Limitations of Classic Thermodynamics

Although descriptions of chemical change are permeated with the terms and language of molecular theory, the concepts of classic thermodynamics are independent of molecular theory thus, these concepts do not require modification as our knowledge of molecular structure changes. This feature is an advantage in a formal sense, but it is also a distinct limitation because we cannot obtain information at a molecular level from classic thermodynamics. 

In contrast to molecular theory, classic thermodynamics deals only with measurable properties of matter in bulk (for example, pressure, temperature, volume, cell potential, 

For example, classic thermodynamic methods predict that the maximum equUi-brium yield of ammonia from nitrogen and hydrogen is obtained at low temperatures. Yet, under these optimum thermodynamic conditions, the rate of reaction is so slow that the process is not practical for industrial use. Thus, a smaller equilibrium yield at high temperature must be accepted to obtain a suitable reaction rate. However, although the thermodynamic calculations provide no assurance that an equUibrium yield will be obtained in a finite time, it was as a result of such calculations for the synthesis of ammonia that an intensive search was made for a catalyst that would allow equilibrium to be reached. 

Similarly, specific catalysts called enzymes are important factors in determining what reactions occur at an appreciable rate in biological systems. For example, adenosine triphosphate is thermodynamically unstable in aqueous solution with respect to hydrolysis to adenosine diphosphate and inorganic phosphate. Yet this reaction proceeds very slowly in the absence of the specific enzyme adenosine triphosphatase. This combination of thermodynamic control of direction and enzyme control of rate makes possible the finely balanced system that is a hving cell. 

In the case of the graphite-to-diamond transformation, thermodynamic results predict that graphite is the stable allotrope at a fixed temperature at all pressures below the transition pressure and that diamond is the stable aUotrope at all pressures above the transition pressure. But diamond is not converted to graphite at low pressures for kinetic reasons. Similarly, at conditions at which diamond is the thermodynamically stable phase, diamond can be obtained from graphite only in a narrow temperature range just below the transition temperature, and then only with a catalyst or at a pressure sufficiently high that the transition temperature is about 2000 K. 

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The first order division is based on the system with or without reaction coupling, so the modern thermodynamics in a broad sense has been divided into the classical thermodynamics and the modem thermodynamics in a narrow sense. It is very clear that classical thermodynamics should only be used for simpler systems without reaction coupling, because the second law of thermodyneimics, such as dG)r,p 0, is only concerned with the whole system and not concerned with individual processes. That is a severe limitation of classical thermodynamics, because systems in modem inorganic synthesis and in life science are usually complex with multi-reaction processes including reaction coupling. 

While the limitations of classical thermodynamics are clear enough, the potentially vast possibilities opened by statistical thermodynamics are still far from realized. Just what modern physics can do for us will be discussed later in the week for now,... 

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