Acid-base reactions defined

Two very important classes of chemical reactions are oxidation-reduction (redox) reactions and acid-base reactions, which are defined by molecules or ions accepting and donating electrons or protons, respectively. 

According to this theory, an acid is defined as a proton donor and a base as a proton acceptor (a base must have a pair of electrons available to share with the proton this is usually present as an unshared pair, but sometimes is in a 7t orbital). An acid-base reaction is simply the transfer of a proton from an acid to a base. (Protons do not exist free in solution but must be attached to an electron pair). When the acid gives up a proton, the species remaining still retains the electron pair to which the proton was formerly attached. Thus the new species, in theory at least, can reacquire a proton and is therefore a base. It is referred to as the conjugate base of the acid. All acids have a conjugate base, and all bases have a conjugate acid. All acid-base reactions fit the equation... 

Acid strength may be defined as the tendency to give up a proton and base strength as the tendency to accept a proton. Acid-base reactions occur because acids are not equally strong. If an acid, say HCI, is placed in contact with the conjugate base of a weaker acid, say acetate ion, the proton will be transferred because the HCI has a greater tendency to lose its proton than acetic acid. That is, the equilibrium... 

Proton transfers between oxygen and nitrogen acids and bases are usually extremely fast. In the thermodynamically favored direction, they are generally diffusion controlled. In fact, a normal acid is defined as one whose proton-transfer reactions are completely diffusion controlled, except when the conjugate acid of the base to which the proton is transferred has a pA value very close (differs by g2 pA units) to that of the acid. The normal acid-base reaction mechanism consists of three steps ... 

Boyle (1661) attempted to provide a more definite concept and attributed the sour taste of acids to sharp-edged acid particles. Lemery, another supporter of the corpuscular theory of chemistry, had similar views and considered that acid-base reactions were the result of the penetration of sharp acid particles into porous bases (Walden, 1929 Finston Rychtman, 1982). However, the first widely accepted theory was that of Lavoisier who in 1 111 pronounced that oxygen was the universal acidifying principle (Crosland, 1973 Walden, 1929 Day Selbin, 1969 Finston Rychtman, 1982). An acid was defined as a compound of oxygen with a non-metal. 

The limitation of using such a model is the assumption that the diffusional boundary layer, as defined by the effective diffusivity, is the same for both the solute and the micelle [45], This is a good approximation when the diffusivities of all species are similar. However, if the micelle is much larger than the free solute, then the difference between the diffusional boundary layer of the two species, as defined by Eq. (24), is significant since 8 is directly proportional to the diffusion coefficient. If known, the thickness of the diffusional boundary layer for each species can be included directly in the definition of the effective diffusivity. This approach is similar to the reaction plane model which has been used to describe acid-base reactions. 

The equivalent is defined in terms of a chemical reaction. It is defined in one of two different ways, depending on whether an oxidation-reduction reaction or an acid-base reaction is under discussion. For an oxidation-reduction reaction, an equivalent is the quantity of a substance that will react with or yield 1 mol of electrons. For an acid-base reaction, an equivalent is the quantity of a substance that will react with or yield 1 mol of hydrogen ions or hydroxide ions. Note that the equivalent is defined in terms of a reaction, not merely in terms of a formula. Thus, the same mass of the same compound undergoing different reactions can correspond to different numbers of equivalents. The ability to determine the number of equivalents per mole is the key to calculations in this chapter. 

In the Bronsted theory, an acid is defined as a substance that donates a proton to another substance. In this sense, a proton is a hydrogen atom that has lost its electron it has nothing to do with the protons in the nuclei of other atoms. (The nuclei of 2H are also considered protons they are also hydrogen ions.) A base is a substance that accepts a proton from another substance. The reaction of an acid and a base produces another acid and base. The following reaction is thus an acid-base reaction according to Bronsted ... 

To overcome this difficulty, we can introduce a new variable defined in terms of a linear combination of A and B such that the chemical source term for ( is null. Consider an acid-base reaction of the form... 

The mixture fraction as defined above describes turbulent mixing in the reactor and does not depend on the chemistry. However, by comparing Eqs. (45) and (46), we can note that they have exactly the same form. Thus, for the acid-base reaction, the mixture fraction is related to rA—B by... 

The Lewis acid-base reaction leading to complex formation910 has been recently11 considered in relation to the role of solvation effects. Many scales of thermodynamic parameters have been suggested. The concept of donor number (DN) was proposed by Gutmann12, and defined as the AH (kcalmol-1) for the interaction of a basic solvent with SbCL in 1,2-dichloromethane at room temperature ... 

The limitations of the Arrhenius theory of acids and bases are overcome by a more general theory, called the Bronsted-Lowry theory. This theory was proposed independently, in 1923, by Johannes Br0nsted, a Danish chemist, and Thomas Lowry, an English chemist. It recognizes an acid-base reaction as a chemical equilibrium, having both a forward reaction and a reverse reaction that involve the transfer of a proton. The Bronsted-Lowry theory defines acids and bases as follows ... 

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 ... 

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. 

S. A. Arrhenius defined an acid as any hydrogen-containing species able to release protons and a base as any species able to form hydroxide ions [71]. The aqueous acid-base reaction is the reaction between hydrogen ions and hydroxide ions with water formation. The ions accompanying the hydrogen and hydroxide ions form a salt, so the overall Arrhenius acid-base reaction can be written ... 

For the treatment of acid-base reactions, especially neutralizations, it is often convenient to define an add as a species that increases the concentration of the... 

Proton transfer reactions can be regarded as a special type of Lewis acid-base reaction. Recall from Sections 2.12 and 2.13 that a Lewis acid is defined as an electron pair acceptor and a Lewis base is defined as an electron pair donor. A proton (H+) is an electron pair acceptor and therefore a Lewis acid, because it can attach to a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. A Bronsted base is a special kind of Lewis base, one that can use a lone pair to bond to a proton. 

According to the Arrhenius theory, acids (HA) are substances that dissociate in water to produce H + (aq). Bases (MOH) are substances that dissociate to yield OH aq). The more general Bransted-Lowry theory defines an acid as a proton donor, a base as a proton acceptor, and an acid-base reaction as a proton-transfer reaction. Examples of Bronsted-Lowry acids are HC1, NH4+, and HSO4- examples of Bronsted-Lowry bases are OH-, F-, and NH3. 

The theory of IEs was formulated by Bigeleisen and Mayer.9 The IE on the acid-base reaction of Equation (1) is defined as the ratio of its acidity constant KA to the acidity constant of the isotopic reaction, Equation (2). The ratio KJ KA is then the equilibrium constant XEIE for the exchange reaction of Equation (3). That equilibrium constant may be expressed in terms of the partition function Q of each of the species, as given in Equation (4), which ignores symmetry numbers. 

All the reactions discussed in the previous section could be described as acid/base phenomena, defining acids and bases quite liberally. The importance of ionic equilibria in aqueous solution was recognised in the 1880s by Arrhenius, who proposed that acids were sources of H+(aq) while bases were sources of OH-(aq), and it was soon realised that this definition was closely related to the self-dissociation of water ... 

Acid/base reactions can be defined in non-protonic solvents. A popular solvent in this category is POCl3, which is believed - although there has been some controversy over this - to undergo self-ionisation ... 

On the basis of these properties, the three approaches summarized in Figure 3 have been elaborated specifically for the introduction of photoactive M—— M cores into layered phosphate host structures. These are (1) the direct intercalation of solvated M—— M cores into layered phosphates wherein the phosphate groups of the layers form the ligation sphere for the bimetallic core (2) acid-base reaction of specially functionalized ligands on the bimetallic core with protons from the layers and (3) replacement of the phosphate groups with functionalized phosphonates that offer well-defined coordination sites for the M—M core. We now discuss each of these methodologies. 

Usanovic acid-base theory — A general theory of acids and bases taking into consideration electron exchange processes (redox reactions) was proposed in 1939 by Usanovic. His definition is a symmetrical one and includes all concepts discussed above, i.e., an acid is defined as a substance which is able to liberate protons or other cations (cation donator) or to take up anions or electrons (anion acceptor, electron acceptor). A base is defined as a substance which is able to release anions or electrons (anion donator, electron donator) or to take up protons or other cations (cation acceptor). According to that theory, all chemical reactions (excluding reactions between radicals resulting in covalent bindings) can be considered as acid-base reactions. 

The Br0nsted-Lowry definition of acids and bases does not replace the Arrhenius definition, but extends it. The Bronsted-Lowry definition of acids and bases requires you to take a closer look at the reactants and products of an acid-base reaction. In this case, acids and bases are not easily defined as having hydronium and hydroxide ions. Instead, you are asked to look and see which substance has lost a proton and which has gained the very same proton that was lost. 

In Chapter 4, the ionization constant (i.e., the reaction constant of dissolution) for weak acids and acid phosphates was defined. The concept of the ionization constant is very general and useful while discussing dissolution of sparsely soluble oxides in acid-base reactions. We assign the symbol K for this constant. 

So far we have considered solids dissolving in aqueous solutions. Now we will consider the reverse process—the formation of precipitates. When solutions are mixed, various reactions can occur. We have already considered acid-base reactions in some detail. In this section we show how to predict whether a precipitate will form when two solutions are mixed. We will use the ion product, which is defined just like the Ksp expression for a given solid except that initial concentrations are used instead of equilibrium concentrations. For solid CaF2 the expression for the ion product (Q) is written... 

Equivalent mass based on acid-base reactions. In this method, the equivalent mass is defined as (Snoeyink and Jenkins, 1980) ... 

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