Conjugate acid-base pairs concept

Along with the Bronsted-Lowery concept of a proton donor (acid) and a proton acceptor (base), arises the concept of conjugate acid-base pairs. For... 

KEY CONCEPT PROBLEM 15.3 For the following reaction in aqueous solution, identify the Bronsted-Lowry acids, bases, and conjugate acid-base pairs ... 

The ionization of the acid HA in solvent S leads to a new acid HS+ and a base A. Equation (1.1) has a very wide scope and can be very well applied to neutral and positively and negatively charged acid systems. The acid-base pair that differs only by aproton is referred to as the conjugate acid-base pair. Thus, H20 is the conjugate base of the acid H30+. An obvious consequence of the concept is that the extent to which an acid ionizes depends on the basicity of the solvent in which the ionization takes place. This shows the difficulty in establishing an absolute acidity scale. Acidity scales are energy scales, and thus they are arbitrary with respect to both the reference point and the magnitude of units chosen. 

The Bronsted-Lowry concept looks at the equilibrium reaction and ties the acid on the left to a base on the right, called a conjugate acid-base pair or, more simply, a conjugate pair. Suppose we were to consider an acid reacting with a compound in equilibrium with the acid s anion and the products, as... 

When a base accepts a proton, it becomes an acid capable of returning that proton. When an acid donates its proton, it becomes a base capable of accepting that proton back. One of the most important principles of the Br0nsted-Lowry definition is this concept of conjugate acids and bases. For example, NH4 and NH3 are a conjugate acid-base pair. NH3 is the base when it accepts a proton, it is transformed into its conjugate acid, NH4. Many compounds (water, for instance) can react either as an acid or as a base. Here are some additional examples of conjugate acid-base pairs. 

Arrhenius in 1887 was the first person to give a definition of an acid and a base. According to him, an acid is one that gives rise to excess of in aqueous solution, whereas a base gives rise to excess of OH in solution. This was modified by Bronsted-Lowry in 1923 such that a proton donor was defined as an acid and a proton acceptor as a base. They also introduced the familiar concept of the conjugate acid-base pair. The final refinement to the acid-base theory was completed by Lewis in 1923, who extended the concept that acid is an acceptor of electron pairs while base is a donor of electron pairs. 

An extension of the Brpnsted definition of acids and bases is the concept of the conjugate acid-base pair, which can be defined as an acid and its conjugate base or a base and its conjugate acid. The conjugate base of a Brpnsted acid is the species that remains when one proton has been removed from the acid. Conversely, a conjugate acid results from the addition of a proton to a Brpnsted base. 

Arrhenius concept Bronsted-Lowry model hydronium ion conjugate base conjugate acid conjugate acid-base pair acid dissociation constant Section 14.2 strong acid weak acid diprotic acid oxyacids organic acids carboxyl group monoprotic acids amphoteric substance autoionization... 

Lewis theory of acids Lewis introduced the idea that acids and bases had a reciprocal relationship and he introduced the concept of Lewis conjugate acid-base pairs. 

Arrhenius concept of acids and bases (16.1) Bronsted-Lowry model (16.1) conjugate acid (16.1) conjugate base (16.1) conjugate acid-base pair (16.1)... 

Arrhenius concept Br0nsted-Lowry model hydronium ion conjugate base conjugate acid conjugate acid-base pair acid dissociation constant... 

Br0nsted-Lowry Acid H+ donor Base H-" acceptor Conjugate acid-base pairs No concept of neutraiity... 

CONCEPT CHECK 14.3 Conjugate Acid-Base Pairs... 

The theory proposed by Brpnstcd (1923) defines an acid as any substance that can ionize in solution to give a solvated hydrogen ion (i.e. a proton stabilized by interaction with either the solvent or a substance in solution). Conversely a base is a substance which can accept a hydrogen ion. Thus an acid is a proton donor and a base a proton acceptor and the ionization process always involves the two which are known as conjugate acid-base pairs. This concept may be generalized by the equation... 

Concept Assessment 16-1. (a) is a conjugate acid/base pair HCQs can transfer a proton to a base (e.g., OH yielding C03 , and C03 can react with an acid (e.g., H3O ) to reform HCO3. (b) is not a conjugate acid/base pair S04 can be produced from HSO3" only through an oxidation process not in an acid-base reaction, (c) is not a conjugate acid/base pair it is a pair of unrelated acids. (d> yes ... 

The exact verbal definition of qualitative concepts is more often the province of philosophy than of physical science. However, the various definitions suggested for acids and bases have been closely linked with the development of physical chemistry and have often served to stimulate experimental work and to further our understanding of chemical processes, and we shall therefore devote some time to this subject. The definitions used in the remainder of this book will be those proposed by Bronsted in 1923, namely, An acid is a species having a tendency to lose a proton, and a base is a species having a tendency to add on a proton. This can be represented schematically by A B-j-H", where A and B are termed a conjugate or corresponding) acid-base pair. Before examining the consequences of this definition and its relation to more recent concepts we shall consider briefly the previous history of the terms acid and base . 

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