Acidity Bronsted definition

The Bronsted definition also includes the possibility that an ion is an acid (an option not allowed by the Arrhenius definition). For instance, a hydrogen carbonate ion, HC03, one of the species present in natural waters, can act as an acid and lose a proton, and the resulting carbonate ion is removed by precipitation if suitable cations are present (Fig. 10.2) ... 

The products of proton transfer in aqueous solution may also react with water. For example, the CIST ion produced when HCN loses a proton to water can accept a proton from a water molecule and form HCN again. Therefore, according to the Bronsted definition, CN is a base it is called the conjugate base of the acid HCN. In general, a conjugate base of an acid is the species left when the acid donates a proton ... 

The Bronsted definitions of acids and bases are more general than the Arrhenius definitions they also apply to species in nonaqueous solvents and even to gas-phase reactions. For example, when pure acetic acid is added to liquid ammonia, proton transfer takes place and the following equilibrium is reached ... 

An important implication of the Bronsted definitions of acids and bases is that the same substance may be able to function as both an acid and a base. For example, we have seen that a water molecule accepts a proton from an acid molecule (such as HC1 or HCN) to form an H30+ ion. So water is a base. I lowever, a water molecule can donate a proton to a base (such as O2- or NH3) and become an OH ion. So water is also an acid. We describe water as amphiprotic, meaning that an H20 molecule can act both as a proton donor and as a proton acceptor. 

The various acid-base definitions are summarized in the Venn diagram (Fig. 2.1). From this it can be seen that the Usanovich definition subsumes the Lewis definition, which in turn subsumes all other definitions (i.e. Arrhenius, Bronsted-Lowry, Germann-Cady-Elsey, Lux-Flood). 

In silicate melts and other nonprotonated solvents, the Bronsted-Lowry equation is not applicable and is conveniently replaced by the Lux-Flood acid-base definition (Lux, 1939 Flood and Forland, 1947), according to which free oxygen 0 replaces A basic oxide is one capable of furnishing oxygen ions, and an acidic oxide is one that associates oxygen ions ... 

Figure 7.4 is the graphical expression of the Henderson-Hassel-BALCH equation. The graph may he used for estimation of buffer composition, if the pK of the buffering substance is known. For example, the base A of an acetate buffer is the anion CHsCOO", the acid HA is CH3 COOH, and in a Tris buffer, TrisH" is the acid HA and unprotonated Tris is the base A (according to the Bronsted definition, the acid is that substance which has bound the hydrogen cation) ... 

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. 

C Choice A is the definition of a strong acid, choice B is the definition of a Lewis acid, and choice D is the definition of an Arrhenius acid. By definition, all Arrhenius bases form OH- ions in water, and all Bronsted bases are proton acceptors. But not all Bronsted bases use OH- as a proton acceptor. NH3 is a Bronsted base for example. 

This is an amphoteric compound because it behaves as both an acid and a base in the Bronsted definition. As an acid, it can donate an H+ and becomes the conjugate base ... 

However, many species which are acids under Lewis definition cannot be termed so according to Bronsted definition. A few examples are sulphur trioxide and halides of boron, aluminium, iron (ferric) and zinc. The central atom in each is able to accept a pair of electrons to complete is octet. 

Many substances in solution have the capability of releasing or absorbing protons. According to the classic Bronsted definition, acids are those substances that release protons and bases (alkalis) are those that accept protons. Thus, acetic acid is an acid, but acetate is a base. Ammonium ion (NH4+) is an acid, whereas ammonia (NH3) is a base. When an acid loses its proton, its conjugate base is formed, whereas when a base gains protons, its conjugate acid is generated. 

Acids and bases were defined and described by early chemists, including Boyle, Lavoisier, Davy, Berzelius, Liebig, and Arrhenius. At the present time, depending on objectives, one of two definitions of acids and bases is likely to be accepted. These two definitions, by Bronsted and Lowry and by Lewis, were proposed about the same time. According to the Bronsted definition acids are substances having a tendency to lose a proton, and bases are those having a tendency to accept a proton. Thus, for an acid HA the acid-base half-reaction is... 

Where Ka is the acid dissociation constant. (Note that the definition of Ka is based on the Bronsted definition.) Values of Ka can vary tremendously (IQis to 10-60). after all anything with at least one proton can be considered an acid under some circumstances with this definition. The common definition of a strong acid is an acid which dissociates completely in a 1 M solution. The common strong acids in aqueous solution, such as sulphuric, nitric and hydrochloric acids have Ka values (for the first dissociation in the case of sulfuric) of 10 to 10. Thus they all dissociate completely (first dissociation only for sulphuric) in aqueous solution, though they wiU have different strengths in some other solvents. Most common organic acids are weak in aqueous solution, having Ka values of lO-s to Note that... 

Other definitions of acids and bases are usefui, the most notabie being those of Lewis, aiso proposed in 1923. However, for this chapter, the Bronsted definition is entireiy adequate. 

Scheme 3.1 is not a realistic expression, however, since protons are too reactive to exist independently and are rapidly taken up by the solvent. The proton-accepting entity, hy the Lowry-Bronsted definition, is a base, and the product formed when the proton has heen accepted by the solvent is an acid. Thus a second acid-base equilibrium occurs when the solvent accepts the proton, and this may be represented by... 

Notice tliat in the Bronsted definition, die acid donates , and in the Lewis definition the acid accepts. ... 

In these four sections the terms Bronsted and Lewis acid sites have been widely used. While Bronsted sites can be reasonably well defined as proton-donating sites, the properties of Lewis acids are not so clear. A status report on Lewis acid-base definitions is highly relevant for clarifying these problems. 

THE BRONSTED definition OF ACIDITY AND BASICITY. PROPERTIES OF ACIDS AND BASES... 

It is evident from equation (1) that the acidic and basi properties are independent of the electric charge of the molecul of the acid or base. Whereas formerly only electrically neutra molecules could be regarded as acids and bases, Bronsted definition admits that molecules which bear an electric charg may also behave as acids and bases. 

Most ions constituting ionic liquids can be categorized according to their Lewis acid/base properties (i.e., their capability to accept or to donate an electron pair) nevertheless, some ions may be considered according to the Bronsted definition, i.e., on the basis of their ability to accept or donate a proton. Typical ionic liquids are those based on neutral or very weakly basic anions (BF4, PF, NOf, CHsSO, 4 f2N ) and neutral (tetraalkylammonium, dialkyl-pyrrolidium, trialkylsulfonium) or weakly acidic cations (1,3-dialkylimidazolium and 1,2,3-trialkylimidazolium) (Figure 4.1). 

According to the Bronsted definition an acid is a proton donor, and a base is a proton acceptor. It follows that acid-base reactions can also be called proton transfer reactions. Thus, the reaction between acid HA and base B is written as... 

It has been established from these studies that the different catalytic properties of transition metal oxides (chromium, cobalt) on zirconium dioxide are attributed to their different acidic properties determined by TPDA and IR-spectroscopy. The most active catalyst is characterized by strong acidic Bronsted centers. The cobalt oxide deposited by precipitation on the zirconium-containing pentasils has a considerable oxidative activity in the reaction N0+02 N02, and for SCR-activity the definite surface acidity is necessary for methane activation. Among the binary systems, 10% CoO/(65% H-Zeolite - 35% Z1O2)... 

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