Energy and the first law of thermodynamics

In this chapter we look at the way energy may be converted from one form to another, by breaking and forming bonds and interactions. We also look at ways of measuring these energy changes. 

Finally, we look at indirect ways of measuring these energies. Both internal energy and enthalpy are state functions, so energy cycles may be constructed according to Hess s law we look also at Bom-Haber cycles for systems in which ionization processes occur. 

Obviously, we cannot actually conduct a transformation reversibly. An infinite length of time would be required if the volume increment in each stage were truly infinitesimal. Reversible processes therefore are not real processes, but ideal ones. Real processes are always irreversible. With patience and skill the goal of reversibility can be very closely approached, but not attained. Reversible processes are important because the work effects associated with them represent maximum or minimum values. Thus limits are set on the ability of a specified transformation to produce work in actuality we will get less, but we must not expect to get more. 

In the isothermal cycle described above, the net work produced in the irreversible cycle was negative, that is, net work was destroyed. This is a fundamental characteristic of every irreversible and therefore every real isothermal cyclic transformation. If any system is kept at a constant temperature and subjected to a cyclic transformation by irreversible processes (real processes), a net amount of work is destroyed in the surroundings. This is in fact a statement of the second law of thermodynamics. The greatest work effect will be produced in a reversible isothermal cycle, and this, as we have seen, is Wcy = 0. Therefore we cannot expect to get a positive amount of work in the surroundings from the cyclic transformation of a system kept at a constant temperature. 

Examination of the arguments presented above shows that the general conclusions reached do not depend on the fact that the system chosen for illustration consisted of a gas the conclusions are valid regardless of how the system is constituted. Therefore to calculate the expansion work produced in the transformation of any system whatsoever we use Eq. (7.4), and to calculate the work produced in the reversible transformation, we set Pop = p and use Eq. (7.5). 

By appropriate modification of the argument, the general conclusions reached could be shown to be correct f or any kind of work electrical work, work done against a magnetic field, and so on. To calculate the quantities of these other kinds of work we would not, of course, use the integral of pressure over volume, but rather the integral of the appropriate force over the corresponding displacement. 

The work produced in a cyclic transformation is the sum of the small quantities of work 4W produced at each stage of the cycle. Similarly, the heat withdrawn from the surroundings in a cyclic transformation is the sum of the small quantities of heat dQ withdrawn at each stage of the cycle. These sums are symbolized by the cyclic integrals of 4W and dQ - 

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The Entropy and Irreversible Processes.—Unlike the internal energy and the first law of thermodynamics, the entropy and the second law are relatively unfamiliar. Like them, however, their best interpretation comes from the atomic point of view, as carried out in statistical mechanics. For this reason, we shall start with a qualitative description of the nature of the entropy, rather than with quantitative definitions and methods of measurement. 

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