Chemical change, direction equilibrium

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. 

CHANGES IN FREE ENERGY DETERMINE THE DIRECTION EQUILIBRIUM STATE OF CHEMICAL REACTIONS... 

Le Chatelier s principle to predict the direction in which a chemical system at equilibrium will shift when concentration changes... 

The use of AG as a criterion for deciding the direction of a chemical change has been briefly mentioned. AG"" refers to the reaction with both products and reactants in their standard states and industrial processes are, of course, often carried out under conditions far removed from standard. The significance of AG"" in relation to the technical feasibility of a reaction is illustrated in Table 1 where it is seen that AG = + 20 kJ mol" means an equilibrium constant of 3 x 10 at 298 K. With such a small equilibrium constant, a reaction is unlikely to be of much industrial value. At 2000 K, however, the same value of AG"" leads to an equilibrium constant of 0.3, potentially a very much more attractive situation. 

The facts that have just been described lend considerable support to the Lindemann theory. If this theory is to be applicable, the rate of activation and deactivation at higher pressures ought to be great compared with the rate of chemical change, in order that there may be little disturbance of the statistical equilibrium and hence an absolute rate of reaction directly proportional to the total concentration. At first some difficulty was felt about this point, but the solution appears to have been found, and indeed the solution itself constitutes a rather strong piece of evidence in favour of the theory. 

In the range of temperatures and pressures where the reaction is substantially reversible, the kinetics is much more complicated. There is no grounds to consider chemical changes described by (272) and (273) as independent, not interconnected, reactions. Conversely, if processes (272) and (273) occur on the same surface sites, then free sites will act as intermediates of both processes. Thus one must use the general approach, treating (272) and (273) as overall equations of a certain single reaction mechanism. But if a reaction is described by two overall equations, its mechanism should include at least two basic routes hence, the concept of reaction rate in the forward and reverse directions can be inapplicable in this case. However, experiments show that water-gas equilibrium (273) is maintained with sufficient accuracy in the course of the reaction. Let us suppose that the number of basic routes of the reaction is 2 then, as it has been explained in Section VIII, since one of the routes is at equilibrium, the other route, viz., the route with (272) as overall equation, can be described in terms of forward, r+, and reverse, r, reaction rates. The observed reaction rate is then the difference of these... 

Calorimetric measurements yield enthalpy changes directly, and they also yield information on heat capacities, as indicated by equation 10.4-1. Heat capacity calorimeters can be used to determine Cj , directly. It is almost impossible to determine ArCp° from measurements of apparent equilibrium constants of biochemical reactions because the second derivative of In K is required. Data on heat capacities of species in dilute aqueous solutions is quite limited, although the NBS Tables give this information for most of their entries. Goldberg and Tewari (1989) have summarized some of the literature on molar heat capacities of species of biochemical interest in their survey on carbohydrates and their monophosphates. Table 10.1 give some standard molar heat capacities at 298.15 K and their uncertainties. The changes in heat capacities in some chemical reactions are given in Table 10.2. 

This formula is of great importance in kinetics, for frequently it is not possible to determine the rate of a given chemical change because it is too fast, or too slow, or because the measurements of concentrations can not be determined by direct experimental methods yet devised. The determination of the equilibrium constant K is usually comparatively simple and a knowledge of it and one of the velocity constants permits a ready calculation of the other by equation (13). It is surprising to realize that, except possibly in one or two cases, this important formula has not been subjected to complete experimental test, by measuring all three quantities. There is, however, no question as to the validity of the relation if the reaction proceeds in the manner indicated. 

The influence of concentration (or pressure if the species are gases) on the position of a chemical equilibrium is conveniently described in quantitative terms by means of an equilibrium-constant expression. Such expressions are derived from thermodynamics. They are important because they permit the chemist to predict the direction and completeness of a chemical reaction. An equilibrium-constant expression, however, yields no information concerning the rate at which equilibrium is approached. In fact, we sometimes encounter reactions that have highly favorable equilibrium constants but are of little analytical use because their rates are low. This limitation can often be overcome by the use of a catalyst, which speeds the attainment of equilibrium without changing its position. 

The rearrangement of atoms that occurs in a chemical reaction is virtually always accompanied by the liberation or absorption of heat. If the purpose of the reaction is to serve as a source of heat, such as in the combustion of a fuel, then these heat effects are of direct and obvious interest. We will soon see, however, that a study of the energetics of chemical reactions in general can lead us to a deeper understanding of chemical equilibrium and the basis of chemical change itself. 

Nitrogen atoms of the azo bond are directly engaged in the azo-hydrazone tautomeric system and it can be expected that l4/l5N chemical shifts should reflect the changes in equilibrium. This basic idea was proposed by Berrie et al.,m who used l4N chemical shifts of NHQ/NQ atoms and calculated hydrazone contents (Table 15). They obtained naturally only one set of data measurable by double irradiation technique. The intramolecular hydrogen bond has a marked effect on <5N, as can be seen after comparison of l5NQ chemical shifts in trans-azobenzene (6 = 128), and compounds 87 and 4-hydroxyazobenzene (91)110 (Table 20). From these data it follows that the proper model compounds for tautomeric systems with intramolecular... 

That is, considering a physical or chemical system in equilibrium, THE EQUILIBRIUM BEING FIXED BY THE NATURE OF THE SYSTEM AND CONDITIONS SUCH AS TEMPERATURE AND PRESSURE, THE PRINCIPLE STATES THAT IF WE ALTER ONE OF THESE CONDITIONS OR PARAMETERS, SAY THE TEMPERATURE, THE SYSTEM WILL CHANGE IN SUCH A DIRECTION AS TO TEND TO ANNUL THIS CHANGE IN TEMPERATURE The principle will be made clearer by a few examples... 

Does the reaction take place to a sufficient extent to produce useful (or even detectable) quantities of products This refers to the thermodynamics (energetics) of the reaction, which controls its tendency to occur. The concept of chemical equilibrium which we treat in this chapter addresses this question directly. In a later chapter we will see that the tendency of a reaction to occur can be predicted entirely from the properties of the reactants and products through the law of thermodynamics. This is the macroscopic aspect of chemical change in that it makes no assumptions about the mechanistic details of how the atoms rearrange themselves as the reactants are transformed into products. 

Here we have to expect, from the smallness of the heat change, that an equilibrium should be obtained even at low temperatures, and that, though it must be displaced in the direction of the formation of hydriodic acid, yet all the molecular species take part in the equilibrium with appreciable partial pressures. Experience confirms this fully. In addition to the examples mentioned, we may recall the formation of an ester, the classical case of chemical equilibrium since the formation of gaseous ethyl acetate and water from alcohol and acetic acid vapours takes place without any considerable heat change, an equilibrium must be produced just as with hydriodic acid, in which all the components participate in considerable concentrations since the vapour pressures of the four substances are not very different, this equilibrium must also be found for the liquid mixture. 

By analogy with the thermodynamic treatment of the solute equilibrium, transition-state theory describes rates of chemical change by postulating that a transition state lies somewhere on the pathway between the reactants and the products and that this transition state can be characterized by its own thermodynamic parameters, including its partial molar volume. The difference between the partial molar volume of the transition state and that of the reactants is the activation volume, AU. The activation volume cannot be measured by a direct density measurement, because the transition state is not a chemical species, not even a short-lived one. It can be measured only by the effect of the pressure on some rate constant k that characterizes the chemical process ... 

Enzymes do not change the equilibrium of a chemical reactionist the rate. Many reactions in the body can go in either direction, as indicated by the use of a double-headed arrow ( ) in equations. In some cases, the concentrations of the reactants in the body are such that the reaction always proceeds only in one direction. Such reactions are said to be physiologically irreversible and are written with a single-headed arrow ( ). This is not a property of the enzyme, but depends on the reaction and the reactant concentrations. 

For the reverse change (condensation), A5 niv also equals zero, but AS°ys and ASsun have signs opposite those for vaporization. A similar treatment of a chemical change shows the same result the entropy change of the forward reaction is equal in magnitude but opposite in sign to the entropy change of the reverse reaction. Thus, when a system reaches equilibrium, neither the forward nor the reverse reaction is spontaneous, and so there is no net reaction in either direction. 

A system in chemical equilibrium can be subjected to changes in conditions that displace it from equilibrium. The shift will be from left to right or right to left depending on the nature of the stress. We do not need to know the equilibrium constant expression or its numerical value to predict the direction of the shift. It can be predicted by applying the Le Chatelier principle If a chemical reaction at equilibrium is subjected to a change in conditions which displaces it from equilibrium, the chemical system will readjust to a new equilibrium state which tends to reduce the stress. 

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