CONCLUDING REMARKS

CHAPTER 23 CONCLUDING REMARKS

We have now concluded our consideration of the theory and methods of chemical thermodynamics. Our primary objective, to establish the principles and procedures by which the thermodynamic properties associated with a given transformation can

be determined, has been acheived, and we have learned how these quantities can be used to judge the feasibility of that transformation. However, in emphasizing these aspects of the subject, we have neglected numer- ous broad fields in the realm of thermodynamics. Even within the areas to which we have limited ourselves, we have omitted any discussion of surface reactions [1], and we have paid only brief attention to problems of phase equilibria [2] and to electro- chemical processes [3]. We also could have examined some topics of more theoretical interest, such as relativity and cosmology [4]. Similarly, we could have considered phase equilibria at high temperature and pressure [5].

Although we have indicated some applications of thermodynamics to biological systems, more extensive discussions are available [6]. The study of equilibrium involving multiple reactions in multiphase systems and the estimation of their thermodynamic properties are now easier as a result of the development of computers and appropriate algorithms [7].

The point of view adopted toward thermodynamics in this book is the classic or phenomenological one. This approach is the most general but also the least illuminat- ing in molecular insight. The three basic principles of phenomenological thermo- dynamics are extracted as postulates from general experience, and no attempt is made to deduce them from equations describing the mechanical behavior of material

Chemical Thermodynamics: Basic Concepts and Methods, Seventh Edition . By Irving M. Klotz and Robert M. Rosenberg Copyright # 2008 John Wiley & Sons, Inc.

CONCLUDING REMARKS

bodies. As it is independent of the laws governing the behavior of material bodies, classic thermodynamics cannot be used to derive any of these laws. Generally, thermodynamic reasoning leads to relationships between certain physical quantities, but classic thermodynamics does not allow us to calculate a priori actual values of any of the quantities appearing in these relationships.

The phenomenological approach was inaugurated a century and a half ago and reached its fruition in theoretical formulation near the end of the nineteenth century. Since then, the major extension has been toward an analysis of nonequi- librium, nonisothermal processes. With the aid of additional phenomenological postulates, such as linear relationships between certain rates and appropriate forces, plus the Onsager reciprocity relationships, a conceptual system has been developed that is capable of analyzing a broad class of irreversible processes [8]. The laws of classic thermodynamics also have been recast in the form of

a Euclidean metric geometry whereby its formulas can be read from simple dia- grams. It has been suggested that the relationship between the geometric represen- tation of thermodynamics and the differential equations of Gibbs is analogous to the relationship between the matrix mechanics of Heisenberg and the wave mechanics of Schrodinger [9].

Parallel with the phenomenological development, an alternative point of view has developed toward thermodynamics, a statistical – mechanical approach. Its philos- ophy is more axiomatic and deductive than phenomenological. The kinetic theory of gases naturally led to attempts to derive equations describing the behavior of matter in bulk from the laws of mechanics (first classic, then quantum) applied to molecular particles. As the number of molecules is so great, a detailed treatment of the mechanical problem presents insurmountable mathematical difficulties, and statistical methods are used to derive average properties of the assembly of molecules and of the system as a whole.

In the field of thermodynamics, statistical mechanics has provided a molecular model, which leads to a more concrete visualization of some of the abstract concepts (such as entropy) of classic thermodynamics. In addition, it has developed means for the analysis of microscopic fluctuation phenomena, such as Brownian motion and the density fluctuations that are the basis of light scattering. Furthermore, it has extended the range of thermodynamic reasoning to new kinds of experimental data such as spectroscopic properties of matter, and it has been fundamental to the building of a bridge between the thermodynamics and the kinetics of chemical reactions. For these reasons, a knowledge of statistical thermodynamics is essential as a companion to phenomenological thermodynamics for the effective solution of many current problems and for the formulation of stimulating new questions [10].

In principle, quantum mechanics permits the calculation of molecular energies and therefore thermodynamic properties. In practice, analytic solutions of the equations of wave mechanics are not generally accessible, especially for molecules with many atoms. However, with the advances in computer technology and programming, and the development of new computational methods, it is becoming feasible to calculate energies of molecules by ab initio quantum mechanics [11]. Furthermore, molecular modeling with substantial complexity and molecular mechanics treatments for

529 finding the minimum potential energy configurations of molecules are increasingly

REFERENCES

successful in predicting a range of thermodynamic properties for large molecules with complex structures [12]. Clearly, these procedures will occupy a dominant position as we enter the twenty-first century.

Scientists are frequently tempted or encouraged to predict what new discoveries will appear in the coming decades or century. Past attempts of this kind have proved almost invariably disappointing. For example, the reader might look back at the prediction of August Comte quoted in the preface of this edition in the light of the current status of theoretical energetics. It behooves us to recall the famous epigram (attributed to Niels Bohr):

Prediction is very difficult—especially of the future.

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CONCLUDING REMARKS

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APPENDIX A