Processes and Paths

It is important not to confuse an equilibrium state with a steady state, a state that is constant during a time period during which the system exchanges matter or energy with the surroundings. 

A process is a change in the state of the system over time, starting with a definite initial state and ending with a definite final state. The process is defined by a path, which is the 

T/jermod/nam/cs and C/iem/s(ry, second edition, version 3 2011 by Howard De foe. Latest version Hww.chem.umd.edu/thennobook 

Expansion is a process in which the system volume increases in compression, the volume decreases. 

An isothermal process is one in which the temperature of the system remains uniform and constant. An isobaric or isopiestic process refers to uniform constant pressure, and an isochoric process refers to constant volume. Paths for these processes of an ideal gas are shown in Fig. 2.9. 

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Define or explain the following terms energy, system, closed system, nonflow system, open system, flow system, surroundings, property, extensive property, intensive property, state, heat, work, kinetic energy, potential energy, internal energy, enthalpy, initial state, final state, point (state) function, state variable, cyclical process, and path function. 

Experiment 2 We start with the system in the same initial state as in experiment 1, and again surround it with thermal insulation. This time, instead of releasing the weight we close the switch to complete an electrical circuit with the resistor and allow the same quantity of electrical work to be done on the system as the mechanical work done in experiment 1. We discover the final temperature (300.10 K) is exactly the same as at the end of experiment 1. The process and path are different from those in experiment 1, but the work and the initial and final states are the same. 

FIGURE 9.49 Rearrangements are common in many reactions in which carhocations are involved. Path a shows the stepwise process and path h is the concerted process. 

Election nuclear dynamics theory is a direct nonadiababc dynamics approach to molecular processes and uses an electi onic basis of atomic orbitals attached to dynamical centers, whose positions and momenta are dynamical variables. Although computationally intensive, this approach is general and has a systematic hierarchy of approximations when applied in an ab initio fashion. It can also be applied with semiempirical treatment of electronic degrees of freedom [4]. It is important to recognize that the reactants in this approach are not forced to follow a certain reaction path but for a given set of initial conditions the entire system evolves in time in a completely dynamical manner dictated by the inteiparbcle interactions. 

Solution. Figure 12-8 shows the path on a psychrometric chart. The leaving dry-bulb temperature is obtained directly from Fig. 12-2 as 72.2 F. Since the spray water enters at the wet-bulb temperature of 70 F and there is no heat added to or removed from it, this is by definition an adiabatic process and there will be no change in wet-bulb temperature. The only change in enthalpy is that from the heat content of the makeup water. This can be demonstrated as follows ... 

The importance of emphasizing the essential difference between simple rate-dependent and path-dependent processes is that in the former case one does not have to follow the actual time-resolved deformation path in numerical computation, while in the latter case it is essential. 

The FMEA is executed deliberately, and systematically to reduce the possi All tre modes for one component should be completed before going to the n tabular format begins with drawing references of the system boundaries and sy stei the components as / appear in the process flow path using the following cati... 

Granular and other bulk materials processing and conveying, bag emptying and disposal, and similar operations air into the process enclosure. Induced air picks up dust. If the system component, such as a bin, is tight, the induced air will reverse its path and carry the entrained dust back through the upstream opening, as shown in Fig. 7.4. ... 

Tyndall lamp A parallel light beam pro jected onto a cloud of dust particles gen crated from a process to produce scattering of the light, allowing an assessment of the magnitude and path of the cloud. 

Tracer Studies on the Nitro lysis of Hexamine to RDX and HMX. The formation of RDX and/or HMX molecules from the nitration or nitrolysis of Hexamethylenetetramine (Hexamine) is a complex process and has been postulated to take place via two separate paths. One involves the selective cleavage of the Hexamine molecule to the appropriate cyclic nitramine (RDX, HMX or both) depending on the specific... 

In earlier days, A was called the work function because it equals the work performed on or by a system in a reversible process conducted at constant temperature. In the next chapter we will quantitatively define work, describe the reversible process and prove this equality. The name free energy for A results from this equality. That is, A A is the energy free or available to do work. Work is not a state function and depends upon the path and hence, is often not easy to calculate. Under the conditions of reversibility and constant temperature, however, calculation of A A provides a useful procedure for calculating u ... 

When we go through a cyclic process and end up where we started, AZ = 0, since we have returned to the same state, and therefore, the sum of changes in Z for the steps in the closed cyclic path must also equal 0. We can write this mathematically as... 

Because entropy is a state function, the change in entropy of a system is independent of the path between its initial and final states. This independence means that, if we want to calculate the entropy difference between a pair of states joined by an irreversible path, we can look for a reversible path between the same two states and then use Eq. 1 for that path. For example, suppose an ideal gas undergoes free (irreversible) expansion at constant temperature. To calculate the change in entropy, we allow the gas to undergo reversible, isothermal expansion between the same initial and final volumes, calculate the heat absorbed in this process, and use it in Eq.l. Because entropy is a state function, the change in entropy calculated for this reversible path is also the change in entropy for the free expansion between the same two states. 

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