Conjugate acid/base pair definitions

The definition of pH is pH = —log[H+] (which will be modified to include activity later). Ka is the equilibrium constant for the dissociation of an acid HA + H20 H30+ + A-. Kb is the base hydrolysis constant for the reaction B + H20 BH+ + OH. When either Ka or Kb is large, the acid or base is said to be strong otherwise, the acid or base is weak. Common strong acids and bases are listed in Table 6-2, which you should memorize. The most common weak acids are carboxylic acids (RC02H), and the most common weak bases are amines (R3N ). Carboxylate anions (RC02) are weak bases, and ammonium ions (R3NH+) are weak acids. Metal cations also are weak acids. For a conjugate acid-base pair in water, Ka- Kb = Kw. For polyprotic acids, we denote the successive acid dissociation constants as Kal, K, K, , or just Aj, K2, A"3, . For polybasic species, we denote successive hydrolysis constants Kbi, Kb2, A"h3, . For a diprotic system, the relations between successive acid and base equilibrium constants are Afa Kb2 — Kw and K.a Kbl = A w. For a triprotic system the relations are A al KM = ATW, K.d2 Kb2 = ATW, and Ka2 Kb, = Kw. 

Definition of Acids and Bases 222 Conjugate Acid-Base Pairs 222 Amphoteric Species 224 Strong Acids 225 Strong Bases 225 Weak Acids 226 Weak Bases 226 Polyprotic Acids 227 Acid and Base Strength Ka and Kb 228 Acid/Base Strength of Conjugate Acid-Base Pairs 230 Acid-Base Reactions 231... 

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. 

Such an equilibrium system is termed a conjugate (or corresponding) acid-base system. A and B are termed a conjugate acid-base pair. It is important to realize that the symbol H+ in this definition represents the bare proton (unsolvated hydrogen ion), and hence the new definition is in no way connected to any solvent. The equation expresses a hypothetical scheme for defining the acid and base - it can be regarded as a half reaction which takes place only if the proton, released by the acid, is taken up by another base. 

Conjugate add-base pairs Recall from Chapter 3 that the definition of adds and bases most useful to analytical chemistry is that of Bronsted, in which a conjugate acid-base pair is related by the reaction... 

By definition, a buffer solution resists changes in pH with dilution or with addition of acids or bases. Generally, buffer solutions are prepared from a conjugate acid/base pair, such as acetic acid/sodium acetate or ammonium chloride/ammonia. Chemists use buffers to maintain the pH of solutions at a relatively constant and predetermined level. You will find many references to buffers throughout this text. 

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. 

The Br0nsted-Lowry definitions of acids and bases are widely used in organic chemistry. As noted in the preceding equation, the conjugate acid of a substance is formed when it accepts a proton from a suitable donor. Conversely, the proton donor is converted to its conjugate base. A conjugate acid-base pair always differ by a single proton. 

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. 

Understand the Brpnsted-Lowry definitions of an acid and a base discuss how water can act as a base or as an acid and how an acid-base reaction is a proton-transfer process involving two conjugate acid-base pairs, with the stronger acid and base forming the weaker base and acid ( 18.3) (SPs 18.4, 18.5) (EPs 18.24-18.39)... 

In 1923, two chemists, Johannes Bronsted and Thomas Lowry, described acids and bases in the scientific literature. They were studying how the transfer of hydrogen ions (protons) took place between reacting molecules. A Bronsted-Lowry acid donates a proton in a reaction, while a base is on the receiving end of the proton transfer. In the Bronsted-Lowry definition of acids and bases, ions as well as larger more complex molecules are included. The pairs of (H2O, OH ) and (NH3, NH4) are called Bronsted-Lowry conjugate acid-base pairs. 

According to the Bronsted-Lowry definitions, any pair of molecules or ions that can be interconverted by transfer of a proton is called a conjugate add-base pair. When an acid transfers a proton to a base, the acid is converted to its conjugate base. When a base accepts a proton, it is converted to its conjugate acid. Fundamental to these definitions is the fact that the members of a conjugate acid-base pair differ by a proton. 

According to the Bronsted-Lowry definition of acids and bases, what is a conjugate acid-base pair Provide an example. 

Br0nsted-Lowry acid-base definition (588) proton donor (588) proton acceptor (588) conjugate acid-base pair (589) Section 18.4... 

According to the Bronsted-Lowry definition of acids and bases, when an acid donates a proton, what is left of the acid is itself a base—we call it the conjugate base of the acid. Likewise, when abase accepts a proton, it becomes an acid—the conjugate acid of the base. The idea of conjugate acid-base pairs is illustrated in the following reaction ... 

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. 

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 . 

By definition, every carbanion possesses an unshared pair of electrons and is therefore a base. When a carbanion accepts a proton, it is converted to its conjugate acid (see Chapter 8). The stability of the carbanion is directly related to the strength of the conjugate acid. The weaker the acid, the greater the base strength and the lower the stability of the carbanion. Here, by stability we mean stability toward a proton donor the lower the stability, the more willing the carbanion is to accept a... 

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