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12.1: Brønsted–Lowry Acids and Bases

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    Learning Objectives

    • Identify acids, bases, and conjugate acid-base pairs according to the Brønsted-Lowry definition
    • Write equations for acid and base ionization reactions
    • Describe the acid-base behavior of amphiprotic substances
    • Understand acid-base reactivity as a proton transfer process.

    In previous chapters, we have understood acids and bases in terms of the Arrhenius definition: an acid is a compound that dissolves in water to yield hydronium ions (\(H_3O^+\)) and a base is a compound that dissolves in water to yield hydroxide ions (\(\ce{OH-}\)). This definition is not wrong; it is simply limited. In this chapter, we will extend the definition of an acid or a base using the more general definition proposed in 1923 by the Danish chemist Johannes Brønsted and the English chemist Thomas Lowry. Their definition centers on the proton, \(\ce{H^+}\). A proton is what remains when a normal hydrogen atom, \(\ce{^1_1H}\), loses an electron. A compound that donates a proton to another compound is called a Brønsted-Lowry acid, and a compound that accepts a proton is called a Brønsted-Lowry base.

    Table \(\PageIndex{1}\): Definitions of Acids and Bases
    Definition Acid Base
    Arrhenius \(H^+\) donor \(OH^−\) donor
    Brønsted–Lowry \(H^+\) donor \(H^+\) acceptor

    Brønsted-Lowry Acids

    Brønsted-Lowry acids may be neutral molecules such as \(\ce{HCl}\), \(\ce{H_2SO_4}\), or acetic acid (\(\ce{CH_3COOH}\)). Anions (such as \(\ce{HSO_4^-}\), \(\ce{H_2PO_4^-}\), \(\ce{HS^-}\), and \(\ce{HCO_3^-}\)) and cations (such as \(\ce{H_3O^+}\), \(\ce{NH_4^+}\), and \(\ce{[Al(H_2O)_6]^{3+}}\)) may also act as acids. In general, we can symbolize any acid as \(\ce{HA}\) and describe the donation of a proton to a water molecule as follows:

    \[\ce{HA + H_2O \rightleftharpoons H_3O^+ + A^-}\]

    We call the product, \(\ce{A^-}\), that remains after an acid donates a proton the conjugate base of the acid. In the generalized example above, the anion (\(\ce{A^-}\)) is the conjugate base of the acid (\(\ce{HA}\)). This species is called a base because it can accept a proton (to re-form the original acid).

    The reaction between a Brønsted-Lowry acid and water is called acid ionization. For example, when hydrogen fluoride dissolves in water and ionizes, protons are donated from hydrogen fluoride molecules to water molecules, yielding hydronium ions and fluoride ions:

    This figure has two rows. In both rows, a chemical reaction is shown. In the first, structural formulas are provided. In this model, in purple, an H atom is connected to an F atom with a single bond. The F atom has pairs of electron dots at the top, right, and bottom. This is followed by a plus sign, which is followed in green by an O atom which has H atoms singly bonded above and to the right. The O atom has pairs of electron dots on its left and lower sides. A double arrow follows. To the right, in brackets is a structure with a central O atom in green, with green H atoms singly bonded above and to the right. A pair of green electron dots is on the lower side of the O atom. To the left of the green O atom, a purple H atom is singly bonded. This is followed by a plus sign and an F atom in purple with pairs of electron dots above, right, below, and to the left. This atom also has a superscript negative sign. The reaction is written in symbolic form below. H F is labeled in purple below as “Acid subscript 1.” This is followed by plus H subscript 2 O, which is labeled in green below as “Base subscript 2.” A double sided arrow follows. To the right is H subscript 3 O superscript plus, which is labeled in green as below in as “Acid subscript 2.” This is followed by plus and F surrounded by 4 pairs of dots and superscript negative. The label below in purple reads, “Base subscript 1.” To the right of the reactions is the formula, K subscript a equals left bracket H subscript 3 O superscript plus right bracket left bracket F superscript negative right bracket all over left bracket H F right bracket.

    The fluoride ion (\(\ce{F^-}\)) is the conjugate base of hydrofluoric acid (\(\ce{HF}\)). Here are several other examples of acid ionization in water:

    \[\ce{H_2SO_4 + H_2O \rightleftharpoons H_3O^+ + HSO_4^{−}}\]

    \[\ce{NH_4^+ + H_2O \rightleftharpoons H_3O^+ + NH_3} \]

    In each case, the acid ionization process results in the formation of the conjugate base! Identifying acid-base conjugate pairs will help you understand acid-base reactivity.

    Example \(\PageIndex{1}\): Conjugate Acid-Base Pairs

    Identify the conjugate base for each of the following acids:

    a) acetic acid (\(\ce{HC_2H_3O_2}\))

    b) hydrocyanic acid (\(\ce{HCN}\))

    c) phosphoric acid (\(\ce{H_3PO_4}\))

    Solution

    a) \(\ce{C_2H_3O_2^-}\)

    b) \(\ce{CN^-}\)

    c) \(\ce{H_2PO_4^{-}}\)

    Brønsted-Lowry Bases

    Brønsted-Lowry bases may, likewise, be neutral molecules (such as \(\ce{NH_3}\) and \(\ce{CH_3NH_2}\)), anions (such as \(\ce{OH^-}\), \(\ce{HS^-}\), \(\ce{HCO_3^-}\), \(\ce{CO_3^{2−}}\), \(\ce{F^-}\), and \(\ce{PO_4^{3−}}\)), or cations (such as \(\ce{[Al(H_2O)_5OH]^{2+}}\)). The most familiar bases are ionic compounds such as \(\ce{NaOH}\) and \(\ce{Ca(OH)_2}\), which contain the hydroxide ion, \(\ce{OH^-}\).

    When we add a base to water, a base ionization reaction occurs in which a proton is transferred from a water molecule and accepted by the base molecule (or ion). For example, adding ammonia to water yields hydroxide ions and ammonium ions. We call the product that results when a base accepts a proton the base’s conjugate acid. This species is an acid because it can give up a proton (and thus re-form the base).


    This figure has three parts in two rows. In the first row, two diagrams of acid-base pairs are shown. On the left, a space filling model of H subscript 2 O is shown with a red O atom at the center and two smaller white H atoms attached in a bent shape. Above this model is the label “H subscript 2 O (acid)” in purple. An arrow points right, which is labeled “Remove H superscript plus.” To the right is another space filling model with a single red O atom to which a single smaller white H atom is attached. The label in purple above this model reads, “O H superscript negative (conjugate base).” Above both of these red and white models is an upward pointing bracket that is labeled “Conjugate acid-base pair.” To the right is a space filling model with a central blue N atom to which three smaller white H atoms are attached in a triangular pyramid arrangement. A label in green above reads “N H subscript 3 (base).” An arrow labeled “Add H superscript plus” points right. To the right of the arrow is another space filling model with a blue central N atom and four smaller white H atoms in a tetrahedral arrangement. The green label above reads “N H subscript 3 superscript plus (conjugate acid).” Above both of these blue and white models is an upward pointing bracket that is labeled “Conjugate acid-base pair.” The second row of the figure shows the chemical reaction, H subscript 2 O ( l ) is shown in purple, and is labeled below in purple as “acid,” plus N H subscript 3 (a q) in green, labeled below in green as “base,” followed by a double sided arrow arrow and O H superscript negative (a q) in purple, labeled in purple as “conjugate base,” plus N H subscript 4 superscript plus (a q)” in green, which is labeled in green as “conjugate acid.” The acid on the left side of the equation is connected to the conjugate base on the right with a purple line. Similarly, the base on the left is connected to the conjugate acid on the right side.

    Notice that both these ionization reactions are represented as equilibrium processes. The relative extent to which these acid and base ionization reactions proceed is an important topic treated in a later section of this chapter.

    Acid-Base Reactions

    Acid-base reactivity under the Brønsted-Lowry definition can be considered a proton transfer process. In this process the proton is transferred from the acid to the base. Consider the following reaction between hydrocyanic acid (\(\ce{HCN}\)) and ammonia (\(\ce{NH_3}\)):

    Example \(\PageIndex{2}\): Acid-Base Reaction (Proton Transfer)

    Write and equation for the reaction that would occur between hydrocyanic acid (\(\ce{HCN}\)) and ammonia (\(\ce{NH_3}\)).

    Solution

    This is a proton transfer reaction. The acid will donate a proton and the base will accept a proton.

    \(\ce{HCN(aq)+ NH_3(aq) \rightleftharpoons CN^-(aq) + NH_4^+ (aq)}\)

    Amphiprotic Species

    Many molecules and ions may either gain or lose a proton under the appropriate conditions. Such species are said to be amphiprotic. Another term used to describe such species is amphoteric, which is a more general term for a species that may act either as an acid or a base by any definition (not just the Brønsted-Lowry one). Consider for example the bicarbonate ion, which may either donate or accept a proton as shown here:

    \[\ce{HCO_3^- (aq) + H_2O (l) \rightleftharpoons CO_3^{2-} (aq) + H_3O^+ (aq)}\]

    \[\ce{HCO_3^- (aq) + H_2O (l) \rightleftharpoons H_2CO_3 (aq) + OH^- (aq)}\]

    Example \(\PageIndex{3}\): The Acid-Base Behavior of an AmphoPROTIC Substance

    Write separate equations representing the ionization in water of \(\ce{HSO3-}\)

    1. as an acid
    2. as a base

    Solution

    1. \(\ce{HSO_3^- (aq) + H_2O (aq) \rightleftharpoons SO_3^{2-}(aq) + H_3O^+ (aq)}\)
    2. \(\ce{HSO_3^- (aq) + H_2O (aq) \rightleftharpoons H_2SO_3(aq)+ OH^-(aq)}\)

    Exercise \(\PageIndex{1}\)

    Write separate equations representing the reaction of \(\ce{H2PO4-}\)

    1. as a base with HBr
    2. as an acid with \(\ce{OH^-}\)
    Answer a

    \(\ce{H2PO4-}(aq)+\ce{HBr}(aq)\rightleftharpoons \ce{H3PO4}(aq)+\ce{Br-}(aq)\)

    Answer b

    \(\ce{H2PO4-}(aq)+\ce{OH^-} (aq)\rightleftharpoons \ce{HPO4^2-}(aq)+ \ce{H_2O}_{(l)} \)

    In the preceding paragraphs we saw that water can function as either an acid or a base, depending on the nature of the solute dissolved in it. The amphoprotic nature of water plays a central role in aqueous acid-base reactivity and will be discussed further in the next section.

    Summary

    A compound that can donate a proton (a hydrogen ion) to another compound is called a Brønsted-Lowry acid. The compound that accepts the proton is called a Brønsted-Lowry base. The species remaining after a Brønsted-Lowry acid has lost a proton is the conjugate base of the acid. The species formed when a Brønsted-Lowry base gains a proton is the conjugate acid of the base. Thus, an acid-base reaction occurs when a proton is transferred from an acid to a base, with formation of the conjugate base of the reactant acid and formation of the conjugate acid of the reactant base. Amphiprotic species can act as both proton donors and proton acceptors. Water is an important amphiprotic species.

    Glossary

    acid ionization
    reaction involving the transfer of a proton from an acid to water, yielding hydronium ions and the conjugate base of the acid
    amphiprotic
    species that may either gain or lose a proton in a reaction
    amphoteric
    species that can act as either an acid or a base
    base ionization
    reaction involving the transfer of a proton from water to a base, yielding hydroxide ions and the conjugate acid of the base
    Brønsted-Lowry acid
    proton donor
    Brønsted-Lowry base
    proton acceptor
    conjugate acid
    substance formed when a base gains a proton
    conjugate base
    substance formed when an acid loses a proton

    Contributors and Attributions

    12.1: Brønsted–Lowry Acids and Bases is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.