Enthalpy of acid-base reaction

The major disadvantage of the HSAB principle is its qualitative nature. Several models of acid-base reactions have been developed on a quantitative basis and have application to solvent extraction. Once such model uses donor numbers [8], which were proposed to correlate the effect of an adduct on an acidic solute with the basicity of the adduct (i.e., its ability to donate an electron pair to the acidic solute). The reference scale of donor numbers of the adduct bases is based on the enthalpy of reaction. A//, of the donor (designated as B) with SbCb when they are dissolved in 1,2-dichloroethane solvent. The donor numbers, designated DN, are a measure of the strength of the B—SbCb bond. It is further assumed that the order of DN values for the SbCb interaction remains constant for the interaction of the donor bases with all other solute acids. Thus, for any donor base B and any acceptor acid A, the enthalpy of reaction to form B A is ... 

Direct calorimetric method or temperatnre dependence of eqnilibrium constants can be nsed to measnre enthalpies and entropies of acid-base reactions. Calorimetric techniqnes allow obtaining an interesting quantification and evaluation of the gas-solid interactions and more details on use of data from these measurements will be given in the following section. 

However, in many cases, electronegativity difference alone cannot account for the stability of the molecule. For example, according to the electronegativity criterion, the CsF molecule should be very stable as the electronegativity difference between Cs and F is very large. But the reaction enthalpy data indicate that Fil and CsF will react to form Csl and FiF. In order to predict the direction of acid-base reactions and to account for the stability of the products, Pearson introduced two parameters hardness and softness in the vocabulary of chemistry. The qualitative definitions of hard and soft acids and bases are as follows [28-32] ... 

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. ... 

Bronsted s definition acknowledges the existence of acid-base reactions in the gas phase, in which the complicating effects of solvation are absent. The acidity of a substance B is then measured by the proton affinity PA of the molecule, i.e. the variation of enthalpy involved in the reaction ... 

The sensitivity of the equilibrium constant to temperature, therefore, depends upon the enthalpy change AH . This is usually not a serious limitation, because most reaction enthalpies are sufficiently large and because we commonly require that the perturbation be a small one so that the linearization condition is valid. If AH is so small that the T-jump is ineffective, it may be possible to make use of an auxiliary reaction in the following way Suppose the reaction under study is an acid-base reaction with a small AH . We can add a buffer system having a large AH and apply the T-jump to the combined system. The T-jump will alter the Ka of the buffer reaction, resulting in a pH jump. The pH jump then acts as the forcing function on the reaction of interest. 

On the other hand, Arnett and his coworkers have reported both the enthalpies of the protonation (AHJ and the hydrogen bond (AHf) for acid-base reactions. They calculated Hj by measuring the association constants for the proton transfer (ionization) in a number of bases by using FSO3H as the acid and determined Ai/j by calorimetric measurements of the heat of dissolution of P-FC6H4OH in various hydrogen bond acceptors, including sulphoxides, in They have also tried to correlate and... 

The relative stabilities of the dioxides, sesquioxides and monoxides for first period transition metals are given in Figure 7.11(c). The stability of the higher oxidation state oxides decreases across the period. As we will discuss later, higher oxidation states can be stabilized in a ternary oxide if the second metal is a basic oxide like an alkaline earth metal. The lines in Figure 7.11(c) can in such cases be used to estimate enthalpies of formation for unstable oxidation states in order to determine the enthalpy stabilization in the acid-base reactions see below. Finally, it should be noted that the relative stability of the oxides in the higher oxidation states increases from the 3d via 4d to the 5d elements, as illustrated for the Cr, Mo and W oxides in Figure 7.11(d). 

A reaction is said to be enthalpy-driven if it involves a large, negative AH" with a smaller and usually unfavourable TAS at all accessible temperatures. In a thermochemical analysis of such a reaction, and in comparing several such related reactions, only the enthalpy terms need normally be considered. Most redox reactions and acid-base reactions come into this category. The latter term can be interpreted liberally to include many instances of complex formation, e.g. ... 

Tab. 13.4 Thermochemistry of Selected Acid/Base Reactions Deprotonation Enthalpies (kcal/mol) for Deprotonations of /PrOH with Various Organolithium Compounds and Lithium Amides...

Corrosion of refractories and ceramics involves consideration of (i) acid-base effects involved in the corrosion of refractory (ii) verification of the thermal stability of each constituent (iii) calculation of the free enthalpy of all possible reactions that might occur in the corrosion of the refractory or ceramic compound. Kinetic data are also useful in understanding the corrosion and selection of the refractory compound for a specific application. 

By definition, heat of hydration is the heat generated during setting of the cement due to hydration. In the case of CBS, however, heat of hydration may not be an appropriate term, because CBS sets by acid-base reaction and not hydration. The most appropriate term would be heat of formation, which is the net change in the enthalpy during the reaction that... 

On the basis of these parameters, it is possible to predict the enthalpies of Lewis acid/base reactions, even those reactions which might be inaccessible experimentally, with remarkable accuracy (within +0.8 kJ mol ) [216],... 

The standard enthalpies of the dissociation reactions are those determined by Antonini, et al (11) for horse hemoglobin. Their p fs are based on the assumption that the oxygen-linked groups are independent. However, pKi and p/fa in equation 14.2-1 are the usual thermodynamic p fs independent of any assumptions. Actually some of the acid dissociations are cooperative, which indicates that they are not independent. The p s used in the current calculations were obtained using... 

The enthalpy change of some reactions can be measured directly, but for those that do not go to completion (as is common in acid-base reactions), thermodynamic data from reactions that do go to completion can be combined using Hess s law to obtain the needed data. For example, the enthalpy and entropy of ionization of a weak acid, HA, can be found by measming (1) the enthalpy of reaction of HA with NaOH, (2) the enthalpy of reaction of a strong acid (such as HCl) with NaOH, and (3) the equilibrium constant for dissociation of the acid (usually determined from the titration curve). 

Often the enthalpy of reaction is obtained by measuring the equilibrium constant of an acid-base reaction over a range of temperatures. If In K is plotted versus l/T. the slope will be equal to -AH/R. Thus various experimental methods have been devised to measure the equilibrium constant by spectrophotometric methods. Any absorption which differs between one of the reactants (either acid or base) and the acid-base adduct is a potential source of information on the magnitude of the equilibrium constant since it gives the concentration of two of the three species involved in the equilibrium directly and the third indirectly from a knowledge of the stoichiometry of the reaction. For example, consider the extensively studied reaction between organic carbonyl compounds and iodine. The infrared carbonyl absorption is shifted in frequency in the adduct with respect to the free carbonyl compound. Thus the equilibrium mixture exhibits two absorption bands in the carbonyl region of the spectrum (Fig. 9.1) and the relative concentrations of the free carbonyl and the adduct can be obtained.3- Alternatively, one can observe the absorption of the iodine molecule, U, in... 

The solubility of carbon dioxide in aqueous and non-aqueous solutions depends on its partial pressure (via Henry s law), on temperature (according to its enthalpy of solution) and on acid-base reactions within the solution. In aqueous solutions, the equilibria forming HCO3 and CO3 depend on pH and ionic strength the presence of metal ions which form insoluble carbonates may also be a factor. Some speculation is made about reactions in nonaqueous solutions, and how thermodynamic data may be applied to reduction of CO2 to formic acid, formaldehyde, or methanol by heterogenous catalysis, photoreduction, or electrochemical reduction. 

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