System, closed

Let s start with the closed system, which is defined as one in which there is no movement of fluid. Heat can transfer into the system from the surroundings, and it s possible to do work on the system, but energy cannot enter or leave the system with the fluid, since there is no movement of fluid. Thus, any energy entering or leaving the system must be related to heat (0 or work (W), and the energy balance is written as 

In other words, the change in the internal energy of the system is strictly related to the amount of energy that is added to the system by heat transfer or mechanical work. Often, one will use the symbol A to describe the difference between two points, in which case the first law of thermodynamics for a closed system is written as 

The basic assumption in the closed system is that the total quantity of each species is fixed. Consider, as an example, the cloud formation (liquid water mixing ratio wl) in an air parcel that has initially a H202 partial pressure p )Hl0l and assume that no reactions take place. If we treat the system as open, then at equilibrium the aqueous-phase concentration of H202 will be given by 

Note that for an open system, the aqueous phase concentration of H202 is independent of the cloud liquid water content. For a closed system, the total concentration of H202 per liter (physical volume) of air, [H202]total, will be equal to the initial amount of H202 or  

After the cloud is formed, this total H202 is distributed between gas and aqueous phases and satisfies the mass balance  

FIGURE 7.20 Aqueous-phase H202 as a function of liquid water content for 0.S, 1, and 2 ppb H202. Dashed lines correspond to an open system and solid lines to a closed one. 

For species with low solubility, such as ozone, I + HwLRT 1, and the two approaches result in essentially identical concentration estimates (recall that wl 10-6 for most atmospheric clouds). 

if not most, step polymerizations involve equilibrium reactions, and it becomes important to analyze how the equilibrium affects the extent of conversion and, more importantly, the polymer molecular weight. A polymerization in which the monomer(s) and polymer are in equilibrium is referred to as an equilibrium polymerization or reversible polymerization. A first consideration is whether an equilibrium polymerization will yield high-molecular-weight polymer if carried out in a closed system. By a closed system is meant one where none of the products of the forward reaction are removed. Nothing is done to push or drive the equilibrium point for the reaction system toward the polymer side. Under these conditions the concentrations of products (polymer and usually a small molecule such as water) build up until the rate of the reverse reaction becomes equal to the polymerization rate. The reverse reaction is referred to generally as a depolymerization reaction other terms such as hydrolysis or glycolysis may be used as applicable in specific systems. The polymer molecular weight is determined by the extent to which the forward reaction has proceeded when equilibrium is established. 

Equation 2-61 yields the extent of conversion as a function of the equilibrium constant. To obtain an expression for the degree of polymerization as a function of K, Eq. 2-61 is combined with Eq. 2-27 to yield 

TABLE 2-5 Effect of Equilibrium Constant on Extent of Reaction and Degree of Polymerization in Closed System 

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Lamp Method the sample is burned in a closed system in an atmosphere of 70% CO2 and 30% oxygen in order to avoid formation of nitrogen oxides. This method was to have been abandoned as it takes three hours to carry out, but remains officially required for jet fuel sulfur analysis. 

In employing this method, an important precaution to take is to use a tip that has been ground smooth at the end and is free tom any nicks. In the case of liquids that do not wet the tip, r is the inside radius. Volatile liquids are studied in a closed system as described by Harkins and Brown [21] to minimize evaporation losses. 

More generally, without considermg the various possible kinds of work, one can write for an isothennal change in a closed system (dn. = 0)... 

If a themiodynamic system includes species that may undergo chemical reactions, one must allow for the fact that, even in a closed system, the number of moles of a particular species can change. If a chemical reaction (e.g. N2 + 3H2 INHg) is represented by the symbolic equation... 

An explicit example of an equilibrium ensemble is the microcanonical ensemble, which describes closed systems with adiabatic walls. Such systems have constraints of fixed N, V and E < W< E + E. E is very small compared to E, and corresponds to the assumed very weak interaction of the isolated system with the surroundings. E has to be chosen such that it is larger than (Si )... 

This is a collection of closed systems with the same number of particles A and volume V (constant density) for each system at temperature T. The partition fiinction... 

Lebowitz J L and Percus J 1961 Long range correlations in a closed system with applications to nonuniform fluids Phys. Rev. 122 1675... 

While, in principle, a tricritical point is one where three phases simultaneously coalesce into one, that is not what would be observed in the laboratory if the temperature of a closed system is increased along a path that passes exactly tlirough a tricritical point. Although such a difficult experiment is yet to be perfomied, it is clear from theory (Kaufman and Griffiths 1982, Pegg et al 1990) and from experiments in the vicinity of tricritical points that below the tricritical temperature only two phases coexist and that the volume of one slirinks precipitously to zero at T. ... 

Haupt S, Collisi U, Speckmann H D and Strehblow H-H 1985 Specimen transfer from the electrolyte to the UHV in a closed system and some examinations of the double layer on Cu J. Electroanal. Chem. 194 179-90... 

Most chemically reacting systems tliat we encounter are not tliennodynamically controlled since reactions are often carried out under non-equilibrium conditions where flows of matter or energy prevent tire system from relaxing to equilibrium. Almost all biochemical reactions in living systems are of tliis type as are industrial processes carried out in open chemical reactors. In addition, tire transient dynamics of closed systems may occur on long time scales and resemble tire sustained behaviour of systems in non-equilibrium conditions. A reacting system may behave in unusual ways tliere may be more tlian one stable steady state, tire system may oscillate, sometimes witli a complicated pattern of oscillations, or even show chaotic variations of chemical concentrations. 

It is reported that mild carbon steels may be effectively protected by as little as 55 ppm of KTc04 in aerated distilled water at temperatures up to 250oC. This corrosion protection is limited to closed systems, since technetium is radioative and must be confined. 9sTc has a specific activity of 6.2 X lOs Bq/g. Activity of this level must not be allowed to spread. 99Tc is a contamination hazard and should be handled in a glove box. 

In an excimer laser the mixture of inert gas, halogen gas, and helium, used as a buffer, is pumped around a closed system consisting of a reservoir and the cavity. 

Recirculating systems can make aquaculture feasible in locations where conditions would otherwise not be conducive to successful operations. Such systems can also be used to reduce transportation costs by making it possible to grow animals near markets. In areas where there are concerns about pollution or the use of exotic species, closed systems provide an alternative approach to more extensive types of operations. 

Furfural is very thermally stable in the absence of oxygen. At temperatures as high as 230°C, exposure for many hours is required to produce detectable changes in the physical properties of furfural, with the exception of color (17). However, accelerating rate calorimetric data shows that a temperature above 250°C, in a closed system, furfural will spontaneously and exothermically decompose to furan and carbon monoxide with a substantial increase in pressure. The pressure may increase to 5000 psi or more, sufficient to shatter the container (18). 

Wines can be made effervescent by carbonation rather than refermentation in a closed system, but that must be stated on the label. Even though... 

Molybdenum hexafluoride [7783-77-9] MoF, is a volatile liquid at room temperature. It is very moisture sensitive, hydrolysing immediately upon contact with water to produce HF and molybdenum oxyfluorides. MoF should therefore be handled in a closed system or in a vacuum line located in a chemical hood. The crystals possess a body-centered cubic stmcture that changes to orthorhombic below —96° C (1,2). The known physical properties are Hsted in Table 1. 

The problem with all the mirror approaches is that none has achieved the degree of confinement quaUty that the closed systems have. Closed systems ate characterized by magnetic field lines that close on themselves so that charged particles following the field lines remain confined within the system. 

Atomic Absorption Spectroscopy. Mercury, separated from a measured sample, may be passed as vapor iato a closed system between an ultraviolet lamp and a photocell detector or iato the light path of an atomic absorption spectrometer. Ground-state atoms ia the vapor attenuate the light decreasiag the current output of the photocell ia an amount proportional to the concentration of the mercury. The light absorption can be measured at 253.7 nm and compared to estabUshed caUbrated standards (21). A mercury concentration of 0.1 ppb can be measured by atomic absorption. 

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