Acid base concept

The acid-base concept of Lux and Flood/ acid + = base 

2AlCl4- Al2Cl7- +Clin which Al2Cl7 is a strong acid and Cl is a strong base. The basicity is usually expressed as pC,  

in the modified Lewis (or Lux-Flood) concept, pure alkali halides represent the highest degree of basicity as the solvent composition changes from alkali halide-rich to alkali halide-deficient melts, the solvent becomes acidic. Acid-base properties of molten halides may be used to explain stabilization of unusually low (or high) oxidation states, the differences in stability of the same oxidation state in related melts, and the effects on coordination observed spectrally for certain metal ions. Or, restating the idea in other terms, the redox potentials depend on melt basicity. Thus, the systematic variation of melt composition is a useful technique in the arsenal of the molten salt electrochemist who is interested in the chemistry of solute species in molten salt solvents. In this respect, it is important to note that variation of temperature may be used to serve the same purpose for example, it has been shown that in neutral chloro-aluminates C1- decreases with temperature. 

Much attention, particularly among the Russian workers, has been given to the applicability of the Heyrovsky-Ilkovic versus the Kolthoff-Lingane equation for electrode reactions involving metal deposition. In practice, one plots the electrode potential versus log[(/i — /)//] or log(/ — i) and examines the linearity of the plot and also the value of the slope (theoretically 2.3RTjnF). It is expected that the Heyrovsky-Ilkovic equation should be applicable when alloy formation with the electrode material takes place and the metal formed diffuses away from the surface so that its surface activity is a function of the current density. Alloying and diffusion in the electrode will be functions of the metal deposited, electrode material, temperature, and the rate of deposition (current density) therefore, comparison is difficult or meaningless if several of these variables are varied simultaneously. 

Both linear sweep voltammetry (oscillographic polarography, stationary electrode polarography, chronoamperometry with linear sweep) and chrono-potentiometry have been extensively applied for studies in molten salts. The advantages of linear sweep voltammetry include (1) extensively developed theory enabling the experimentalist to interpret the mechanisms of relatively complex electrode reactions (2) well-defined mass transfer conditions, particularly when faster scan rates ( 1 V/sec) are employed (3) the decrease of the faradaic charge with the square root of the scan rate and the resulting decrease of any modifications of the solid electrode caused by the faradaic process. Chronopotentiometry, 29) related electroanalytical 

Bremsted-Lowry In 1923 J. N. Brpnsted and T. M. Lowry independently1 suggested that acids be 

Def nition defined as proton2 donors and bases as proton acceptors. For aqueous solutions the 

1 Brpnsied, J. N. Reel. Trav. Chim. Pays-Bas 1923,42.718-728 Lowry. T. M. Chem. Ind. (London1 1923, 42, 43. 

2 As this book was going to press the International Union of Pure and Applied Chemistry recommended that the word proton be used only when the H isotope was intended, and that the more general hydron be used everywhere else, as in hydron donor. See Appendix 1. Section 8. We have not attempted at the last minute to change all of these protons to hydrons. Like the SI system or units, this change, if accepted by the world s chemists, will lake some time, and the term proton donor wilt not soon disappear. 

Br0nsted-Lowry definition does not differ appreciably from the Arrhenius definition of hydrogen ions (acids) and hydroxide ions (bases)  

The usefulness of the Brpnsted-Lowry definition lies in its ability to handle any protonic solvent such as liquid ammonia or sulfuric acid  

The cement-forming reaction is a special case of an acid-base reaction so that concepts of acid, base and salt are central to the topic. In AB cement theory, we are concerned with the nature of the acid-base reaction and how the acidity and basicity of the reactants are affected by their constitution. Thus, it is appropriate at this stage to discuss the various definitions and theories available. 

Although acids and bases have been recognized since antiquity, our concepts of them are still the subject of debate and development (Walden, 1929 Hall, 1940 Bell, 1947, 1973 Luder, 1948 KolthofF, 1944 Bjerrum, 1951 Day Selbin, 1969 Jensen, 1978 Finston Rychtman, 1982). The history of these concepts is a long one and can be seen as a prolonged and continuous refinement of inexact and commonsense notions into precise scientific theories. It has been a long and difficult journey and one that is by no means ended. 

There are various definitions of acids and bases, and in discussing them it should be emphasized that the question is not one of validity but one of utility. Indeed, the problem of validity does not arise because of the fundamental nature of a definition. The problem is entirely one of choosing a definition which is of greatest use in the study of a particular field of acid-base chemistry. One point that needs to be borne in mind is that a concept of acids and bases is required that is neither too general nor too restrictive for the particular field of study. 

These reactions of HCI and NH3, in benzene and in the gas phase, are similar to the reaction in aqueous solution but cannot be explained by the Arrhenius concept. Broader acid-base concepts are needed. 

In the first part of this chapter, we will discuss the Arrhenius, the Bronsted-Lowry, and the Lewis concepts of acids and bases.The Bronsted-Lowry and Lewis concepts apply to nonaqueous as well as aqueous solutions and also enlarge on the Arrhenius concept in other ways.This chapter expands on what you learned in Chapter 4 about acids and bases. 

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R. G. Pearson, J. Am. Chem. Soc. 85 3533 (1963) T. L. Ho, Hard and Soft Acids and Bases in Organic Chemistry, Academic Press, New York, 1977 W. B. Jensen, The Lewis Acid-Base Concept, Wiley-Interscience, New York, 1980, Chapter 8. 

The strength of the complexation is a function of both the donor atom and the metal ion. The solvent medium is also an important factor because solvent molecules that are potential electron donors can compete for the Lewis acid. Qualitative predictions about the strength of donor-acceptor complexation can be made on the basis of the hard-soft-acid-base concept (see Section 1.2.3). The better matched the donor and acceptor, the stronger is the complexation. Scheme 4.3 gives an ordering of hardness and softness for some neutral and ionic Lewis acids and bases. 

The subject is also closely related to fuel-ash corrosion which in most cases is caused by a layer of fused salts such as sulphates and chlorides Attention has been focused on the electrochemistry of this type of corrosion and the relevant thermodynamic data summarised in the form of diagrams . Fluxing and descaling reactions also resemble in some respects reactions occurring during the corrosion of metals in fused salts. A review of some of the more basic concepts underlying corrosion by fused salts (such as acid-base concepts and corrosion diagrams) has appeared. ... 

We see in Table 11-IV that the equilibrium view of acid strengths suggests that we regard water itself as a weak acid. It can release hydrogen ions and the extent to which it does so is indicated in its equilibrium constant, just as for the other acids. We shall see that this type of comparison, stimulated by our equilibrium considerations, leads us to a valuable generalization of the acid-base concept. 

The dissolution of A1C13 in the NaAlCl4 melt makes the melt acidic. The acidic-base concept has been discussed by Blander et al. [40]. An acidic melt influences the solubility of the nickel chloride in the ZEBRA cell the solubility of the nickel chloride increases. 

For a monograph on Lewis acid-base theory, see Jensen, W.B. The Lewis Acid-Base Concept Wiley NY, 1980. For a discussion of the definitions of Lewis acid and base, see Jensen, W.B. Chem. Rev, 1978, 78, 1. 

W. B. Jensen, The Lewis Acid-Base Concepts, An Overview, John Wiley Sons, Hoboken, NJ, 1980. 

C21-0089. The fluorides BF3, AIF3, SiFq, and PF5 are Lewis acids. They all form very stable fluoroanions when treated with lithium fluoride. In contrast, three other fluorides, CF4, NF3, and SFg, do not react with lithium fluoride. Use Lewis acid-base concepts to explain this behavior. 

This theory was a milestone in the development of acid-base concepts it was the first to define acids and bases in terms other than that of a reaction between them and the first to give quantitative descriptions. However, the theory of Arrhenius is far more narrow than both its predecessors and its successors and, indeed, it is the most restrictive of all acid-base theories. 

The Lux-Flood theory relates to oxide melts. Geologists have often used acid-base concepts for the empirical classification of igneous silicate rocks (Read, 1948). Silica is implicitly assumed to be responsible for acidity, and the silica content of a rock is used as a measure of its acid-base balance ... 

The relative ease with which proton transfer is accomplished is responsible for the importance of the generalized acid-base concept in solution chemistry. The Br0nsted concept of acidity is most useful in this respect. Br0nsted defined an acid as a species that tends to give up a proton and a base as a species that tends to accept a proton. In this sense any proton transfer process having the general form... 

The HSAB concept is the hard-soft acid-base concept. 

Resonance such as (5.28a)-(5.28c) is inherently a quantal phenomenon, with no classical counterpart. In NBO language, each of the resonance interactions (5.28a)-(5.28c) corresponds to a donor-acceptor interaction between a nominally filled (donor Lewis-type) and unfilled (acceptor non-Lewis-type) orbital, the orbital counterpart of G. N. Lewis s general acid-base concept. As mentioned above, Lewis and Werner (among others) had well recognized the presence of such valence-like forces in the dative or coordinative binding of free molecular species. Thus, the advent of quantum mechanics and Pauling s resonance theory served to secure and justify chemical concepts that had previously been established on the basis of compelling chemical evidence. 

Classification of biologically important metal ions and ligands according to the hard-soft acid-base concept and their general characteristics... 

I. Definition of Basicity A. General Comments on the Acid-base Concept The current general definition of the acid-base concept is based on the definitions by Bronsted and Lewis. According to Bronsted s theory (1923) an acid-base interaction can be described by the general relation (1) ... 

These and similar solvents are distinguished by the fact that they themselves cannot eliminate any protons. On the other hand one is still dealing with water-like solvents in the case of alcohols, since the selfdissociation of the alcohols resembles that of water in order of magnitude (see Gurney, 1953). The ability of free electron pairs to function prompted Lewis (1923) to a more general definition of the acid-base concept. 

Jensen WB (1980) The Lewis acid-base concept. Wiley, New York... 

Four-membered rings display a variety of reactivity modes ranging from thermolysis to attack by nucleophiles and electrophiles. Almost all reactions are predictable, simply from consideration of orbital symmetry or simple acid-base concepts. 

According to the acid-base concept of Pearson, A -phosphorins can be viewed as soft bases the lone electron pair at phosphoms is much more delocalized than the lone pair at nitrogen in pyridine. Thus, such soft Lewis acids as Hg ions are more likely to react with A -phosphorins (see p. 84). 

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