Gases acid-base reactions

A. (The gas phase estimate is about 100 picoseconds for A at 1 atm pressure.) This suggests tliat tire great majority of fast bimolecular processes, e.g., ionic associations, acid-base reactions, metal complexations and ligand-enzyme binding reactions, as well as many slower reactions that are rate limited by a transition state barrier can be conveniently studied with fast transient metliods. 

This is an acid-base reaction, in which the base is the oxide ion (p. 89) the acidic oxide SiOj displaces the weaker acidic oxide CO2 in the fused mixture. But in aqueous solution, where the 0 ion cannot function as a strong basefp. 89),carbon dioxide displaces silica, which, therefore, precipitates when the gas is passed through the aqueous silicate solution. In a fused mixture of silica and a nitrate or phosphate, the silica again displaces the weaker acidic oxides N2O5 and P4OJ0 ... 

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... 

Another way of analyzing these combinations is by type of substance. Parts a and c involve mixing of two aqueous solution, parts b and d involve adding a metal to an aqueous system, and part e is the interaction of a metal with O2 gas. When solutions mix, we look first for acid-base reactions (part c), then for formation of a precipitate (part b). When a metal contacts an aqueous system, the most likely reaction, if any, is oxidation of the metal. Any time molecular oxygen is present, we can expect oxidation to be one possibility. 

In solutions neither H+ nor e can exist in a free state they will be donated only if they are accepted within the solution, e.g., by another acceptor, which may be the solvent and thus cause solvation here the mere solvation of electrons is an exceptional case, but may occur, e.g., in liquid ammonia, where according to Kraus82 the strongly reducing alkali metals dissolve while dissociating into cations M+ and solvated electrons e, which, however, are soon converted into NH2" and H2 gas. Further, from the analogy with acid-base reactions and the definition of... 

In each case we indicate whether the compound is more soluble in acid than in water. We write the net ionic equation for the reaction in which the solid dissolves in acid. Substances are more soluble in acid if either (1) an acid-base reaction occurs [as in (b-d)] or (2) a gas is produced, since escape of the gas from the reaction mixture causes the reaction to shift to the right. 

Acidity or causticity Acid gas release (e.g., anhydrous HC1) Corrosive liquid or solid spill Uncontrolled acid/base reaction... 

For the reaction acid(g) + base(g) —> adduct (sum of gas-phase acid-base reaction and the heat of sublimation ). b No reaction observed. c No simple adduct formation. 

Proton transfer is one of the prominent representatives of an ion-molecule reaction in the gas phase. It is employed for the determination of GBs and PAs (Chap. 2.11.2) by either method the kinetic method makes use of the dissociation of proton-bound heterodimers, and the thermokinetic method determines the equilibrium constant of the acid-base reaction of gaseous ions. In general, proton transfer plays a crucial role in the formation of protonated molecules, e.g., in positive-ion chemical ionization mass spectrometry (Chap. 7). 

Redman, E.W. Morton, T.H. Product-Determining Steps in Gas-Phase Breasted Acid-Base Reactions. D rotonation of 1-Methylcyclopentyl Cation by Amine Bases. J. Am. Chem. Soc. 1986,108, 5701-5708. 

Jnst as free protons do not exist in solution in acid-base reactions, there are no free electrons in redox reactions. However it is possible to define the activity of electrons relative to a specified standard state and thereby treat electrons as discrete species in equilibrinm calcnlations in the same way as ions and molecules. The standard state of electron activity for this pnrpose is by convention defined with respect to the redox conple made by hydrogen ions and hydrogen gas ... 

The gas-solid reaction can thus be more appropriately described as a special kind of solvation rather than as a heterogeneous acid-base reaction. In the reversible supramolecular reaction the zwitterion reacts with gaseous HCOOH,... 

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. 

It can be seen from the foregoing discussion that the interpretations of the observed acidities leave something to be desired even for such a fundamental series of compounds as the simple hydrides. The matter has been reopened in recent—, years by the development of techniques for measuring acidities in the gas phase.86 The available results reemphasize the fact, already well known from previous work, that solvation factors have a profound influence on the course of acid-base reactions. But the gas-phase experiments do more than this they call into question some of the fundamental assumptions and interpretations that haVe long been used to account for observed acidities in terms of molecular structure. 

III. ACID-BASE REACTIONS A. Gas-phase Proton Affinities2... 

The systematic study of gas-phase proton exchange reactions between bases has led to the development of a new formalism describing the quantitative effects of substitution on the GBs of organic compounds200. Because of their formal simplicity and general importance, proton transfer reactions are excellent models for the study of other acid-base reactions, both in the gas phase and in solution201. 

The reaction between hydrogen chloride gas and ammonia can be described as an acid-base reaction under this theory. The hydrogen chloride molecule acts as a proton donor and the ammonia molecule acts as the proton acceptor (Figure 8.8). 

That the entropy change is unfavourable could be confidently predicted, given the presence of two moles of gas on the left-hand side. As the temperature is increased, the TAS" term becomes more important neglecting the small temperature dependence of AH° and A5°, it can be easily shown that AG° will become zero at about 850 K, at which temperature the decomposition of the complex should be complete. Such decomposition can be achieved at lower temperatures if the partial pressure of ammonia is kept low, by pumping. Most thermal decompositions-which are often the reverse of acid-base reactions (see Section 9.2) - are entropy-driven. All substances containing chemical bonds can be decomposed by heating to a sufficiently high temperature. 

In recent years the question of acidity and basicity has been reopened by the development of techniques to measure them in the gas phase(f ). The results available reemphasize the fact that solvation factors have a profound influence on the course of acid-base reactions. 

Chlorine gas is added to the water in some large commercial swimming pools to kill bacteria. However, in most home swimming pools, either solid calcium hypochlorite (Ca(OCl)2) or an aqueous solution of sodium hypochlorite (NaOCl) is used to treat the water. Both compounds dissociate in water to form the weak acid hypochlorous acid (HOC1). Hypochlorous acid is a highly effective bactericide. By contrast, the hypochlorite ion (OC1-) is not a very effective bactericide. Use the information above to answer the following questions about the acid-base reactions that take place in swimming pools. 

There is an interesting effect of pH on the solubility of acid gas in water. In a solution with a high pH (a basic solution), the solubility of the acid gas components is dramatically increased. This is due to the acid-base reactions that occur between the dissolved acid gas and the base in the original solution. 

Eluent solvents should be able to give gas phase acid-base reactions with analyte... 

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