Aqueous solution acid-base reactions

Much important chemistry, including most of the chemistry of the natural world, occurs in aqueous solution. We have already introduced one very significant class of aqueous equilibria, acid-base reactions. In this chapter we consider more applications of acid-base chemistry and introduce two additional types of aqueous equilibria, those involving the solubility of salts and the formation of complex ions. 

Reviewed herein are some of the fundamental concepts associated with chemical equilibrium, chemical thermodynamics, chemical kinetics, aqueous solutions, acid-base chemistry, oxidation-reduction reactions and photochemistry, all of which are essential to an understanding of atmospheric chemistry. The approach is primarily from the macroscopic viewpoint, which provides the tools needed by the pragmatist. A deeper understanding requires extensive treatment of ihe electronic structure of matter and chemical bonding, topics that are beyond the scope of this introductory text. This book can be used for either self-instruction, or as the basis for a short introductory class... 

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 ... 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

The acidic and basic properties of aqueous solutions are dependent on an equilibrium that involves the solvent, water. The reaction involved can be regarded as a Bransted-Lowry acid-base reaction in which the H20 molecule shows its amphiprotic nature ... 

Now by taking one more step we can view acid-base reaction in a broader sense. Suppose we mix aqueous solutions of ammonium chloride, NH4CI, and sodium acetate, CH3COONa. A sniff indicates ammonia has been formed. Reaction occurs,... 

When we mix two solutions the result is often simply a new solution that contains both solutes. However, in some cases the solutes can react with each other. For instance, when we mix a colorless aqueous solution of silver nitrate with a clear yellow aqueous solution of potassium chromate, a red solid forms, indicating that a chemical reaction has occurred (Fig. 1.1). This section and the next two introduce three of the main types of chemical reactions precipitation reactions, acid-base reactions, and redox reactions, all of which are discussed in more depth in later chapters. (The fourth type of reaction discussed in this text, Lewis acid-base reactions, is introduced in Section 10.2.) Because many chemical reactions take place in solution, particularly in water, in this section we begin by considering the nature of aqueous solutions. 

FIGURE 15.4 When aqueous ammonia is added to a copper(ll) sulfate solution, first a light-blue precipitate ot Cu(OH)2 forms (the cloudy region at the top, which appears dark because it is backlit). The precipitate disappears when more ammonia is added to form the dark blue complex ru(NH))4 T by a Lewis acid-base reaction. 

Many of the d-block elements form characteristically colored solutions in water. For example, although solid copper(II) chloride is brown and copper(II) bromide is black, their aqueous solutions are both light blue. The blue color is due to the hydrated copper(II) ions, [Cu(H20)fJ2+, that form when the solids dissolve. As the formula suggests, these hydrated ions have a specific composition they also have definite shapes and properties. They can be regarded as the outcome of a reaction in which the water molecules act as Lewis bases (electron pair donors, Section 10.2) and the Cu2+ ion acts as a Lewis acid (an electron pair acceptor). This type of Lewis acid-base reaction is characteristic of many cations of d-block elements. 

Using Environmental Examples to Teach About Acids. Acid-base reactions are usually presented to secondary students as examples of aqueous equilibrium (2). In their study of acids and bases, students are expected to master the characteristic properties and reactions. They are taught to test the acidity of solutions, identify familiar acids and label them as strong or weak. The ionic dissociation of water, the pH scale and some common reactions of acids are also included in high school chemistry. All of these topics may be illustrated with examples related to acid deposition (5). A lesson plan is presented in Table I. 

The quantitative aspects of acid-base chemistry obey the principles Introduced earlier in this chapter. The common acid-base reactions that are important in general chemistry take place in aqueous solution, so acid-base stoichiometry uses molarities and volumes extensively. Example Illustrates the essential features of aqueous acid-base stoichiometry. 

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. 

Equilibria that occur in aqueous solution are of particular interest, because water is the medium of life and a major influence on the geography of our planet. Many substances dissolve in water, and the solutes in an aqueous solution may participate in a number of different types of equilibria. Solubility itself is one important type of equilibrium, as we describe in Chapter 18. Acid-base reactions, considered in detail in Chapter 17, are another. To conclude this chapter, we describe how to determine which equilibria are most important in any particular aqueous solution. 

Thus, the relationship between acid and base is a reciprocal one and an acid-base reaction involves the transfer of a proton. This concept is not restricted to aqueous solutions and it discards Arrhenius prerequisite of ionization. 

This concept covers most situations in the theory of AB cements. Cements based on aqueous solutions of phosphoric acid and poly(acrylic acid), and non-aqueous cements based on eugenol, alike fall within this definition. However, the theory does not, unfortunately, recognize salt formation as a criterion of an acid-base reaction, and the matrices of AB cements are conveniently described as salts. It is also uncertain whether it covers the metal oxide/metal halide or sulphate cements. Bare cations are not recognized as acids in the Bronsted-Lowry theory, but hydrated... 

The phosphate bonded cements described in this chapter are the products of the simple acid-base reaction between an aqueous solution of orthophosphoric acid and a basic oxide or silicate. Such reactions take place at room temperature. Excluded from this chapter are the cementitious substances that are formed by the heat treatment of aqueous solutions of acid metal phosphates. 

Oxysalt bonded cements are formed by acid-base reactions between a metal oxide in powdered solid form and aqueous solutions of metal chloride or sulphate. These reactions typically give rise to non-homo-geneous materials containing a number of phases, some of which are crystalline and have been well-characterized by the technique of X-ray diffraction. The structures of the components of these cements and the phase relationships which exist between them are complex. However, as will be described in the succeeding parts of this chapter, in many cases there is enough knowledge about these cements to enable their properties and limitations to be generally understood. 

The reversibility of reactions is another important characteristic in assessing the fate of deep-well-injected wastes. Depending on environmental conditions, reversible reactions readily proceed in either or both directions. Most acid-base reactions exemplify reversible processes. In aqueous solutions, relatively minor changes in such factors as pH or concentration can change the direction of these reactions. Irreversible reactions, typified by hydrolysis, have a strong tendency to go in one direction only. 

In Chapter 4, we introduced the concept of acids and bases. Acids and bases may be strong or weak. Strong acids completely dissociate in water and weak acids only partially dissociate. For example, consider two acids HC1 (strong) and CH3COOH (weak). If we add each to water to form aqueous solutions, the following reactions take place ... 

In this chapter, you will continue your study of acid-base reactions. You will find out how ions in aqueous solution can act as acids or bases. Then, by applying equilibrium concepts to ions in solution, you will be able to predict the solubility of ionic compounds in water and the formation of a precipitate. 

The acidic or basic property of an aqueous solution of a salt results from reactions between water and the dissociated ions of the salt. Some ions do not react with water. They are neutral in solution. Ions that do react with water produce a solution with an excess of HsO iaq) or OH (aq). The extent of the reaction determines the pH of the solution. As you will see, the reaction between an ion and water is really just another acid-base reaction. 

Changes in the pH of subsurface aqueous solutions may lead to an apparent increase or decrease in the solubility of organic contaminants. The pH effect depends on the structure of the contaminant. If the contaminant is sensitive to acid-base reactions, then pH is the governing factor in defining the aqueous solubility. The ionized form of a contaminant has a much higher solubility than the neutral form. However, the apparent solubility comprises both the ionized and the neutral forms, even though the intrinsic solubility of the neutral form is not affected. 

Cesium hydroxide is the strongest base known. Its aqueous solution undergoes neutralization reactions with acids. Precipitation reactions don t yield crystaUine cesium salts because of their high solubility. 

In this chapter, we will focus on acid-base reactions in aqueous solutions. Such solutions play important roles in our daily lives. Vinegar, oranges, apples, grapes, and lemons are some familiar acid-containing substances, and most household cleaning products are base-containing substances. 

When strong bases neutralize strong acids in solutions that have molar concentrations of 1 mol dm-3, the enthalpy of the reaction is observed to be -55.83 kJ mol - irrespective of the counter ions (e.g. the chloride ion derivable from HC1 and the sodium ion contained in NaOH) present. For example, when a standard solution (1 mol dm-3) of hydrochloric acid is neutralized by a standard solution (1 mol dm-3) of sodium hydroxide, the change in enthalpy of the reaction is -55.83 kJ mol-1. Because the strong acid HCI and the strong base NaOH are 100% dissociated in aqueous solution, theucutruli/atiun reaction may be written as ... 

This book was written to provide readers with some knowledge of electrochemistry in non-aqueous solutions, from its fundamentals to the latest developments, including the current situation concerning hazardous solvents. The book is divided into two parts. Part I (Chapters 1 to 4) contains a discussion of solvent properties and then deals with solvent effects on chemical processes such as ion solvation, ion complexation, electrolyte dissociation, acid-base reactions and redox reactions. Such solvent effects are of fundamental importance in understanding chem-... 

This chapter discusses acid-base reactions in lion-aqueous solvents, with particular emphasis on how they differ from those in aqueous solutions. The problem of pH in non-aqueous solutions is also discussed. The Bmisted acid-base concept is adopted, i.e. an acid is a proton donor and a base is a proton acceptor. The relation between an acid A and its conjugate base B is expressed by ... 

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