Affinity of a reaction

Early chemists thought that the beat of reaction, —AH. should be a measure of the "chemical affinity" of a reaction. With the introduction of the concepl of netropy (q.v.) and ihe application of the second law of thermodynamics lo chemical equilibria, it is easily shown that the true measure of chemical affinity and Ihe driving force for a reaction occurring at constant temperature and pressure is -AG. where AG represents the change in thermodynamic slate function, G. called Gibbs free energy or free enthalpy, and defined as the enthalpy, H, minus the entropy. S. times the temperature, T (G = H — TS). For a chemical reaction at constant pressure and temperature ... 

A second qualification is that the OSPE parameter, as well as the CFSE(crystal glass) difference parameters, eq. (8.11), are enthalpy terms and as such cannot be used to determine the affinity of a reaction. Free energy changes must be considered. However, since entropy changes are expected to be similar for all transition metal ions of the same valency, the use of an enthalpy term such as CFSE to interpret element fractionation may be a valid approximation when comparisons are made between transition metal ions of similar radius and charge. 

The physical quantity that we usually call the affinity of a reaction corresponds to the average affinity of the reaction. Generally, the affinity of a reaction at constant T and V differs numerically from that at constant 2"and p, as compared to the heat of reaction whose... 

The most important property of the chemical potential is that the affinity of a reaction is expressed by the difference in the chemical potential between the reactants and the products as shown in Eq. 5.13 and that the condition of reaction equilibrium is also expressed in terms of the chemical potentials of these reactants and products as shown in Eq. 5.14. 

The unitary affinity of a reaction in the standard state (298 K, 101.3 kPa, and unit activities) is normally called the standard affinity A0. 

The unitary affinity of a reaction can be obtained, as mentioned in the foregoing chapter 5, from the unitary chemical potentials of the reactants and products. 

Affinity of a reaction, A — The decrease in - Gibbs Alcoa process/Alcoa electrolyzer — This is a process energy on going from the reactants to the products of to manufacture metallic aluminum by electrolysis of... 

As we have seen the affinity of a reaction is at any instant a function of state of the system, and does not depend upon the conditions under which changes in the system occur. If we consider not the instantaneous value of the affinity but the average value in the course of a reaction, then this average will depend upon the conditions under which the reaction occurs e.g, whether at constant T and p, or T and V. In this book we shall employ the instantaneous values of the affinity, but to show the relationship between the present methods, and those introduced by Lewis and Randall we now proceed to express the average values of both heat of reaction and affinity in terms of the thermodynamic functions Z7, H, F and G. 

Let us first consider an ideal single phase system. For the affinity of a reaction occurring in this phase we have, from (6.22) and (7.1)... 

We now extend the above results to an ideal system of cj) phases. Using equations (6.69) and (7.3) we have for the affinity of a reaction taking place in the ideal system... 

Equation (7.80) holds whether a system is ideal or not, for the standard affinity of a reaction in a non-ideal system is the same as that in the corresponding reference system. 

In this chapter we shall consider the application of tabulated values of affinities, heats and entropies of reaction to the calculation of equilibrium constants. As we have pointed out already it is much more convenient to consider standard affinities of reaction than equilibrium constants. This is because standard affinities can be added and subtracted in just the same way as stoichiometric equations, so that the standard affinity of a reaction not included in the table is easily calculated. This means, as we shall see, that the only reactions which need to be included are those relating to the formation of compounds from their elements. 

Hence if the standard affinity of a reaction is known to be negative at a given temperature, then in a large number of cases it is possible to predict qualitatively whether the reaction can be made feasible by an increase or decrease of temperature. 

We can now make use of the above expressions for chemical potentials to calculate the affinity of a reaction in a perfect gas mixture, and to deduce the conditions for chemical equilibrium. 

The affinity of a reaction taking place in a non-reactive solvent is given by (7.74) and the law of mass action by (7.75). If the reaction occurs between dissolved molecules in very dilute solution, the affinity is readily expressed as a function of the molar concentrations (20.47) or the molalities (20.51). Instead of (7.27) we now have to define two new equilibrium constants and ... 

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. 

It will be observed that the magnitude of A depends on how far the arbitranly chosen concentration of the system is from the equilibrium concentration state If the arbitranly chosen state happens to correspond to the equilibrium state naturally no work will be done in reaching the equilibrium state This the above formula expresses It will also be observed that the above formula is deduced on the assumption that the system obeys the gas laws Numerical illustration of this formula will be given when we come to study the affinity of a reaction The most general mode of expressing the isotherm when seveial substances react, eg—... 

For the present we have of course to restrict the applications of equations (3) and (4) to the case of solid or liquid systems, for at the absolute zero, or in its neighbourhood, gases have no possible existence This restriction is, however, not of such importance as it seems, for it is possible by the aid of the first two laws of thermodynamics to calculate the affinity of a reaction occurring in a gaseous system (or in a dilute solution) if we know the affinity and heat relations for the same reaction when it occurs m the solid state... 

The notions of the total and standard affinity of a reaction introduced in the thermodynamics, and defined as... 

This shows the relation between the change in free energy, or the chemical affinity of a reaction, the heat of reaction at constant volume, the absolute temperature at which the reaction proceeds, and the temperature coefficient of the chemical affinity, dAjdTj of the reaction. These three... 

The quantity dV/dA)A=o is known as the phenomenological coefficient of a reaction. It can be written as Vq/RT, where Vq is the exchange rate and the coefficient RT renders value A dimensionless. The affinity of a reaction is determined according to de Donder as the chemical (or electrochemical, for charged particles) potential difference of products (/) and initial substances (/), multiplied by the respective stoichiometric factors ... 

At equilibrium, this difference is known to be equal to zero hence, the affinity of a reaction determines the degree of its deviation from equilibrium. At low deviations (A RT), the linear relationship (67) is true. For electrode reactions involving transfer of n electrons,... 

Affinity of a reaction is a state function, completely defined by the chemical potentials. It can be expressed as a function of V, T and Nk- For a closed system, since all the changes in Nk can only be due to chemical reactions, it can be expressed in terms of V, T, and the initial values of the mole numbers Nj. There are some general properties of affinities that follow firom the fact that chemical reactions can be interdependent when a reactant is involved in more than one reaction. 

In this case the decrease in entropy due to one reaction is compensated by the increase in entropy due to the other. Such coupled reactions are common in biological systems. The affinity of a reaction is driven away from zero at the expense of another reaction whose affinity tends to zero. 

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