Thermodynamic Processes and Thermochemistry

Experience shows that heat is the most important factor influencing the extent of chemical reactions. Heat drives some reactions toward completion, but retards the progress of others. Therefore, it is appropriate to launch our study of chemical equilibrium by learning how to measure the heat transfer in a chemical reaction. This objective leads us into the branch of physical science called thermodynamics, which describes the meaning of heat and gives procedures for measuring heat transfer quantitatively. 

Thermodynamics is a broad and general subject with applications in all branches of the physical and biological sciences and engineering thus, we limit our discussion to those aspects necessary for chemical equilibrium. In this chapter, we demonstrate that heat—which on first examination appears mysterious despite 

Thermodynamics is an operational science, concerned with macroscopic, measurable properties and the relations among them. Its goal is to predict what types of chemical and physical processes are possible, and under what conditions, and to calculate quantitatively the properties of the equilibrium state that ensues when a process is conducted. For example, with thermodynamics we can answer the following types of chemical questions  

If hydrogen and nitrogen are mixed, is it possible for them to react If so, what 

How will a particular change in temperature or pressure affect the extent of the 

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CHAPTER 12 Thermodynamic Processes and Thermochemistry 507 TABLE 12.2 Enthalpy Changes of Fusion and Vaporizationt... 

Calorimetry is the basic experimental method employed in thermochemistry and thermal physics which enables the measurement of the difference in the energy U or enthalpy of a system as a result of some process being done on the system. The instrument that is used to measure this energy or enthalpy difference (At/ or A//) is called a calorimeter. In the first section the relationships between the thermodynamic functions and calorimetry are established. The second section gives a general classification of calorimeters in terms of the principle of operation. The third section describes selected calorimeters used to measure thermodynamic properties such as heat capacity, enthalpies of phase change, reaction, solution and adsorption. 

Energetics. Regardless of the gas phase combustion kinetics and thermochemistry, burning will only be possible if the energy balance is favorable. The first law of thermodynamics for a constant pressure gas phase process in which all of the work is pressure-volume (P-V) work states that the internal energy change dLT is related to the change in heat content dQ ... 

Thermodynamic properties of metal compounds and alloys are being measured in quite a large number of laboratories all over the world. There are two main reasons for this amount of work. Systematic measurements of enthalpies and entropies of formation provide the data which are needed to test the various models designed to explore the causes of chemical stability. Secondly, the data converted into Gibbs energies are needed to assess the feasibility of industrial processes and to deal with other practical problems. These two major aims are, of course, common to chemical thermodynamics in general, but metallurgical thermochemistry presents special problems which will be briefly indicated. 

Problems of interest include the effects of solvation on molecular properties such as geometries (e.g., molecular conformations and bond lengths), energies and thermodynamic stability, vibrational frequencies, dipole moments, nmr chemical shifts, etc., as well as the role of solvent in electronic excitation processes and the thermochemistry and rates of chemical and biochemical reactions. 

It is now be pertinent to move on to chemical reactions which, as mentioned earlier, are central to the subject of thermochemistry. Metallurgical processes are mostly carried out at constant pressure and, therefore, enthalpy assumes the role of a very important thermodynamic parameter in metallurgical thermochemistry. 

The second important aspect in thermodynamic studies is the determination of the enthalpy. A knowledge of the thermochemistry of epoxy-amine interactions is important also as a prerequisite for rational curing processes as manufacturing methods. The solution of this problem is also important for the application of the calorimetric method to the kinetic investigations. In fact, in the case of reactions with continuously varying concentrations of the donors and acceptors, the observed heat release (Q) may depend nonlinearly on conversion (a) as of the general case... 

THERMOCHEMISTRY. That aspect of chemistry which deals with die heat changes which accompany chemical reactions and processes, the heal produced by them, and die influence of temperature and odier thermal quantities upon them. Tt is closely related to chemical thermodynamics. The heat of formation of a compound is the heat absorbed when it is formed from its elements in their standard states. An exothermic reaction evolves heat and endothermic reaction requires heat for initiation. 

The sources and magnitudes of thermochemical data have been the subject of many entries in this Encycl. The use of the data presupposes a general acquantance with chemical thermodynamics (next article) and with detonation theory (Vol 4, D268-L to D298-R). The principle difference between classic thermodynamics and the thermochemistry of reactive systems is that expins and deflagrations do not represent equilibrium processes. In principle, the heat of reaction is obtained by ... 

Thermochemistry is concerned with the determination of the heat absorbed by a system when some process occurs within the system. The quantity of heat absorbed may be determined experimentally by the use of calorimeters or by calculation from prior knowledge of the thermodynamic properties of the system. The equations relating the heat absorbed by a system for a given process to the change of energy or enthalpy of the system for the change of state that occurs during the process are the mathematical statements of the first law of thermodynamics. They are Equations (2.26) and (2.30), written here as... 

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