Thermodynamics chemical changes

This chapter introduces the first law of thermodynamics and its applications in three main parts. The first part introduces the basic concepts of thermodynamics and the experimental basis of the first law. The second part introduces enthalpy as a measure of the energy transferred as heat during physical changes at constant pressure. The third part shows how the concept of enthalpy is applied to a variety of chemical changes, an important aspect of bioenergetics, the use of energy in biological systems. 

Like physical equilibria, all chemical equilibria are dynamic equilibria, with the forward and reverse reactions occurring at the same rate. In Chapter 8, we considered several physical processes, including vaporizing and dissolving, that reach dynamic equilibrium. This chapter shows how to apply the same ideas to chemical changes. It also shows how to use thermodynamics to describe equilibria quantitatively, which puts enormous power into our hands—the power to control the And, we might add, to change the direction of a reaction and the yield of products,... 

Although thermodynamics can be used to predict the direction and extent of chemical change, it does not tell us how the reaction takes place or how fast. We have seen that some spontaneous reactions—such as the decomposition of benzene into carbon and hydrogen—do not seem to proceed at all, whereas other reactions—such as proton transfer reactions—reach equilibrium very rapidly. In this chapter, we examine the intimate details of how reactions proceed, what determines their rates, and how to control those rates. The study of the rates of chemical reactions is called chemical kinetics. When studying thermodynamics, we consider only the initial and final states of a chemical process (its origin and destination) and ignore what happens between them (the journey itself, with all its obstacles). In chemical kinetics, we are interested only in the journey—the changes that take place in the course of reactions. 

There are two principal chemical concepts we will cover that are important for studying the natural environment. The first is thermodynamics, which describes whether a system is at equilibrium or if it can spontaneously change by undergoing chemical reaction. We review the main first principles and extend the discussion to electrochemistry. The second main concept is how fast chemical reactions take place if they start. This study of the rate of chemical change is called chemical kinetics. We examine selected natural systems in which the rate of change helps determine the state of the system. Finally, we briefly go over some natural examples where both thermodynamic and kinetic factors are important. This brief chapter cannot provide the depth of treatment found in a textbook fully devoted to these physical chemical subjects. Those who wish a more detailed discussion of these concepts might turn to one of the following texts Atkins (1994), Levine (1995), Alberty and Silbey (1997). 

The science of chemical kinetics is concerned primarily with chemical changes and the energy and mass fluxes associated therewith. Thermodynamics, on the other hand, is concerned with equilibrium systems. .. systems that are undergoing no net change with time. This chapter will remind the student of the key thermodynamic principles with which he should be familiar. Emphasis is placed on calculations of equilibrium extents of reaction and enthalpy changes accompanying chemical reactions. 

The solid state polymerisation of diacetylenes (2) with U.V. radiation, heating or shear force is most indicative of the predominant influence of electron-lattice coupling. The details of the chemical changes that occur during th polymerisation process are crucial (2,40) but the overall description only needs part of this chemical information. The kinetics and thermodynamics of the polymerisation process using an elastic strain approach have been worked out in (41). 

The qualitative picture of chemical change is clear. The reactant system, in an otherwise fixed environment, approaches an activated, or valence state, at a critical temperature. In addition to the appearance of normal critical phenomena, the chemical system is further prepared for reaction by long-range quantum-mechanical activation. This feature falls outside the scope of statistical thermodynamics and needs elucidation in terms of molecular quantum fields. 

We may be able to infer information about the mechanism of chemical change from kinetics but not from thermodynamics the rate of chemical change is dependent on the path of reaction, as exemplified by the existence of catalysis thermodynamics, on the other hand, is not concerned with the path of chemical change, but only with state and change of state of a system. 

Nevertheless, equilibrium can be an important aspect of kinetics, because it imposes limits on the extent of chemical change, and considerable use is made of thermodynamics as we proceed. 

At this point, the final specifications for the one or two chosen product(s) after selection process should be identified. This can be done using a three-step procedure. First, we define the product micro and macrostructure. Second, we rank the product s most important attributes, an effort that forces a review of how the product will be used. Third, we review any chemical triggers, that is, chemical stimuli which cause major changes in product properties. Finally, we turn to the manufacture itself, which relies on technical know-how of thermodynamics, chemical reaction kinetics, transport phenomena, and unit operations. Some of these ideas are illustrated in the following example. Except for a schematic of the manufacturing process, the many details related to the manufacturing plant are omitted in this discussion. 

Irradiation of a compound may produce different products than do thermal reactions. In a thermal reaction the chemical change favours the more thermodynamically-stable compound but the fate of a photochemical reaction is governed by using a wavelength that is absorbed by the reactant but not by the product. Thus the reaction pathways for excited-state species and ground-state species... 

All molecules have an amount of energy determined mainly by their chemical structure. Metabolism involves chemical change. Inevitably therefore, energy changes always accompany the chemical changes which occur in metabolism. Our understanding of bioenergetics arises from physics and the laws of 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. 

The second law of thermodynamics can be expressed in terms of another state function, the entropy (S). The thermodynamics definition considers the change in entropy dS that occurs as a result of a physical or chemical change, and is based on the expression... 

A representation of all of the elementary reactions that lead to the overall chemical change being investigated. This representation would include a detailed analysis of the kinetics, thermodynamics, stereochemistry, solvent and electrostatic effects, and, when possible, the quantum mechanical considerations of the system under study. Among many items, this representation should be consistent with the reaction rate s dependence on concentration, the overall stoichiometry, the stereochemical course, presence and structure of intermediate, the structure of the transition state, effect of temperature and other variables, etc. See Chemical Kinetics... 

At this stage it is worth re-emphasizing that, although thermodynamics can furnish information about the direction of a chemical change, by its very nature it can say nothing about the rate at which the change will take place. 

Often thermodynamic calculations for complex systems are made assuming that all chemical changes can take place that are allowed within the framework of the atomic material balances. This approximation may be appropriate at high temperatures but is often not true for catalytic systems. 

Equation (6.35b) shows that the new intensive variable, chemical potential pi, as introduced in this chapter, is actually superfluous for the case c = 1, because its variations can always be expressed in terms of the old variations dT dP. Thus, as stated in Inductive Law 1 (Table 2.1), only two degrees of freedom (independently variable intensive properties) suffice to describe the thermodynamic variability of a simple c = 1 system. This confirms (as expected) that chemical potential pu only becomes an informative thermodynamic variable when chemical change is possible, that is, for c > 2 chemical components. 

Arrhenius interpreted the equation by suggesting that there exists an equilibrium between normal molecules and what he called active molecules, and that only the active molecules undergo chemical change. The active molecules were supposed to be formed endothermically from the normal molecules. The rapid increase in the rate of chemical change with rising temperature is therefore caused by the shift in the equilibrium between the two kinds of molecules, and, since k is proportional to the number of active molecules, the equation d log k/dT = A/RT2 represents this shift in the ordinary thermodynamic way. A is the heat absorbed in the formation of an active molecule from a normal one and is therefore called the heat of activation. 

Thermochemistry is the study of the thermal (heat) changes that are associated with physical and chemical changes. Thermodynamics is much broader in scope because it includes the study of all forms of energy, including work. Some of these aspects are considered in later chapters. 

Because energy underlies all chemical change, thermodynamics—the study of the transformations of energy—is central to chemistry. Thermodynamics explains why reactions occur at all. It also lets us predict the heat released or required by chemical reactions. Heat output is an essential part of assessing the usefulness of compounds as fuels and foods, and the first law of thermodynamics allows us to discuss these topics systematically. The material in this chapter provides the foundation for the following chapters, in particular Chapter 7, which deals with the driving force of chemical reactions—why they occur and in which direction they can be expected to go. 

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