Bronsted acids, definition

A useful definition of acids and bases is that independently introduced by Johannes Bronsted (1879-1947) and Thomas Lowry (1874-1936) in 1923. In the Bronsted-Lowry definition, acids are proton donors, and bases are proton acceptors. Note that these definitions are interrelated. Defining a base as a proton acceptor means an acid must be available to provide the proton. For example, in reaction 6.7 acetic acid, CH3COOH, donates a proton to ammonia, NH3, which serves as the base. 

The Lewis definition of acids and bases is broader and more encompassing than the Bronsted-Lowry definition because it s not limited to substances that donate or accept just protons. A Lewis acid is a substance that accepts an electron pair, and a Lewis base is a substance that donates an electron pair. The donated electron pair is shared between the acid and the base in a covalent bond. 

The Lewis definition of a base as a compound with a pair of nonbonding electrons that it can use to bond to a Lewis acid is similar to the Bronsted-Lowry definition. Thus, H20, with its two pairs of nonbonding electrons on oxygen, acts as a Lewis base by donating an electron pair to an H+ in forming the hydronium ion, H30+. 

The problem with the Arrhenius definitions is that they are specific to one particular solvent, water. When chemists studied nonaqueous solvents, such as liquid ammonia, they found that a number of substances showed the same pattern of acid-base behavior, but plainly the Arrhenius definitions could not be used. A major advance in our understanding of what it means to be an acid or a base came in 1923, when two chemists working independently, Thomas Lowry in England and Johannes Bronsted in Denmark, came up with the same idea. Their insight was to realize that the key process responsible for the properties of acids and bases was the transfer of a proton (a hydrogen ion) from one substance to another. The Bronsted-Lowry definition of acids and bases is as follows ... 

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. 

Bronsted-Lowry definition A definition of acids and bases in terms of the ability of molecules and ions to participate in proton transfer. 

Any reaction in which a proton is transferred from one substance to another is an acid-base reaction. More specifically, the proton-transfer view is known as the Bronsted-Lowiy definition of acids and bases. In an acid-base reaction, an acid donates a proton, and a base accepts that proton. Any species that can give up a proton to another substance is an acid, and any substance that can accept a proton from another substance is a base. The production of two water molecules from a hydroxide anion (a base) and a hydronium ion (an acid) is just one example of an acid-base reaction acids and bases are abundant in chemistry. 

In an acid-base reaction, a proton (H ) is transferred from one chemicai species to another. A species that donates a proton is an acid, and a species that accepts a proton is a base. This identification of acids and bases is the Bronsted-Lowry definition of acid-base reactions. From this perspective, every acid-base reaction has two reactants, an acid and a base. Every acid-base reaction aiso forms two products ... 

KolthoflF, I. M. (1944). The Lewis and Bronsted-Lowry definitions of acids and bases. Journal of Physical Chemistry, 48, 51-7. 

Water as the solvent is essential for the acid-base setting reaction to occur. Indeed, as was shown in Chapter 2, our very understanding of the terms acid and base at least as established by the Bronsted-Lowry definition, requires that water be the medium of reaction. Water is needed so that the acids may dissociate, in principle to yield protons, thereby enabling the property of acidity to be manifested. The polarity of water enables the various metal ions to enter the liquid phase and thus react. The solubility and extent of hydration of the various species change as the reaction proceeds, and these changes contribute to the setting of the cement. 

The Bronsted-Lowry definition of an acid is essentially the same as Arrhenius idea An acid is any substance that releases a hydrogen ion. Their idea has come to be known as the Bronsted-Lowry theory of acids and bases. 

According to the Bronsted-Lowry definition of acids and bases, an acid is a proton donor. The particle that is left over after an acid donates its proton, however, can now accept a proton and,... 

The chlorine ion can now accept a proton (and become hydrochloric acid again). If the chlorine can accept a proton, according to the Bronsted-Lowry definition, it is a base. Chemists actually call this chlorine ion the conjugate base of hydrochloric acid. Any time an acid gives up its proton, the substance that is left over can act as a base. So every acid has a conjugate base. 

Thus, according to the above definitions, Lewis bases are also Bronsted bases but Lewis acids do not correspond to Bronsted acids. 

Thus, a reducing agent donates electrons, while an oxidizing agent receives them. The Bronsted-Lowry definitions of acid and base specify that... 

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 ... 

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. 

Lewis defined a base as an electron pair donor and an acid as an electron pair acceptor. Lewis electron pair donor was the same as Bronsted-Lowry s proton acceptor, and therefore, was an equivalent way of defining a base. Lewis acids were defined as a substance with an empty valence shell that could accommodate a pair of electrons. This definition broadened the Bronsted-Lowry definition of an acid. The three definitions of acids and bases are summarized in Table 13.3. 

Under the Bronsted-Lowry definition, an acid is a substance that donates a hydrogen ion (H+) in an acid-base reaction, while a base is a substance that accepts that hydrogen ion from the acid. When ionized to form a hydrogen cation, hydrogen loses its one and only electron and is left with only a single proton. For this reason, Bronsted-Lowry acids are often called proton donors, and Bronsted-Lowry bases are called proton acceptors. 

This is a simple double replacement reaction (see Chapter 8 for an introduction to these types of reactions). A hydrogen ion from water switches places with the sodium of sodium carbonate to form the products carbonic acid and sodium hydroxide. By the Bronsted-Lowry definition, water is the acid because it donates its hydrogen to Na2COj. This makes Na2C03 the base because it accepts the hydrogen from H2O. 

All Br0nsted-Lowry acids are Lewis acids, but in practice, the term Lewis acid is generally reserved for Lewis acids that don t also fit the Bronsted-Lowry definition. The best way to spot a Lewis acid-base pair is to draw a Lewis dot structure of the reacting substances, noting the presence of lone pairs of electrons. (We introduce Lewis structures in Chapter 5.) For example, consider the reaction between ammonia (NH3) and boron trifluoride (BFj) ... 

In practice, it s much simpler to use the Arrhenius or Bronsted-Lowry definition of acid and base, but you ll need to use the Lewis definition when hydrogen ions aren t being exchanged. You can pick and choose among the definitions when you re asked to identify the acid and base in a reaction. 

Drugs cross biological membranes most readily in the unionised state. The unionised drug is 1000-10000 times more lipid-soluble than the ionised form and thus is able to penetrate the cell membrane more easily. Chemical compounds in solution are acids, bases or neutral. The Bronsted-Lowry definition of an acid is a species that donates protons (H+ ions) while bases are proton acceptors. Strong acids and bases in solution dissociate almost completely into their conjugate base and H+. Weak acids and weak bases do not completely dissociate in solution, and exist in both ionised and unionised states. Most drugs are either weak acids or weak bases. For an acid, dissociation in solution is represented by ... 

What are the Bronsted-Lowry definitions of acid and base ... 

Which definition of acids and bases is more universal the Bronsted-Lowry definition or the Lewis definition ... 

Whenever we refer simply to acids and bases in this text, we use the Bronsted-Lowry definitions. 

The proton is fundamental to both the Arrhenius and the Bronsted-Lowry definitions of an acid. Dissociation of an Arrhenius acid HA gives an aqueous hydrogen ion, or hydrated proton, written as H + (aq) ... 

Since all proton acceptors have an unshared pair of electrons, and since all electron-pair donors can accept a proton, the Lewis and the Bronsted-Lowry definitions of a base are simply different ways of looking at the same property. All Lewis bases are Bronsted-Lowry bases, and all Bronsted-Lowry bases are Lewis bases. The Lewis definition of an acid, however, is considerably more general than the Bronsted-Lowry definition. Lewis acids include not only H+ but also other cations and neutral molecules having vacant valence orbitals that can accept a share in a pair of electrons donated by a Lewis base. 

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