The Driving Force of Chemical Reactions

It might be thought that the heat of a reaction is its driving force, and that a reaction will proceed if it evolves heat, and not proceed if it would absorb heat. In fact, however, many reactions take place with absorption of heat. We have mentioned some of these reactions in the preceding sections of this chapter another example is that when mercuric oxide is heated it decomposes into mercury and oxygen, with absorption of heat. 

Accordingly we see that this effect of probability can be made to cause more of the liquid to evaporate, simply by increasing the volume of the system. 

In the branch of science called thermodynamic chemistry a more detailed consideration is given to the relative effects of energy and probability. It has been found that the effect of probability can be described quantitatively by a new property of substances. This new property, which represents the probability of a substance in various states, is called entropy. 

It is clear from these considerations that the driving force of a reaction depends not only on the chemical formulas of the reactants and the structure of their molecules, hut also on the concentrations of the reactants and of the products. 

The relation between free energy, enthalpy, and entropy is 

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Further insight into the driving forces of chemical reactions can be gained by considering major physicochemical effects at the reaction center. 

Why Do We Need to Know This Material The laws of thermodynamics govern chemistry and life. They explain why reactions take place and let us predict how much heat reactions release and how much work they can do. Thermodynamics plays a role in every part of our lives. For example, the energy released as heat can be used to compare fuels, and the energy resources of food lets us assess its nutritional value. The material in this chapter provides a foundation for the following chapters, in particular Chapter 7, which deals with the driving force of chemical reactions. 

Chemical "affinity" remained part of the tool kit of the chemist, however badly defined and understood. Affinity cannot simply be explained away as heat, insisted Wurtz, a leading advocate of chemical and physical atomism in France in the generation following Dumas.58 As we will see in chapter 5, "energy" replaced "affinity" in the late 1800s as the driving force of chemical reactions. In addition, the concepts of spontaneity and irreversibility entered the domain of physics, undermining the classical mechanics of matter and force in which processes are, in principle, reversible. Conceptually, the notions of spontaneity and irreversibility were more closely allied with experimental results in classical chemistry than in classical physics. 

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. 

The energy factor (enthalpy) and the probability factor (entropy) in chemical reactions. The driving force of chemical reactions—free energy. Oxidation-reduction potentials and their uses. 

The structural approach will also contribute to the analysis of the thermodynamics of nonequilibrium systems. It is the aim and purpose of thermod5mamics to describe structural features of systems in terms of macroscopic variables. Unfortunately, classical thermodynamics is concerned almost entirely with the equilibrium state it makes only weak statements about nonequilibrium systems. The nonequihbrium thermodynamics of Onsager (f), Prigogine (2), and others introduces additional axioms into classical thermodynamics in an attempt to obtain stronger and more useful statements about nonequilibrium systems. These axioms lead, however, to an expression for the driving force of chemical reactions that does not agree with experience and that is only applicable, as an approximation, to small departures from equilibrium. A way in which this situation may be improved is outlined in Section VII. 

In the previous section, we explored how Kohn-Sham density-functional theory can be used to provide accurate quantitative characterizations of chemical systems. In the present section, we focus instead on density-functional theoretic tools for qualitative descriptions of chemical reactivity, with particular focus on describing the driving forces of chemical reactions. 

Chemical Potential and Affinity the Driving Force of Chemical Reactions... 

The reason for this violation of the PLM requirements lies, apparently, in the fact that the driving force of chemical reactions has a much more complicated nature than that given by a condition of the Eq. (1.31) type. Indeed, this condition may be used in calculating a reaction coordinate only as a crude approximation. 

The driving force for chemical reactions definition of affinity... 

In theory, any chemical reaction could proceed at the same time in the reverse direction to some extent. In practice, this is not usually the case. Often, the driving force of a reaction favors one direction so greatly that the extent of the reverse reaction is so small that it is impossible to measure. The driving force of a chemical reaction is the change in free energy accompanying the reaction and it is an exact measure of the tendency of the reaction to go to completion. The possibilities are ... 

On a microscopic scale, atoms and molecules travel faster and, therefore, have more collisions as the temperature of a system is increased. Since molecular collisions are the driving force for chemical reactions, more collisions give a higher rate of reaction. The kinetic theory of gases suggests an exponential increase in the number of collisions with a rise in temperature. This model fits an extremely large number of chemical reactions and is called an Arrhenius temperature dependency, or Arrhenius law. The general form of this exponential relationship is... 

An example of a chemically induced substitution reaction is represented by the halide removal in MnCl(CO)5 by AICI3 leading to the hexacarbonylmanganese(I) cation. The driving force of this reaction, carried ont at room temperature, is represented by the high affinity of aluminum for chloride. Similarly, the corresponding rhenium(I) derivative [Re(CO)6]AlCLi can be prepared. This complex has considerable stability in that it can be dissolved in water without prompt decomposition (equation 32). 

The rate at which free energy changes as the concentration of a particular substance changes is the chemical potential of that substance, and Gibbs could show that it was the chemical potential that acted as the driving force behind chemical reactions. A chemical reaction moved spontaneously from a point of high chemical potential to one of low, as heat flowed spontaneously from a point of high temperature to one of low. 

Chemical kinetics and its relation to chemical equilibrium is a subject of monographs and reviews [108, 131, 132, 154, 157]. Classical non-equilibrium thermodynamics [3, 4, 119, 120] smdies this subject starting from entropy production (4.178) and therefore taking the affinity as a driving force of chemical reaction rates [158] but this seems (at least) insufficient because of the decomposition (4.174), cf. discussion... 

The driving force for chemical reaction j, Xrx,j, is a function of the chemical affinity and temperature, according to the expression,... 

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