Acid–Base Theories (Arrhenius, Bronsted, and Lewis)
** Several
acid–base theories have been proposed to explain or classify acidic and basic
properties of substances.
** You are
probably most familiar with the Arrhenius theory, which is applicable only to
water.
** Other
theories are more general and are applicable to other solvents or even the gas
phase.
** We
describe the common acid–base theories here.
(1) ARRHENIUS THEORY → H+ and OH−
** Arrhenius,
as a graduate student, introduced a dramatically new theory that:
An acid: is any substance that
ionizes (partially or completely) in water to give hydrogen ions (which
associate with the solvent to give hydronium ions, H3O+)
A base: is any substance
that ionizes in water to give hydroxide ions.
Weak
(partially ionized) bases generally ionize as follows:
while strong
bases such as metal hydroxides (e.g., NaOH) dissociate as:
** This
theory is obviously restricted to water as the solvent.
(2) THEORY OF SOLVENT SYSTEMS → Solvent cations and anion
** In 1905,
Franklin was working in liquid NH3 as solvent and noticed the
similarity with acid-base behavior in water.
** In 1925,
Germann, working with liquid COCl2 as solvent is
observed the similarities as well and formulated a general solvent system
concept of acids and bases.
** This
theory recognizes the ionization of a solvent to give a cation and an anion;
for example:
An acid: is defined as a
solute that yields the characteristic cation of the solvent
A base: is a solute that yields the characteristic anion of the
solvent.
Examples:
NH4Cl (which produces
ammoniated NH4+, i.e., [NH4(NH3)+],
and Cl−) is a strong acid in liquid ammonia (similar to HCl in
water: HCl + H2O → H3O+ + Cl−)
NaNH2 is a strong base
in ammonia (similar to NaOH in water); both of these compounds ionize to give
the characteristic solvent cation and anion, respectively.
Ethanol ionizes as
follows: 2C2H5OH ⇔ C2H5OH2+ +
C2H5O−. Hence, sodium ethoxide, NaOC2H5 ,
is a strong base in this solvent.
Summary: Franklin and
Germann’s theory is similar to the Arrhenius theory but is applicable also to
other ionizable solvents.
(3) BRØNSTED - LOWRY THEORY →Taking and giving protons
** The theory
of solvent systems is suitable for ionizable solvents, but it is not applicable
to acid–base reactions in nonionizable solvents such as benzene or dioxane.
** In 1923,
Brønsted and Lowry independently described what is now known as the
Brønsted–Lowry theory.
** This
theory states that:
An acid: is any substance
that can donate a proton
A base: is any substance
that can accept a proton.
** Thus, we
can write a “half-reaction”a
** The acid
and base of a half-reaction are called conjugate pairs.
** Free
protons do not exist in solution, and there must be a proton acceptor (base)
before a proton donor (acid) will release its proton. That is, there must be a
combination of two half-reactions.
** Another
way to look at it is that an acid is an acid because it can lose a proton.
However, it cannot exhibit its acidic behavior unless there is a base present
to accept the proton. It is like being wealthy on a deserted island with no one
to accept your money.
** Some
acid–base reactions in different solvents are illustrated in the
Table:
(a) In the
first example, acetate ion is the conjugate base of acetic acid and ammonium
ion is the conjugate acid of ammonia.
(b) The first
four examples represent ionization of an acid or a base in a solvent,
(c) the
others represent a neutralization reaction between an acid and a base in the
solvent.
** It is
apparent from the above definition that a substance cannot act as an acid
unless a base is present to accept the protons. Thus, acids will undergo
complete or partial ionization in basic solvents such as water, liquid ammonia,
or ethanol, depending on the basicity of the solvent and the strength of the
acid. But in neutral or “inert” solvents, ionization is insignificant. However,
ionization in the solvent is not a prerequisite for an acid–base reaction, as
in the last example in the table, where picric acid reacts with aniline.
Summary: The
Brønsted–Lowry theory assumes a transfer of protons from an acid to a base,
i.e., conjugate pairs.
(4) LEWIS THEORY →Taking and giving electrons
** In 1923,
G. N. Lewis introduced the electronic theory of acids and bases. In
the Lewis theory:
An acid: is a substance
that can accept an electron pair
A base: is a substance
that can donate an electron pair.
** The latter
frequently contains an oxygen or a nitrogen as the electron donor. Thus,
nonhydrogen-containing substances are included as acids.
** Examples
of acid–base reactions in the Lewis theory are as follows:
In the second
example, aluminum chloride is an acid and ether is a base.
Summary: The Lewis theory
assumes a donation (sharing) of electrons from a base to an acid.
Reference: Analytical chemistry/ Seventh
edition / Gary D. Christian, University of Washington, Purnendu K. (Sandy)
Dasgupta, University of Texas at Arlington, Kevin A. Schug, University of Texas
at Arlington.
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