Acid-base equilibria electron-pair acceptor/donor

Br0nsted-Lowery acids are H+ donors and bases are H+ acceptors. Strong acids dissociate completely in water. Weak acids only partially dissociate, establishing an equilibrium system. Weak acid and base dissociation constants (Ka and Kb) describe these equilibrium systems. Water is amphoteric, acting as both an acid or a base. We describe water s equilibrium by the Kw expression. A pH value is a way of representing a solution s acidity. Some salts and oxides have acid-base properties. A Lewis acid is an electron pair acceptor while a Lewis base is an electron pair donor. 

Lewis Theory of Acids and Bases. According to Lewis, acids are electron-pair acceptors (EPA) and bases electron-pair donors (EPD) connected through the equilibrium (fig 3.2). 

The solubility product is the equilibrium constant for the dissolution of a solid salt into its constituent ions in aqueous solution. The common ion effect is the observation that, if one of the ions of that salt is already present in the solution, the solubility of a salt is decreased. Sometimes, we can selectively precipitate one ion from a solution containing other ions by adding a suitable counterion. At high concentration of ligand, a precipitated metal ion may redissolve by forming soluble complex ions. In a metal-ion complex, the metal is a Lewis acid (electron pair acceptor) and the ligand is a Lewis base (electron pair donor). 

Thus, water functions both as an acid (donor of or proton) and as a base (acceptor of H+ or proton). This description of an acid and a base follows from the Bronsted-Lowry theory. According to the Lewis theory, acids are electron pair acceptors and bases are electron pair donors. The equilibrium constant, K, for the dissociation reaction in Equation (1.1) is... 

The choice of acid or base for solvent is simplified appreciably for melts containing complex ions (as a rule, they are anions), which are prone to the acid-base dissociation. Dissociation of this ion is assumed as the intrinsic acid-base equilibrium of a melt of such kind. In this case, the simpler eliminated anion will be considered as the base of the solvent and the coordinationally unsaturated residue will be the acidic particle of the solvent. Naturally, the division of particles formed by the auto-dissociation into acids and bases is made on the basis of the Lewis definition [13] an acid is the acceptor of an electron pair and a base is the donor of this electron pair. Ionic melts based on complex halides of gallium(III) [28], aluminium(III) [29] and boron(III) [30,31] may serve as examples of successful application of the above approach. The electron-deficient covalent halide (e.g. A1C13, BF3) in these melts is the solvent acid, and the corresponding halide ion is the base of the solvents ... 

Nucleophilicity and electrophilicity are closely related to Lewis basicity and acidity, respectively. Nucleophiles are Lewis bases (electron-pair donors) and electrophiles are Lewis acids (electron-pair acceptors). Now, as discussed previously, nucleophilicity is measured in terms of the rate of a nucleophilic attack, so it s a kinetic concept. Basicity, on the other hand, is measured in terms of the equilibrium constant for protonation (or for association with some Lewis acid), so it is a thermodynamic concept. Another difference is that. 

First, add unshared pairs of electrons on the reacting atoms to give each a complete octet. Then identify the Lewis base (the electron-pair donor) and the Lewis acid (the electron-pair acceptor). The position of equilibrium lies on the side of the weaker acid and weaker base. 

A Lewis base transfers an electron pair to a Lewis acid. A Bronsted acid transfers a proton to a Bnansted base. These exist in conjugate pairs at equilibrium. In an Arrhenius base, the proton acceptor (electron pair donor) is OH-. All Arrhenius acids/bases are Bronsted acids/bases and all Bransted acids/bases are Lewis acids/bases. Each definition contains a subset of the one that comes after it. 

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