Molecular definitions of acids and bases

We have just seen some of the properties of acids and bases. In this section we examine two different models that explain the molecular basis for acid and base behavior the Arrhenius model and the Bronsted-Lowry model. The Arrhenius model, which was developed earlier, is more limited in its scope. The Bronsted-Lowry model was developed later and is more broadly applicable. 

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In the 1880s, the Swedish chemist Svante Arrhenius proposed the following molecular definitions of acids and bases. 

Such considerations led to a more general definition of acids and bases, which was proposed independently by J. N. Brpnsted and T. M. Lowry in 1923. They defined acid as any substance (in either the molecular or the ionic state) which donates protons (H+), and a base as any substance (molecular or ionic) which accepts protons. Denoting the acid by A and the base by B, the acid-base equilibrium can be expressed as... 

Some of the most important processes in chemical and biological SYSTEMS ARE ACID-BASE REACTIONS IN AQUEOUS SOLUTIONS. In THIS FIRST OF TWO CHAPTERS ON THE PROPERTIES OF ACIDS AND BASES, WE WILL STUDY THE DEFINITIONS OF ACIDS AND BASES, THE pH SCALE, THE IONIZATION OF WEAK ACIDS AND WEAK BASES, AND THE RELATIONSHIP BETWEEN ACID STRENGTH AND MOLECULAR STRUCTURE. WE WILL ALSO LOOK AT OXIDES THAT CAN ACT AS ACIDS OR BASES. 

The lower the value of this constant, the larger the deferences in acidity indices (pH) between the standard solutions of strong acids and bases, that results in a wider acid-base range for the solvent. This refers not only to the acid-base equilibria in aqueous solutions but also applies to any donor-acceptor interaction in molecular solvents which are prone to heterolytic dissociation with the formation of acidic and basic particles, as provided by an appropriate definition of acids and bases. It follows from equations (1.1.3) and (1.1.4) that the Arrhenius definition can only be used for the description of acid-base interactions in aqueous solutions, since the reaction between the acid of solvent and the base of solvent can result in the formation only of the solvent molecules. In the case considered, this solvent is water. 

Some acids (e.g., acetic and citric) have a sour taste. In fact, sourness had been a defining property since the 17 " century an acid was any substance that had a sour taste reacted with active metals, such as aluminum and zinc, to produce hydrogen gas and turned certain organic compounds characteristic colors. (We discuss indicators later and in Chapter 19.) A base was any substance that had a bitter taste and slippery feel and turned the same organic compounds different characteristic colors. (Please remember NEVER to taste or touch laboratory chemicals instead, try some acetic acid in the form of vinegar on your next salad.) Moreover, it was known that when acids and bases react, each cancels the properties of the other in a process called neutralization. But definitions in science evolve because, as descriptions become too limited, they must be replaced by broader ones. Although the early definitions of acids and bases described distinctive properties, they inevitably gave way to definitions based on molecular behavior. 

The earliest and simplest definition of acids and bases that reflects their molecular nature was suggested by Svante Arrhenius, whose work on reaction rate we... 

As KjO dissolves in water, the oxide ion reacts with water molecules to form hydroxide ions. Write the molecular and net ionic equations for this reaction. Based on the definitions of acid and base, what ion is the base in this reaction What is the add What is the spectator ion in the reaction ... 

One of the most important types of chemical reactions is the acid-base reaction. However, the definition of which species constitute acids or bases has evolved over the years as the breadth of known chemical reactions has continued to proliferate. For this reason, it is necessary to first introduce the more common historical definitions of acids and bases so that we may better understand how they each fit into the lexicon of chemical reactivity. Just as there were several complimentary models to facilitate our understanding of chemical bonding, so too there are numerous definitions of what it means to be an acid or a base. Which of these definitions we choose will depend on the complexity of the specific acid-base interaction at hand. Ultimately, however, every acid-base reaction entails a change in the way that the valence electrons are arranged in the atomic or molecular orbitals of the participating species. Therefore, the most modem definition of acid-base chemistry builds upon the MO concepts developed in previous chapters and provides the context for a natural continuation of that discussion. 

Some of the most important processes in chemical and biological systems are acid-base reactions in aqueous solutions. In this first of two chapters on the properties of acids and bases, we will study the definitions of acids and bases, the pH scale, the ionization of weak acids and weak bases, and the relationship between acid strength and molecular structure. We will also look at oxides that can act as acids and bases. 

The earliest definition that highlighted the molecular nature of acids and bases is the Arrhenius acid-base definition, which classifies these substances in terms of their... 

Arrhenius and Ostwald played very important roles in the early studies on add-base catalysis, one century ago. Arrhenius contributed to the definition of acids and bases, and established the dependence between the rate constants and the temperature. Additionally, he also formulated an electrolytic theory of dissociation that ultimately led to him receiving the 1903 Nobel Prize in Chemistry. Ostwald proposed useful definitions of catalysis and classifications of catalysts, but he was unable to develop a satisfactory theory of these effects. This is not surprising, in view of the very limited knowledge of the mechanisms of catalysis at his time, and of the lack of understanding of how molecular properties can influence the rates of reactions. Nevertheless, his seminal work on catalysis was rewarded by him receiving the 1909 Nobel Prize in Chemistry. 

The Arrhenius definition was an important contribution to understanding many acids and bases, but it does not explain why a compound such as ammonia (NH3) neutralizes acids, even though it has no hydroxide ion in its molecular formula. In Section 1-13 we discuss a more versatile theory of acids and bases that will include ammonia and a wider variety of organic acids and bases. 

Researchers have devised numerous extraction and fractionation schemes to deal with the heterogeneous nature of humic substances. Traditionally, the operational definition of humic substances as used by the International Humic Substances Society (Hayes et al., 1989) is based on the solubility in a series of acids and bases. In this scheme, humic substances are classified into three chemical groupings (1) fulvic acid, soluble in both alkali and acid solutions, has the lowest molecular weight and is generally considered the most susceptible to microbial degradation (2) humic acid, soluble in alkali but not in acid, is intermediate in molecular weight and decomposability and (3) humin, insoluble in both alkali and acid solutions, is the most... 

Molecular Definitions of Acids 14.8 Water Acid and Base in Combustion 514... 

Another significant difference between definitions is that Bronsted-Lowry acids and bases need not be molecular substances. There are a variety of reactions in which ions donate or accept protons. In the sample below, note how the cyanide ion (CN-) acts as a base by accepting a proton and the bicarbonate ion (HC03 ) acts as an acid by donating a proton ... 

To go into this idea quantitatively, we need definitions of hardness and softness, and a rank order for acids and bases on a scale of hardness. This has been done in two ways one based on molecular orbital theory, and the other on density functional theory. 

The first point to be made concerning acids and bases is that so-called acid-base theories are in reality definitions of what an acid or base is they are not theories in the sense of valence bond theory or molecular orbital theory. In a very real sense, we can make an acid be anything we wish the differences between the various acid-base concepts are not concerned with which is right but which is most convenient to use in a particular situation. All of the current definitions of acid-base behavior are compatible with each other. In fact, one of the objects in the following presentation of many different definitions is to emphasize their basic parallelism and hence to direct the students toward a cosmopolitan attitude toward acids and bases which will stand them in good stead in dealing with various chemical situations, whether they be in aqueous solutions of ions, organic reactions, nonaqueotis titrations, or other situations. 

Hard and soft acid and base theory gives access to an early part of the slope in a reaction profile like that in Fig. 3.3, just as perturbation molecular orbital theory does. Using the definitions of absolute electronegativity and absolute hardness derived in Equations 3.5 and 3.6, the (fractional) number of electrons AN transferred is given by Equation 3.14. 

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