Acid-Base Theories
Arrhenius Theory
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In 1887, a Swedish chemist, Svante Arrhenius, published a paper
concerning acids and bases.
He knew that solutions containing acids or bases conducted an
electric current.
He concluded that these substances released charged particles
when dissolved.
He called these charged particles ions (wanderers).
He concluded that acids were substances which separated
(ionized) in water solution to produce hydrogen ions, H+, or
free protons).
He also believed that bases were substances which ionized to
produce hydroxide ions, OH- in water solution.
HCl --> H+ + ClNaOH --> Na+ + OH-
Bronsted-Lowry Theory
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As the knowledge of catalysts and nonaqueous solutions increased, it became necessary to
redefine the terms acid base.
In l923, an English scientist, T. M. Lowry, and a Danish scientist, J. N. Bronsted, independently
proposed a new definition.
They stated that in a chemical reaction, any substance which donates a proton is an
acid and any substance which accepts a proton is a base.
– E.g. When hydrogen chloride gas is dissolved in water, ions are
formed.
– HCl(aq) (acid) + H20(l) (base) ---> H3O+(aq) + Cl- (aq)
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In this reaction, hydrogen chloride is an acid, and water is a base. Notice that the hydrogen ion
has combined with a water molecule to form the polyatomic ion H30+, which is called the
hydronium.
There is strong evidence that the hydrogen ion is never found free as H+.
It is possible to have isolated H+ ions in the gas phase. But if a free hydrogen ion encounters a
water molecule, it attacks the unshared electron pairs on the oxygen in the water molecule and
forms a hydronium ion, H3O+. The chemical bond that forms between the water and hydrogen
ion is covalent and very strong. In an aqueous solution, essentially all of the H+ exists as H3O+.
H30+(aq) + Cl-(aq) ---> HCl(aq) + H2O(l)
In this reaction, the H3O+ ion is an acid. It acts as an acid because it donates a
proton to the chloride ion, which is a base.
Conjugates
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The hydronium ion is said to be the conjugate acid of the base,
water.
The chloride ion is called the conjugate base of the acid,
hydrochloric acid.
In general, any acid-base reaction is described as:
acid + base ---> conjugate base + conjugate acid
The conjugate base of an acid is the particle that remains after a
proton has been released by the acid. The conjugate acid of a base
is formed when the base acquires a proton from the acid.
Table 24-1 contains a list of some anions and their conjugate acids.
Sample reaction
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Consider what happens when ammonia gas is
added to water.
NH3(ag) + H20(l) ---> NH4+(aq) + OH-(aq)
base + acid ---> conjugate acid +
conjugate base
 In this reaction, water acts as an acid because it
donates a proton to the ammonia molecule. The
ammonium ion is the conjugate acid of
ammonia, a base, which receives a proton from
water. Hydroxide ion is the conjugate base.
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Lewis Theory
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In 1923, the same year that Bronsted and Lowry proposed their
theories, another new idea appeared.
Gilbert Newton Lewis, an American chemist, proposed an even
broader definition of acids and bases.
The same type of reasoning as Bronsted's and Lowry's led to his
proposals.
However, Lewis focused on electron transfer instead of proton
transfer.
He defined an acid as an electron-pair acceptor, and a base as an
electron-pair donor.
This definition is more general than Bronsted's. It applies to
solutions and reactions which do not even involve hydrogen or
hydrogen ions.
Acid-Base Behavior
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Consider a compound having the formula HOX.
If the element X is highly electronegative, it will have a strong
attraction for the electrons it is sharing with the oxygen.
– As these electrons are pulled toward X, the oxygen, in turn, will pull
strongly on the electrons it is sharing with the hydrogen.
– The hydrogen ion, or proton, would then be lost easily. In this case,
HOX is behaving as an acid.
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If the element X has a low electronegativity, the oxygen will tend to
pull the shared electrons away from X.
– The hydrogen will remain joined to the oxygen.
– Since in this case the formation of hydroxide ion, OH-, is likely, HOX is
behaving as a base.
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We know that nonmetals have high electronegativities and metals
low electronegativities.
We can conclude, then, that nonmetals will tend to form
acids, and metals will tend to form bases.
Amphoteric Behavior
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Some substances can react as either an acid or a base.
If one of these substances is in the presence of a proton
donor, then it reacts as a base.
In the presence of a proton acceptor, it acts as an acid.
Such a substance is said to be amphoteric.
Water is the most common amphoteric substance.
HCl (proton donor) + H2O (base) ---> H30+ + ClNH3 (proton acceptor) + H2O (acid) ---> NH4+ +
OH-
Acid-Base Neutralization
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An acid is composed of positive hydrogen ions combined with
negative nonmetallic ions.
Metallic bases are composed of negative hydroxide ions combined
with positive metallic ions.
An acid reacts with a base to form a salt and water.
– The water is formed from the hydrogen ion of the acid and the
hydroxide ion of the base.
– If the water is evaporated, the negative ions of the acid will unite with
the positive ions of the base to form a new compound called a salt.
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It would appear such a reaction should result in removal of all
hydrogen and hydroxide ions from solution.
– The resulting solution should neither an acid nor a base.
– We could say that the solution is neutral (neither acidic nor basic).
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The reaction of an acid and a base is called a neutralization
reaction.
Definition of a Salt
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A salt is a crystalline compound composed of the
negative ion of an acid and the positive ion of a base.
For example, if equivalent amounts of chloric acid and
sodium hydroxide react, sodium chlorate and water are
formed.
HCl03(aq) + NaOH(aq) ---> NaClO3(aq) + H20(l)
acid + base ---> a salt + water
Recall: Binary acids (prefix hydro-, suffix -ic) form salts
ending in -ide. As an example, hydrochloric acid forms
chloride salts. Ternary acids form salts in which -ic acids
form -ate salts and -ous acids form -ite salts. Prefixes
from the acid names remain in the salt names.