Free energy acid, base reactions

Free energy change for reaction HA = H + A" in kcal/mol. Standard state 1 atm., 298 K. For origin of data other than those in parentheses, see Blades et al.58 Estimated acidity value based on relationship with AG U. 

FREE-ENERGY CHANGES IN AN ACID-BASE REACTION. 

Weller s review (1961) is not confined to acid-base reactions but deals with the kinetics of excited state reactions in general. Vander Donckt (1970) covers developments of the acid-base section of the Weller field but pays more attention to physical organic aspects, such as applications of resonance theory to the interpretation of pA-shifts upon excitation and the application of linear free energy relationships. The reviews by Schulman and Winefordner (1970) and by Winefordner et al. (1971a) are directed towards possible analytical applications. 

The standard free energy for this reaction, AG°, is a measure of the gas-phase acidity of BH (AG°cid) or the gas-phase basicity of B (AG +). The standard enthalpy change for this reaction, AH°, is known as the proton affinity (PA) of the base B (a neutral or anionic species, depending on whether v = 1 or v = 0, respectively). The significance of these values lies in the fact that they are intrinsic in nature, as they are free of solvation contributions6 from both neutral and charged species in a bulk condensed phase. 

These relations reveal the nature of chemical reactions involved in formation of CBPCs, which include dissolution of oxides and acid phosphates, and their subsequent acid-base reaction to form ceramics. Therefore, the Gibbs free energy plays an important role in determining which reactions (and hence which components) are most suitable in forming ceramics. 

Let us consider the proton transfer from acetone to a series of bases [3] and compare the logarithm of the rate constant with the pK of the conjugate acid of the base. A linear relationship is observed (Fig. 1) which is an example of a Bronsted correlation. If we had plotted log kf (a measure of the free-energy difference between ground and transition state) versus log k /kf (a measure of the free energy of the reaction) it is clear intuitively that the transition state would be product-like for a slope of unity and reactant-like for zero slope. It is difficult to measure equilibrium constants such as in Eqn. 1 but ionisation constants are easily estimated (using pH-titration equipment for example) so that the majority of comparisons are with these. Inspection of Eqns. 1 and 2 shows that the only identities are the base and the acid comparison of oxonium ion with acetone and water with the conjugate base of acetone is doubtful. 

Enzyme catalyzed mechanisms represent fundamentally familiar reactions from organic chemistry (Figure 2.17). Acid-base catalysis is associated with the donation or subtraction of protons. Acid catalysis is a process in which partial proton transfer from an acid lowers the free energy of the reaction transition state, while base catalysis is a process in which partial proton subtraction by a base lowers the free energy of the reaction transition state. Concerted acid-base catalysis, where both processes occur simultaneously, is a common enzymatic mechanism. 

The acid strength of proton acid-base pairs (Bronsted acids) is given by the pk value (Table 1.4.2). At the pH that corresponds to the pk, half of the molecules are in the acid form, the other half in the form of the corresponding base. Each pH unit then shifts the ratio by a factor of 10. Phenol, for example, is 50% phenolate and 50% phenol at pH 10, corresponding to a 1 1 ratio at its pk. At pH 7 the ratio of phenohphenolate is then 1000 1. The free energy of acidbase reactions can be directly calculated from the pk values. The pk of water (14 at a concentration of 10 M) is also drastically changed by coordination to metal ions, e.g., to 10 in the coordination sphere of... 

For acid-base equilibria the proton exchange reaction between HaO and H2O at standard conditions is assigned a free energy change of zero and an equilibrium constant of unity. It provides the datum to which all other acid base reactions are referenced. For redox equilibria, the... 

EMF measurements on the ZnQa-AQ (A = K, Rb or Cs) systems have yielded partial molar free energies, enthalpies and entropies, and the results have been interpreted in terms of an acid-base reaction. Tlie enthalpy of mixing consists of a contribution from the reaction to form the tetrahedral complex A2ZnCl4, and a contribution from the mixing of the products with remaining reactants to form a solution. The results indicate that as the polarizing power of the alkaU metal cation decreases, the reaction part of the enthalpy becomes more pronounced, reflecting the increased basicity of the alkali metal salt. 

The standard state for reporting enthalpies and free energies of acid-base reactions in the gas phase is 298 K. The spectroscopic parameters must be corrected because homolytic dissociation energies are usually reported at 298 K, but electron affinity and ionization potential values derived from spectroscopic data refer to enthalpies at OK. ... 

We begin our study of mechanisms in the context of acid-base chemistry in Chapter 3. Acid-base reactions are fundamental to organic reactions, and they lend themselves to introducing several important topics that students need early in the course (1) curved arrow notation for illustrating mechanisms, (2) the relationship between free-energy changes and equilibrium constants, and (3) the importance of inductive and resonance effects and of solvent effects. 

In the section on simulation, we first outline some key elements of the methodology used to carry out the simulations and their analysis. We comment specifically on the models used for the solvent, an area in which there has been considerable controversy. We then describe some of the derived results relating to single ion solvation structure and free energy, and the character of local density augmentation in this context. The reflection of this in activation free energies for ionic reactions and for acid-base equilibria, in particular, is then discussed. 

Rustad JR, Dixon DA, Kubicki JD, Felmy AR (2000a). The gas phase acidities of tetrahedral oxyacids from ab initio electronic structure theory. J Phys Chem A 104 4051-4057 Rustad JR, Dixon DA, Rosso KM, Felmy AR (1999a) Trivalent ion hydrolysis reactions A linear free energy relationship based on density functional electronic structure calculations. J Am Chem Soc... 

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