Maximum useful work, thermodynamic

We all widely utilize aspects of the first law of thermodynamics. The first law mainly deals with energy balance regardless of the quality of that part of the energy available to perform work. We define first law efficiency or thermal efficiency as the ratio of the work output to total rate of heat input, and this efficiency may not describe the best performance of a process. On the other hand, the second law brings out the quality of energy, and second law efficiency relates the actual performance to the best possible performance under the same conditions. For a process, reversible work is the maximum useful work output. If the operating conditions cause excessive entropy production, the system will not be capable of delivering the maximum useful output. 

Free energy a thermodynamic function equal to the enthalpy (H) minus the product of the entropy (5) and the kelvin temperature (T) G = H — TS. Under certain conditions the change in free energy for a process is equal to the maximum useful work. (10.7)... 

One measure of the thermodynamic efficiency of a complex process (e.g., an electrical generating station or an automobile engine) is the ratio of the useful work obtained for a specified change of state to the maximum useful work obtainable with the ambient temperature To and pressure Pa. Here by useful work v.e mean the total work done by the system less the work done in expansion of the system boundaries against the ambient pressure. 

Maximum useful work, according to Equation (1.34), reaches its values only imder reversibility conditions, i.e., at thermodynamic equilibrium, when values AS, A U and AF are infinitely small. AAlhlch means that maximum useful work of chemical processes by analogy with Equation (1.22) is equal... 

Thus, maximum useful work may be measured as the function of change in internal energy, enthalpy and free energy or free enthalpy. However, in practical thermodynamic studies most convenient and used is free enthalpy. 

Section 19.5 The Gibbs free energy (or just free energy), G, is a thermodynamic state function that combines the two state functions enthalpy and entropy G = H — TS. For processes that occur at constant temperature, AG = AH — TAS. For a process or reaction occurring at constant temperature and pressiue, the sign of AG relates to the spontaneity of the process. When AG is negative, the process is spontaneous. When AG is positive, the process is nonspontaneous the reverse process is spontaneous. At equilibrium the process is reversible and AG is zero. The free energy is also a measure of the maximum useful work that can be performed by a system in a spontaneous process. 

This remarkable result shows that the efficiency of a Carnot engine is simply related to the ratio of the two absolute temperatures used in the cycle. In normal applications in a power plant, the cold temperature is around room temperature T = 300 K while the hot temperature in a power plant is around T = fiOO K, and thus has an efficiency of 0.5, or 50 percent. This is approximately the maximum efficiency of a typical power plant. The heated steam in a power plant is used to drive a turbine and some such arrangement is used in most heat engines. A Carnot engine operating between 600 K and 300 K must be inefficient, only approximately 50 percent of the heat being converted to work, or the second law of thermodynamics would be violated. The actual efficiency of heat engines must be lower than the Carnot efficiency because they use different thermodynamic cycles and the processes are not reversible. 

This expression shows that the maximum possible useful work (i.e., reversible work) that can be obtained from any process occurring at constant temperature and pressure is a function of the initial and final states only and is independent of the path. The combination of properties U + PV - TS or H - TS occurs so frequently in thermodynamic analysis that it is given a special name and symbol, F, the free energy (sometimes called the Gibbs Free Energy). Using this definition, Equation 2-143 is written... 

The simplest rules of thermodynamics suggest that energy must be expended to do work— You cannot get something for nothing, and that even if work is done some energy is forever lost to useful work— You cannot even get what you paid for . And that this entropy effect is such that the entropy of the universe is forever driving toward a maximum— Nature spontaneously falls into a mess Humor aside, the consequence is that any narrow packet as described above will spread over space in an attempt to make the local and universal mole fraction of A, B or C. .. the same everywhere. 

It should be noted that r y is the maximum thermodynamic efficiency obtained under reversible conditions, i.e., such that the rate of any photochemical reaction from D is infinitesimally slow. Although riy has some theoretical interest, it has no practical interest since we are interested in maximizing the rate of a photochemical reaction from D which will lead to the production of useful work. The rate of energy conversion by such a process can be defined as... 

This is the maximum amount of useful work that can be derived from the system on driving the reaction in the opposite direction. Thus, Vrev corresponds to the reversible work and is consequently called the thermodynamic reversible potential. At 25°C and 1 bar, the AG for the water-splitting reaction is 237.178 kj/mol [10]. Therefore,... 

Thermodynamic Work of Adhesion. One other important aspect of surface energetics (71, 72) is the use of surface free energy to calculate the maximum reversible work of adhesion, Wad, which has been correlated to the adhesive strength (41, 44) and should not be equated to the strength of an adhesive joint (6). Since neither wetting nor adhesion is controlled purely by thermodynamic factors, we should use the maximum reversible work of adhesion, Wad on the basis of an idealistic approach. When all other variables are equal, we can use Wad to compare the effectiveness of adhesives for a specific substrate. 

We can define this position of equilibrium in another way that will be useful in our study of thermodynamics. If we allow the mass Mj to fall through a distance dh it will do work M2gdh, by lifting M2 (where Mi < MJ. If we make M2 closer in magnitude to we shall get correspondingly more work, until at Mx == M2 an infinitesimal displacement will lead to work M1 g dh. This is the maximum work Mx can do, as it is now lifting an equal mass. When Mx = M2 the system is of course at equilibrium therefore we can define the equilibrium condition of the system as that for which a small displacement leads to the system doing the maximum possible work. 

Only in one case does the useful work of spontaneous chemical reactions reach maximum values. It is thermodynamic equilibrium, for which is valid Equation... 

Exergy is the maximum theoretical useful work (shaft work or electrical work) obtainable from a thermal system as it is brought into thermodynamic equilibrium with the environment while interacting with the environment only. Alternatively, exergy is the minimum theoretical work (shaft work or electrical work) required to form a quantity of matter from substances present in the environment and to bring the matter to a specified state. Hence, exergy is a measure of the departure of the state of the system from the state of the enviromnent. 

We encounter another thermodynamic state function, free energy (or Gibbs free energy), a measure of how far removed a system is from equilibrium. The change in free energy measures the maximum amount of useful work obtainable from a process and tells us the direction in which a chemical reaction is spontaneous. 

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