Maximum work available

The overall reactions given in Table 2-2 can be used to produce both electrical energy and heat. The maximum work available from a fuel source is related to the free energy of reaction in the case of a fuel cell, whereas the enthalpy (heat) of reaction is the pertinent quantity for a heat engine, i.e.. 

This reaction, having equal number of mols of gas reactants and products, has a negligible change in entropy and thus a negligible heat effect if carried out reversibly at constant temperature. The maximum work available from a fuel cell under these circumstances would then be approximately the enthalpy change of the reaction, i.e., the heat of combustion of the... 

The value of AG gives the maximum work available from the process, apart from work associated with changes in pressure or volume. AG may be expressed in joules or calories per mole (4.186 J= leal). 

Mf Transition spontaneous for species/ maximum work available per mole of species j is /if - /if... 

This result applies whenever the process is carried out in a nonreversible (irreversible) manner. In other words, in an irreversible cyclic process more work must be input to the system than the system produces. In all the finite gas compressions the work required is greater than I.4P1V1, which is the maximum work available from the expansion. 

The term iG represents the portion of an energy change (AH) that is available to do useful work. If AH is the total energy in a reaction, then AG = AH -T AS indicates that part of AH is alway dissipated as heat (the T AS term) and is therefore unavailable for other things, such as muscle contraction, ion transport, or tissue growth. The remaining amount (AG ) is available for useful work, but may not actually be fully utilized for useful work because the efficiency of a process (the ratio of work actually accomplished to AG, the maximum work available) is always less than 100%. 

Knowing the value of AG for a process thus gives us valuable information about how close the process is to 100% efficiency. For example, when gasoline is burned in a car s engine, the work produced is about 20% of the maximum work available. 

AG= -(maximum work available from the system through the state transition), when this work is given a positive sign. 

This reversible work is the maximum work available in bringing the feed stream to the environmental conditions that is, is the maximum rate of obtaining work, which can be written... 

That is, the change in the maximum work available from the stream is a function solely of its changes in enthalpy and entropy, and the environmental temperature. Like H and. S, is a state function, independent of path, but dependent on the temperature. To, and pressure, T o, of the dead state. If chemical reactions occur, the availability also depends on the composition of the dead state. 

The maximum work available by reversibly mixing two equal volumes of solutions at different salinity is a linear function of the temperature and logarithmic function of the two concentrations. In order to give some value of the aehievable theoretical work, contours for AGjjjix are reported in Figure 9.1 as a function of the two solution salinities. It is possible... 

Calculate the maximum work available from 50.0 g of aluminum in the following cell when the emf is 1.15 V. 

Let us recall the cell in Fig. 16.1, page 314. For this cell, f = 1.10 V. Then the maximum work available when the cell reaction... 

If the concentrations of the ions are 1 mole/liter, AG = AG. If Zn and Cu + simply react with each other, not in a galvanic cell, the total heat liberated is 2.17 X 10 joules AH = —2.17 X 10 joules, the negative sign signifying the emission of heat. The maximum work available from the... 

A° and ArG° are both constant quantities at a constant temperature since the standard chemical potentials are constant in these conditions (see Sect. 2.1). Both quantities can be determined. They are equal (in absolute values) to the useful work, or the maximum work available. Such is the case when the chemical reaction takes place reversibly and when the initial and final states are the standard ones. They are described in Fig. 2.3. 

Electromotive Force of a Galvanic Cell, Cell Potential Difference, Maximum Work Available from a Chemical Reaction, and Nernst s Equation ... 

It is possible to obtain work from many reactions by forming an electrical ceU in which the reaction occurs. The familiar storage battery and dry cell are examples. In many cases it is possible to measure the maximum work available from the reaction quite precisely by measuring the voltage (electromotive force, E) of the cell. E is proportional to the maximum work per electron transferred, and is related to AF by the equation... 

Achieving the maximum work available from a spontaneous process can occur only via a hypothetical pathway. Any real pathway wastes energy. If we could discharge the battery infinitely slowly by an infinitesimal current flow, we would achieve the maximum useful work. Also, if we could then recharge the battery using an inflnitesimally small... 

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