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The Brnsted Definition of Acids and Bases

The Brnsted, or Brnsted-Lowry, model is based on a simple assumption: Acids donate H+ ions to another ion or molecule, which acts as a base. The dissociation of water, for example, involves the transfer of an H+ ion from one water molecule to another to form H3O+ and OH- ions.

According to this model, HCl doesn't dissociate in water to form H+ and Cl+ ions. Instead, an H+ ion is transferred from HCl to a water molecule to form H3O+ and Cl- ions, as shown in the figure below.

Because it is a proton, an H+ ion is several orders of magnitude smaller than the smallest atom. As a result, the charge on an isolated H+ ion is distributed over such a small amount of space that this H+ ion is attracted toward any source of negative charge that exists in the solution. Thus, the instant that an H+ ion is created in an aqueous solution, it bonds to a water molecule. The Brnsted model, in which H+ ions are transferred from one ion or molecule to another, therefore makes more sense than the Arrhenius theory, which assumes that H+ ions exist in aqueous solution.
Even the Brnsted model is naive. Each H+ ion that an acid donates to water is actually bound to four neighboring water molecules, as shown in the figure below.
A more realistic formula for the substance produced when an acid loses an H+ ion is therefore H(H2O)4+, or H9O4+. For all practical purposes, however, this substance can be represented as the H3O+ ion.
The reaction between HCl and water provides the basis for understanding the definitions of a Brnsted acid and a Brnsted base. According to this theory, an H+ ion is transferred from an HCl molecule to a water molecule when HCl dissociates in water.

HCl acts as an H+-ion donor in this reaction, and H2O acts as an H+ ion-acceptor. A Brnsted acid is therefore any substance (such as HCl) that can donate an H+ ion to a base. A Brnsted base is any substance (such as H2O) that can accept an H+ ion from an acid.
There are two ways of naming the H+ ion. Some chemists call it a hydrogen ion; others call it a proton. As a result, Brnsted acids are known as either hydrogen-ion donors or proton donors. Brnsted bases are hydrogen-ion acceptors or proton acceptors.
From the perspective of the Brnsted model, reactions between acids and bases always involve the transfer of an H+ ion from a proton donor to a proton acceptor. Acids can be neutral molecules.

They can also be positive ions

or negative ions.

The Brnsted theory therefore expands the number of potential acids. It also allows us to decide which compounds are acids from their chemical formulas. Any compound that contains hydrogen with an oxidation number of +1 can be an acid. Brnsted acids include HCl, H2S, H2CO3, H2PtF6, NH4+, HSO4-, and HMnO4.
Brnsted bases can be identified from their Lewis structures. According to the Brnsted model, a base is any ion or molecule that can accept a proton. To understand the implications of this definition, look at how the prototypical base, the OH- ion, accepts a proton.

The only way to accept an H+ ion is to form a covalent bond to it. In order to form a covalent bond to an H+ ion that has no valence electrons, the base must provide both of the electrons needed to form the bond. Thus, only compounds that have pairs of nonbonding valence electrons can act as H+-ion acceptors, or Brnsted bases.
The following compounds, for example, can all act as Brnsted bases because they all contain nonbonding pairs of electrons.

The Brnsted model expands the list of potential bases to include any ion or molecule that contains one or more pairs of nonbonding valence electrons. The Brnsted definition of a base applies to so many ions and molecules that it is almost easier to count substances, such as the following, that can't be Brnsted bases because they don't have pairs of nonbonding valence electrons.

Practice Problem 2: Which of the following compounds can be Brnsted acids? Which can be Brnsted bases?
(a) H2O
(b) NH3
(c) HSO4-
(d) OH-
2

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The Role of Water in the Brnsted Theory

The Brnsted theory explains water's role in acid-base reactions.

• Water dissociates to form ions by transferring an H+ ion from one molecule acting as an acid to another molecule acting as a base.

H2O(l)+H2O(l)H3O+(aq)+ OH-(aq)acid base

• Acids react with water by donating an H+ ion to a neutral water molecule to form the H3O+ ion.

HCl(g)+H2O(l)H3O+(aq)+ Cl-(aq) acid base

• Bases react with water by accepting an H+ ion from a water molecule to form the OH- ion.

NH3(aq)+H2O(l)NH4+(aq)+ OH-(aq)base acid

• Water molecules can act as intermediates in acid-base reactions by gaining H+ ions from the acid

HCl(g) +H2O(l)H3O+(aq)+ Cl-(aq)

and then losing these H+ ions to the base.

NH3(aq)+H3O+(aq)NH4+(aq)+ H2O(l)

The Brnsted model can be extended to acid-base reactions in other solvents. For example, there is a small tendency in liquid ammonia for an H+ ion to be transferred from one NH3 molecule to another to form the NH4+ and NH2- ions.

2 NH3NH4++ NH2-

By analogy to the chemistry of aqueous solutions, we conclude that acids in liquid ammonia include any source of the NH4+ ion and that bases include any source of the NH2- ion.
The Brnsted model can even be extended to reactions that don't occur in solution. A classic example of a gas-phase acid-base reaction is encountered when open containers of concentrated hydrochloric acid and aqueous ammonia are held next to each other. A white cloud of ammonium chloride soon forms as the HCl gas that escapes from one solution reacts with the NH3 gas from the other.

HCl(g)+ NH3(g)NH4Cl(s)

This reaction involves the transfer of an H+ ion from HCl to NH3 and is therefore a Brnsted acid-base reaction, even though it occurs in the gas phase