Acids and bases Bronsted

Acids and bases have been known for centuries, but the definitions in common use today are of comparatively recent origin. In 1923 J. N. Bronsted proposed the following definitions 1 

An acid HA is thus any substance that reacts according to Equation 3.1, and a base B is any substance that reacts according to Equation 3.2  

If one confines one s attention to the liquid phase, however, these idealized reactions apparently never occur. The proton, H+, does not exist free in solution, 

If we look again at Equation 3.3, we can see that we should consider the reverse process as an acid-base reaction just as the forward process is. The acetate ion is a base that can accept a proton from the acid HaO+. This reciprocal relationship is emphasized by the terminology applied to processes like that in Equation 3.3 Acetate ion is called thp mnjuont.e hate. of the acid CH3CQQH. and HoO+ is called the coniueate acid of the base HoQ. 

In considering an acid-base reaction, it is important to realize that the choice of which acid is to be called the conjugate acid is completely arbitrary. In Equation 3.4 we could just as well have decided to call HaO the conjugate base of the acid HaO+ and CH3COOH the conjugate acid of the base CH3COO . It would perhaps be better to emphasize the fundamental symmetry of the situation by writing Equation 3.5  

In the Bronsted definition, an acid donates a proton and a base accepts a proton. The strengths of acids and bases are measured by the extent to which they lose or gain protons, respectively. In these reactions acids are converted to their copjugate bases and bases to their conjugate acids. Acid-base reactions go in the direction of forming the weaker acid and the weaker base. 

Problem 3.25 Show the conjugate acids and bases in the reaction of H,0 with gaseous (a) HCI, (b) NH,. 

The conjugate acid-base pairs have the same subscript and are bracketed together. This reaction goes almost to completion because HCI is a good proton donor and hence a strong acid. 

To be called an acid, the species must be more acidic than water and be able to donate a proton to water. Some compounds, such as alcohols, are not acidic toward water, but have an H which is acidic enough to react with very strong bases or with Na. 

Problem 3.26 Write an equation for the reaction of ethanol with (a) NH, a very strong base (h) Na.  

In Chapter 4 we defined a Bronsted acid as a substance capable of donating a proton, and a Bronsted base as a substance capable of accepting a proton. These definitions are generally suitable for discussion of the properties and reactions of acids and bases. 

An extension of the Bronsted definition of acids and bases is the concept of the conjugate add-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 Bronsted 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 Bronsted base. 

Every Bronsted acid has a conjugate base, and every Bronsted base has a conjugate acid. For example, the chloride ion (Cl ) is the conjugate base formed from the acid HCl, and H2O is the conjugate base of the acid HsO . Similarly, the ionization of acetic acid can be represented as 

Electrostatic potential map of the hydronium ion. The proton is always associated with water molecules in aqueous solution. The ion is the simplest formula of a hydrated proton. 

The subscripts 1 and 2 designate the two conjugate acid-base pairs. Thus, the acetate ion (CHsCOO ) is the conjugate base of the acid CH3COOH. Both the ionization of HCl (see Section 4.3) and the ionization of CH3COOH are examples of Bronsted acid-base reactions. 

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We call such substances Bronsted acids and bases or just plain acids and bases because the Bronsted-Lowry definition is the one commonly accepted today and the one used throughout this text. 

Self-Test 10.2A Identify (a) the Bronsted acids and bases in both reactants and products in the proton transfer equilibrium HN02(aq) + HP042 (aq) N02 (aq)... 

Identify (a) the Bronsted acid and base in the following reaction, and (b) the conjugate base and acid formed ... 

Which of the following reactions can be classified as reactions between Bronsted acids and bases For those that can be so classified, identify the acid and the base. (Hint It may help to write the net ionic equations.)... 

Neutralization reactions between Lowry-Bronsted acids and bases are frequently employed in chemical analysis. Methods based on them are sometimes termed acidimetric or alkalimetric. 

Thus, Lewis s definition is a much broader definition that includes coordination compound formation as acid-base reactions, besides Arrhenius and Lowry-Bronsted acids and bases. Examples ... 

Changes in pH are propagated by the movement of protons. Bnt because free protons do not exist they must move by transfer between proton donors and acceptors, i.e. Bronsted acids and bases ... 

The extent to which the pH of a solution is buffered against additions or removals of protons is measured by the solution s pH buffer capacity. This is defined as the amount of strong acid or base required to produce unit change in pH. The buffering depends on the transfer of protons between donors and acceptors, i.e. Bronsted acids and bases, which form conjugate acid-base pairs. The pH buffer capacity of a solution is calculated from the buffer capacities of the individual acid-base pairs present. 

Things are a little more complicated than implied by this statement. Molecules that donate protons and molecules that accept them are formally known as Bronsted acids and bases. There is a more general definition of acid and base— Lewis acids and bases—in which a Lewis acid is any molecule having the ability to accept a pair of electrons and a Lewis base is any molecule having the ability to donate a pair of electrons to a Lewis acid. The Bronsted concept will suffice for all our purposes. 

Periodic table correlations. When comparing BrOnsted acids and bases that differ in the position of an element in the periodic table ... 

Identify Bronsted acids and bases in a chemical reaction, Self-Test 10.2. 

Substituent Effects on Strengths of Bronsted Acids and Bases 149... 

SUBSTITUENT EFFECTS ON STRENGTHS OF BRONSTED ACIDS AND BASES... 

Acid-base reactions have long served as a starting point for consideration of the effects of changes in a structure on the course of chemical reactions. Table 3.2 summarizes solution data for a variety of Bronsted acids and bases because of the problems of measurement, any such table necessarily contains a fair amount of uncertainty. The pKa values that fall between 2 and 10 may be used with considerable confidence, since they are based on accurate measurements in dilute aqueous solutions the values outside this range must be regarded with a certain amount of skepticism. As we have noted in the two previous sections, uncertainties... 

The above two equations describing the behavior of Bronsted acids and bases are not strictly correct because a Bronsted acid does not just dissociate, it donates a proton to something which accepts a proton. The proton does not just dissociate and Boat around in solution but is always attached to something. Furthermore a Bronsted base does not just find a proton to accept it accepts a proton from a Bronsted acid. Thus acidity and basicity are paired behavior— you can t have one without the other. This is the most common misconception about acids and bases and leads to die greatest amount of difficulties when trying to apply die principles of acidity and basicity to real reactions. 

Whereas many reactions catalysed by organometallic compounds are characteristically different from those catalysed by Bronsted acids and bases, and to a degree their investigators have developed distinctive terminologies, the strategies employed in mechanistic studies are... 

In the 1920s, Johannes Bronsted and Thomas Lowry recognized that acids can transfer a proton to bases regardless of whether an OH" ion accepts the proton. In an equilibrium reaction, the direction of proton transfer depends on whether the reaction is read left to right or right to left, so Bronsted acids and bases exist in conjugate pairs with and without a proton. Acids that are able to transfer more than one proton are called polyprotic acids. 

The Lewis theory of acids and bases is more general than Bronsted-Lowry theory, but Bronsted-Lowry s definition is used more frequently. The terms "acid" and "base" most often refer to Bronsted acids and bases, and the term "Lewis acid" is usually reserved for chemicals like BF3 that are not Bronsted acids. 

From these experiments, and on the basis of the Bronsted acids and bases theory, as well as Hantzsch s theory, Lantz has concluded that in a nitrating mixture the hydrated sulphuric acid H2S04.H20 acts as a base towards anhydrous sulphuric acid. Thus an equilibrium acid-base system is established ... 

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