Acidities, Basicities and Thermochemistry

Some thermochemical aspects of reactions and products are included in the theoretical section. The new organic-cation phosphate (101) has been examined by differential scanning calorimetry (DSC).  

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Thermochemistry is the study of the energy of the entire system at every instant of a reaction. It is therefore part of the information gained when deciphering The study of the energy of chemical a complete mechanism. Before examining the mechanisms and thermochemistry of acid-base reactions, we must examine what causes chemical reactions to occur and introduce some basic energy principles that are key to chemical reactivity. 

Much of the available thermodynamic data, snch as bond dissociation energies, gas-phase acidities and basicities, and heats of formation for ions and neutral target species, has been determined nsing TCID [37, 38]. These data inclnde a variety of metals (alkalis, magnesitrm, altrmintrm, and first and second row transition metals), and many types of target molectrles. For instance, Armentront [39] stndied an abso-Inte cation affinity scale, thermochemistry of alkali-metal cation interactions with histidine,and host-gnest interactions of crown ethers with alkali ions nsing TCID. 

The first systematic measurements of the reactions of ions with molecules in the gas phase were initiated largely by workers associated with analytical mass spectrometry.4-6 It was the rapidly expanding area of ion-molecule reactions which led to the origin of Gas-Phase Ion Chemistry as a distinct field.7 The discovery that ion-molecule equilibria in the gas phase can be determined by mass spectrometric techniques8 led to an explosion of thermochemical measurements based on determination of equilibria by a variety of techniques.9 Significantly, for the first time, information could be obtained on the thermochemistry of reactions which had solution counterparts of paramount importance such as acidities and basicities. These were obtained from proton transfer equilibria such as,... 

It was suggested earlier in this section that oxoacid salts such as CaC03 could be viewed as products of reactions between basic oxides (containing O2- discrete ions) and covalent (molecular/polymeric) oxides in which oxide ions are transferred to form oxo-anions. Analysis of the thermochemistry of such reactions has led to the formulation of a numerical scale of acidity for oxides. In Table 9.1 the acidity parameter a is listed for the most important binary oxides. Highly-negative values indicate a basic oxide, while acidic oxides have positive values. 

American 243, 148 (1980) Chemische Reaktionen ohne Ldsungsmittel, Spektrmn der Wissenschaft, January 1981, p. 27ff. [116] R. W. Taft Protonic Acidities and Basicities in the Gas Phase and in Solution Substituent and Solvent Effects, Progr. Phys. Org. Chem. 14, 247 (1983). [117] C. R. Moylan and J. 1. Brauman Gas-Phase Acid-Base Chemistry, Annu. Rev. Phys. Chem. 34, 187 (1983). [118] P. Kebarle Ion Thermochemistry and Solvation from Gas-Phase Ion Equilibrium, Annu. Rev. Phys. Chem. 28, 445 (1977). [119] D. K. Bohme, E. Lee-Ruff, and L. B. Young, J. Am. Chem. Soc. 94, 5153 (1972) D. K. Bohme in P. Ausloos (ed.) Interactions between Ions and Molecules, Plenum, New York, 1974, p. 489ff. [120] M. J. Pellerite and J. I. Brauman Gas-Phase Acidities of Carbon Acids, in E. Buncel and T. Durst (eds.) Comprehensive Carbanion Chemistry, Part A, p. 55ff., Elsevier, Amsterdam, 1980. 

Through these simulations, students appreciate the energetic and kinetic aspects of chemical reactions and why the composition of the atmosphere is critical to the outcome. For example, the class simulates the effect on the yield of amino acids if the atmospheric composition was CO2 instead of CH4 or the nitrogen source was N2 instead of NH3. The simulations also allow students to investigate the consequences if the pre-biotic atmosphere contained free 02. Through these simulations, basic chemical concepts are discussed including thermodynamics, thermochemistry and bond enthalpies, kinetics, and catalysis. 

This chapter is composed of three parts. The first covers principles of thermochemistry that are fundamental to the comprehension of the mechanisms involved in spectral interpretation electronegativity, chemical bonds, acidity and basicity, inductive and mesomeric effects, the Audier-Stevenson rule, and stability rules for radicals in the gas phase. 

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