Br0nsted-Lowry Definition

There are several definitions of acids and bases. We are especially interested here in the Br0nsted-Lowry definition and its consequences. After reviewing definitions, we shall study the strength concept of acids and bases. It is important because it allows us to predict acid-base reactions. 

The Arrhenius definition is considered to have been the first true theory of acids and bases. It results from Arrhenius and Ostwald s idea concerning electrolytes dissociation in aqueous solutions. Arrhenius theory is grounded on the ionization of 

An acid HA is a species that ionizes in water to give one hydrated proton and one anion according to the more or less equilibrated reaction 

There are also polyacids and polybases. Polyacids give several hydrated protons per molecule, while polybases give several hydroxide anions. 

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Achiral (Section 7 1) Opposite of chiral An achiral object is supenmposable on its mirror image Acid According to the Arrhenius definition (Section 1 12) a substance that ionizes in water to produce protons Accord mg to the Br0nsted-Lowry definition (Section 1 13) a sub stance that donates a proton to some other substance According to the Lewis definition (Section 1 17) an electron pair acceptor... 

Although the concepts of specific acid and specific base catalysis were useful in the analysis of some early kinetic data, it soon became apparent that any species that could effect a proton transfer with the substrate could exert a catalytic influence on the reaction rate. Consequently, it became desirable to employ the more general Br0nsted-Lowry definition of acids and bases and to write the reaction rate constant as... 

According to the Br0nsted-Lowry definitions, any species that contains hydrogen can potentially act as an acid, and any compound that contains a lone pair of electrons can act as a base. Therefore, neutral molecules can also act as bases if they contain an oxygen, nitrogen or sulphur atom. Both an acid and a base must be present in a proton transfer reaction, because an acid cannot donate a proton unless a base is present to accept it. Thus, proton-transfer reactions are often called acid-base reactions. 

The usefulness of the Br0nsted-Lowry definition lies in its ability to handle any protonic solvent such as liquid ammonia or sulfuric acid ... 

This is a very useful relationship. You should practice writing equations according to the Br0nsted-Lowry definitions of acids and bases and familiarize yourself with Table 1.7 which gives the pXa s of various Brpn-sted acids. 

Chemical species that differ from each other only to the extent of the transferred proton are termed conjugates. As long as one is dealing with a protonic solvent system, the Br0nsted-Lowry definition is as useful as any. 

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. 

The Br0nsted-Lowry definition of acids and bases depends on the transfer of a proton from the acid to the base. The base uses a pair of nonbonding electrons to form a bond to the proton. G. N. Lewis reasoned that this kind of reaction does not need a proton. Instead, a base could use its lone pair of electrons to bond to some other electron-deficient atom. In effect, we can look at an acid-base reaction from the viewpoint of the bonds that are formed and broken rather than a proton that is transferred. The following reaction shows the proton transfer, with emphasis on the bonds being broken and formed. Organic chemists routinely use curved arrows to show the movement of the participating electrons. 

The Br0nsted-Lowry definition of acids and bases does not replace the Arrhenius definition, but extends it. The Bronsted-Lowry definition of acids and bases requires you to take a closer look at the reactants and products of an acid-base reaction. In this case, acids and bases are not easily defined as having hydronium and hydroxide ions. Instead, you are asked to look and see which substance has lost a proton and which has gained the very same proton that was lost. 

The Lewis definition of acids and bases is more general than the Br0nsted-Lowry definition. 

Still another important concept related to electronegativity and polarity is that of acidity and basicity. We ll soon see that the acid-base behavior of organic molecules helps explain much of their chemistry. You may recall from a course in general chemistry that there are two frequently used definitions of acidity, the Br0nsted-Lowry definition and the Lewis definition. 

We ll look at the Br0nsted Lowry definition in this and the next three sections, and then discuss the Lewis definition in Section 2.11. 

The Br0nsted-Lowry definition of acidity discussed in the previous four sections encompasses all compounds containing hydrogen. Of even more use, however, is the Lewis definition of acids and bases, which is not limited to compounds that gain or lose protons. A Lewis acid is a substance that accepts an electron pair, and a Lewis base is a substance that donates an... 

The Br0nsted-Lowry definition has the advantage of not being limited to aqueous solutions. An example with liquid ammonia as the solvent is... 

Br0nsted-Lowry definition does not differ appreciably from the Arrhenius definition of hydrogen ions (acids) and hydroxide ions (bases) ... 

In contrast to the Br0nsted-Lowry theory, which emphasizes the proton as the principal species in acid-ba.se reactions, the definition proposed by Lux and extended by Floods describes acid-base behavior in terms of the oxide ion. This acid-base concept was advanced to treat nonprotonic systems which were not amenable to the Br0nsted-Lowry definition. For example, in high-temperature inorganic melts, reactions such as the following take place ... 

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. 

The Br0nsted-Lowry definition provides a new way to look at acid-base reactions because it focuses on the reactants and the products. For example, let s examine the reaction between hydrogen sulfide and ammonia ... 

How do the Arrhenius and Br0nsted-Lowry definitions of an acid and a base differ How are they similar Name two Br0nsted-Lowry bases that are not considered Arrhenius bases. Can you do the same for acids Explain. 

The definitions of acid and base that we use now were provided by Brpnsted and Lowry in 1923. In the Br0nsted-Lowry definitions, an acid is a species that donates a proton, and a base is a species that accepts a proton. (Remember that positively charged hydrogen ions are also called protons.) In the following reaction, hydrogen chloride (HCl) meets the Brpnsted-Lowry definition of an acid because it donates a proton to water. Water meets the definition of a base because it accepts a proton from HCl. Water can accept a proton because it has two lone pairs. Either lone pair can form a covalent bond with a proton. In the reverse reaction, H3O is an acid because it donates a proton to CF, and CF is a base because it accepts a proton from H30. ... 

The acid-base concepts described in this section focus on bases that react with protons, the classical Br0nsted-Lowry definition of a base. There are other types of electron deficient molecules that function as acids, called Lewis acids. The following section will discuss the relative strength of Lewis acids. 

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