Thermodynamics of acid-base reactions

Chemical interaction between solids involves the consumption of some compounds and the formation of others. These processes occur in agreement with physicochemical laws and can be characterized using the fundamental thermodynamic equations. The main of them is the equation 

A principal possibility for one or another chemical reaction to occur is determined by the sign of AG. According to the laws of thermodynamics, the process occurs spontaneously in the direction of decreasing the free Gibbs energy. A reaction between the substances proceeds with the formation of products, if 

in order to determine the possibility for a reaction to proceed, it is sufficient to know AG k standard state (p = 0.1 MPa, T = 298 K) 

If a reaction in a mixture of solids is accompanied by the formation of gas or fluid phases (melts, solutions), solid solutions, or by the generation of defects, then, for a more strict thermodynamic forecast, it is necessary to take into account the changes of entropy and specific heat capacity during phase transitions of the components (melting, vaporization, dissolution), changes of volume and other parameters. If these factors are not taken into account, one can come across the contradictions between experimental data and thermodynamic calculations. 

It was mentioned above that the concept of average orbital electronegativity is used to characterize acid-base properties. According to this concept, acidity increases with increasing the numerical value of electronegativity while the basicity decreases, correspondingly. 

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Henry a. Bent, The Second Law, Oxford University Press, New York, 1965, Chapter 32, Thermodynamics of Acid-Base Reactions. ... 

If an acid is added to water, Eq. 5.5 describes the reaction, because the base in solution is water. Further, if a base is added to water, Eq. 5.6 describes the reaction, because now water is the acid. These acid-base reactions are critical to life itself, since nature s solvent is water. Having a good understanding of the thermodynamics of these reactions is not only important for understanding organic reactions in water, but is of the upper most importance in understanding biochemical reactions, almost all of which have acid-base dependencies. The factors that control the thermodynamics of acid-base reactions are the strengths of the acids or bases and the pH of the solution, so these measurements of acidity need to be examined in detail. 

The thermodynamic and analytical aspects of acid-base reactions in aprotic solvents are surveyed in reviews by Davis [1, 2]. The correlation of acid-base strength in water and aprotic solvents is of major importance. Early kinetic work by Bell and co-workers on the acid catalysis of (i) the ethyldiazoacetate-phenol interaction [3] (ii) the rearrangement of N-bromoacetanilide [4] and (iii) the inversion of /-menthone [5] established an order of acid strengths in aprotic media and the importance of intra-molecular hydrogen bonds e.g in picric acid). A thermodynamic method using reference acids and bases is more direct, and Bell and Bayles [6] employed the indicator acid Bromophenol Blue to obtain a basicity order for weak amine bases. Kinetic measurements on these systems have recently been made, and are considered in detail in Section 7. 

Table 4-1 lists some rate constants for acid-base reactions. A very simple yet powerful generalization can be made For normal acids, proton transfer in the thermodynamically favored direction is diffusion controlled. Normal acids are predominantly oxygen and nitrogen acids carbon acids do not fit this pattern. The thermodynamicEilly favored direction is that in which the conventionally written equilibrium constant is greater than unity this is readily established from the pK of the conjugate acid. Approximate values of rate constants in both directions can thus be estimated by assuming a typical diffusion-limited value in the favored direction (most reasonably by inspection of experimental results for closely related... 

First, the simple thermodynamic description of pe (or Eh) and pH are both most directly applicable to the liquid aqueous phase. Redox reactions can and do occur in the gas phase, but the rates of such processes are described by chemical kinetics and not by equilibrium concepts of thermodynamics. For example, the acid-base reaction... 

Proton transfers between oxygen and nitrogen acids and bases are usually extremely fast. In the thermodynamically favored direction, they are generally diffusion controlled. In fact, a normal acid is defined as one whose proton-transfer reactions are completely diffusion controlled, except when the conjugate acid of the base to which the proton is transferred has a pA value very close (differs by g2 pA units) to that of the acid. The normal acid-base reaction mechanism consists of three steps ... 

None of the programs can predict kinetics, that is, the rate of reaction, the activation energy, or the order of the reaction. These parameters can only be determined experimentally. Except for CHETAH, the primary use of the programs is to compute the enthalpies of decomposition and combustion. In fact, acid-base neutralization, exothermic dilution, partial oxidation, nitration, halogenation, and other synthesis reactions are not included in the programs except for CHETAH, which can be used to calculate the thermodynamics of essentially any reaction. 

Anhydrous copper(II) sulfate, 7 773 Anhydrous ethanol, production by azeotropic extraction, 8 809, 817 Anhydrous gaseous hydrogen sulfide, 23 633 Anhydrous hydrazine, 13 562, 585 acid-base reactions of, 13 567-568 explosive limits of, 13 566t formation of, 13 579 vapor pressures of, 13 564 Anhydrous hydrogen chloride, 13 809-813 physical and thermodynamic properties of, 13 809-813 purification of, 13 824-825 reactions of, 13 818-821 uses for, 13 833-834... 

The Lewis acid-base reaction leading to complex formation910 has been recently11 considered in relation to the role of solvation effects. Many scales of thermodynamic parameters have been suggested. The concept of donor number (DN) was proposed by Gutmann12, and defined as the AH (kcalmol-1) for the interaction of a basic solvent with SbCL in 1,2-dichloromethane at room temperature ... 

Relationships of acid-base strengths are in fact relationships between free energies of reaction in terms of the thermodynamic... 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

Note that A is called the conjugate base of HA and BH+ the conjugate acid of B. Proton transfer reactions as described by Eq. 8-1 are usually very fast and reversible. It makes sense then that we treat such reactions as equilibrium processes, and that we are interested in the equilibrium distribution of the species involved in the reaction. In this chapter we confine our discussion to proton transfer reactions in aqueous solution, although in some cases, such reactions may also be important in nonaqueous media. Our major concern will be the speciation of an organic acid or base (neutral versus ionic species) in water under given conditions. Before we get to that, however, we have to recall some basic thermodynamic aspects that we need to describe acid-base reactions in aqueous solution. 

So far, we have used the pure liquid compound as reference state for describing the thermodynamics of transfer processes between different media (Chapter 3). When treating reactions of several different chemical species in one medium (e.g., water) it is, however, much more convenient to use the infinite dilution state in that medium as the reference state for the solutes. Hence, for acid-base reactions in aqueous solutions, in analogy to Eq. 3-34, we may express the chemical potential of the solute i as ... 

There has been a resurgence of interest in proton-coupled redox reactions because of their importance in catalysis, molecular electronics and biological systems. For example, thin films of materials that undergo coupled electron and proton transfer reactions are attractive model systems for developing catalysts that function by hydrogen atom and hydride transfer mechanisms [4]. In the field of molecular electronics, protonation provides the possibility that electrons may be trapped in a particular redox site, thus giving rise to molecular switches [5]. In biological systems, the kinetics and thermodynamics of redox reactions are often controlled by enzyme-mediated acid-base reactions. 

Much of descriptive inorganic chemistry deals with reactions, so Chapter 4 presents a survey of the most important reaction types and the predictive power of thermodynamics. The utility of acid-base chemistry in classifying chemical behavior is described in Chapter 5. The chemistry of the elements follows in Chapters 6-17 based on the periodic table. The remaining chapters are devoted to the transition metals, coordination chemistry, and organometallic compounds. 

Together with acid-base reactions, where a proton transfer occurs (pH-dependent dissolution/ precipitation, sorption, complexation) redox reactions play an important role for all interaction processes in aqueous systems. Redox reactions consist of two partial reactions, oxidation and reduction, and can be characterized by oxygen or electron transfer. Many redox reactions in natural aqueous systems can actually not be described by thermodynamic equilibrium equations, since they have slow kinetics. If a redox reaction is considered as a transfer of electrons, the following general reaction can be derived ... 

The overall reaction is currently regarded as an apparent hydride transfer (for review, see ) because a) hydrogen is transferred directly between the reactants in most cases, without exchange with the solvent, 2) the base-catalyzed disproportionation of benzaldehyde to benzyl alcohol and benzoic acid (Cannizzaro reaction) i ems to be a hydride transfer (3) stereospecificity can be observed in model reactions. In addition, Verhoeven and coworkers have analyzed the thermodynamics of the photoinduced reaction between l-benzyl-3-carbamido-l,4-dihydropyridine and l,l-dimethyl-4,4 -bipyridylium dication, and have concluded that a thermal 1 e reaction between a 1-alkyl-1,4-dihydronicotinamide and a carbonyl compound is unlikely. 

Although nucleophilicity and basicity are interrelated, they are fundamentally different. Basicity is a measure of how readily an atom donates its electron pair to a proton it is characterized by an equilibrium constant Kg in an acid-base reaction, making it a thermodynamic property. Nucleophilicity is a measure of how readily an atom donates its electron pair to other atoms it is characterized by the rate constant, k, of a nucleophilic substitution reaction, making it a kinetic property. 

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