. At each surface element of the boundary, the force exerted by the pressure of the fluid on the container wall is equal in magnitude and opposite in direction to the contact force exerted by the surroundings on the fluid. The horizontal components of the contact forces on opposite sides of the container cancel, but the vertical components do not cancel because of the hydrostatic pressure. The net contact force is mgCz, where m is the system mass and Cz is a unit vector in the vertical +z direction. For this example, Eq. G.7.2 becomes... 

Process Systems. Because of the large number of variables required to characterize the state, a process is often conceptually broken down into a number of subsystems which may or may not be based on the physical boundaries of equipment. Generally, the definition of a system requires both definition of the system s boundaries, ie, what is part of the system and what is part of the system s surroundings and knowledge of the interactions between the system and its environment, including other systems and subsystems. The system s state is governed by a set of appHcable laws supplemented by empirical relationships. These laws and relationships characterize how the system s state is affected by external and internal conditions. Because conditions vary with time, the control of a process system involves the consideration of the system s transient behavior. 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

The words system and surroundings are similarly coupled. A system is taken to be any object, any quantity of matter, any region, and so on, selected for study and set apart (mentally) from everything else, which is called the surroundings. The imaginary envelope which encloses the system and separates it from its surroundings is called the boundary of the system. 

To isolate a system for study, the system is separated from the surroundings by a boundary or envelope that may either be real (e.g., a reactor vessel) or imaginary. Mass crossing the boundaiy and entering the system is part of the mass-in term. The equation may be used for any compound whose quantity does not change by chemical reaction or for any chemical element, regardless of whether it has participated in a chemical reaction. Furthermore, it may be written for one piece of equipment, several pieces of equipment, or around an entire process (i.e., a total material balance). 

Any real sample of a colloidal suspension has boundaries. These may stem from the walls of the container holding the suspension or from a free interface towards the surroundings. One is faced with surface effects that are small compared to volume effects. But there are also situations where surface effects are comparable to bulk effects because of strong confinement of the suspension. Examples are cylindrical pores (Fig. 8), porous media filled with suspension (Fig. 9), and thin colloidal films squeezed between parallel plates (Fig. 10). Confined systems show physical effects absent in the bulk behavior of the system and absent in the limit of extreme confinement, e.g., a onedimensional system is built up by shrinking the size of a cylindrical pore to the particle diameter. 

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