Affinity of a chemical reaction

The affinity of a chemical reaction is in general expressed as a function of the extent of reaction and hence of the number of moles of the chemical substances in the reaction as shown in Eq. 5.12 ... 

The affinity of a chemical reaction is. I Xryr, where v, is the stoichiometric coefficient, which is negative for reactants and positive for products. For a real gas mixture, the affinity is determined by... 

In Chapter IX. it was shown that the affinity of a chemical reaction can be calculated for any temperature, provided its value is known (from experiment) for any one temperature, and provided the heat of reaction and the variation of the heat of reaction with the temperature are known for the range of temperature in which we wish to calculate the affinity. The heat of reaction and its temperature coefficient, which is determined by the specific heats of the reacting substances, can both be determined calorimetrically without difficulty. On the other hand, it is not possible to calculate the affinity or the position of a chemical equilibrium by means of the two laws of thermodynamics and these thermal quantities alone. It is always necessary to know in addition the value of the affinity for some one temperature. The experimental determination of the affinity is often attended with considerable difficulty. It was thereforie eminently desirable to discover a new method which would avoid even this single determination and enable us to calculate the affinity from thermal quantities alone. The valuable researches of Nernst which resulted in the discovery of his heat theorem have placed at our disposal a means of solving this important problem. ... 

In the last chapter we introduced a precise definition of the affinity of a chemical reaction based upon the creation of entropy. The most important property of the affinity is expressed by the fundamental inequality... 

The affinity of a chemical reaction is closely related to the change in the Gibbs energy of micellisation, which is considered to be an irreversible process... 

De Donder also defined the affinity of a chemical reaction that enables us to write expression (4.1.10) in an elegant form, as the product of a thermodynamic force and a thermodynamic flow. The concept of affinity can be understood through the following simple example. 

At equilibrium the thermodynamic flows and hence the entropy production must vanish. This implies that in the state of equilibrium the affinity of a chemical reaction A = 0. Thus we arrive at the conclusion that, at thermodynamic equilibrium, the chemical potentials of the compounds X, Y and Z will reach values such that... 

This theory takes account of the view that the sensation of odour is the result of a chemical reaction in the nose, since in order for a body to be able to enter into chemical reaction the possession of residual affinity is probably a sine qua non, but it should be noted that the converse does not hold. 

Crude chloroform-methanol-water (30 60 8, v/v) extracts of immunostainedTLC bands were analyzed without further purification by nanoelectrospray low-energy mass spectrometry. The authors showed that this effective PLC/MS-joined procedure offers a wide range of applications for any carbohydrate-binding agents such as bacterial toxins, plant lectins, and others. Phenyl-boronic acid (PBA) immobilized on stationary support phases can be put to similar applications. This technology, named boronate affinity chromatography (BAC), consists of a chemical reaction of 1,2- and 1,3-diols with the bonded-phase PBA to form a stable... 

The rate of the active transport of sodium ion across frog skin depends both on the electrochemical potential difference between the two sides of this complex membrane (or, more exactly, membrane system) and also on the affinity of the chemical reaction occurring in the membrane. This combination of material flux, a vector, and chemical flux (see Eq. 2.3.26), which is scalar in nature, is possible according to the Curie principle only when the medium in which the chemical reaction occurs is not homogeneous but anisotropic (i.e. has an oriented structure in the direction perpendicular to the surface of the membrane or, as is sometimes stated, has a vectorial character). 

Equations 27 and 28 permit a simple comparison to be made between the actual composition of a chemical system in a given state (degree of advancement) and the composition at the equilibrium state. If Q K, the affinity has a positive or negative value, indicating a thermodynamic tendency for spontaneous chemical reaction. Identifying conditions for spontaneous reaction and direction of a chemical reaction under given conditions is, of course, quite commonly applied to chemical thermodynamic principle (the inequality of the second law) in analytical chemistry, natural water chemistry, and chemical industry. Equality of Q and K indicates that the reaction is at chemical equilibrium. For each of several chemical reactions in a closed system there is a corresponding equilibrium constant, K, and reaction quotient, Q. The status of each of the independent reactions is subject to definition by Equations 26-28. 

The quantity A = — v p. = —AmlG is often called the affinity of the chemical reaction. It is the thermodynamic driving force of the reaction. 

Example 8.4 Affinity and heat of reaction Describe the relationship between the affinity and heat of a chemical reaction. The affinity is defined as a partial derivative of the Gibbs free energy... 

The sj mbol for the affinity of a reaction has been modified. In the French edition it is denoted by A, but to avoid the possibility of confusion with the American usage of A for Helmholtz free energy, we have adopted the sanserif A. This is not inconsistent with the use of sanserif type for vectors, for the affinity may be regarded formally as the driving force of a chemical reaction correspondingly the velocity of reaction, V, is also printed in sanserif. 

Such reactions are common in inorganic chemistry and serve for the preparation both on a laboratory and on an industrial scale of a number of metals. If this reaction takes place in solution, as for example when zinc is immersed in a solution of copper sulfate, forming zinc sulfate and copper, while the formulation of the mechanism of the reaction is not quite as simple as in the first case, the oxidation-reduction changes are the same. Such reactions also are very common, and are the source of electric currents in galvanic cells using various combinations of metals and solutions, the electromotive forces under certain conditions being a measure of the affinity of the chemical reaction taking place. 

The expression for shows, that the entropy flux for open systems consists of two parts the thermal flux associated with the heat transfer, and the flux due to diffusion. The second expression consists of four terms associated with, respectively, the heat transfer, diffusion, viscosity, and chemical reactions. The expression for the dissipative function a has quadratic form. It represents the sum of products of two factors a flux (specifically, the heat flux /, diffusion flux momentum flux n, and the rate of a chemical reaction and a thermodynamic force, proportional to gradient of some intensive variable of state (temperature, chemical potential, or velocity). The second factor can also include external force F]t and chemical affinity Aj. 

As follows from the assertions in regularity assumptions 2, 3 (see end of Sect. 4.6 and Rem. 19), the matrix of both derivatives in (4.310) has the (maximal) rankn — h (equilibrium values are stressed here by the zero superscript, i.e. these are the values of corresponding quantities at p", T ). Therefore using now zero reaction rates (4.302) in the result (4.310) we obtain a system of homogeneous linear equations torn —h equilibrium affinities with the matrix of rank n - h formed by the equilibrium values of derivatives dJp/dpy. Therefore chemical affinities of independent chemical reactions (and, consequently, also of dependent reactions) must be zero in equilibrium... 

Equation (5.2) implies that when summation of all the fluxes (Ji) of charged species (Zi) across the membrane leading to charging of the membrane capacity has concluded a quasi-equilibrium with a (approximately) constant Vm or Aij/ is attained. Its value depends on the difference in chemical potential Afj, ) of all transported species and on the affinities of coupled chemical reactions. This is embodied in the so-called Goldman diffusion equation but these relations are essentially transcendental with their explicit solution tractable only under defined conditions. If only one species permeates through the membrane, for example, a true equilibrium state is reached and the resulting transmembrane electrical potential difference is described by an expression known as the Nemst equation... 

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