LECTURE-10
ACIDS BASES AND SALT
• Arrhenius acid is a hydrogen-containing compound that, in
water, produces hydrogen ions (H+ ions). The acidic species
in Arrhenius theory is thus the hydrogen ion.
• Arrhenius base is a hydroxide-containing compound that, in
water, produces hydroxide ions (OH- ions). The basic species
in Arrhenius theory is thus the hydroxide ion. For this
reason, Arrhenius bases are also called hydroxide bases.
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IONIZATION
is the process in which individual positive and negative ions are
produced from a molecular compound that is dissolved in
solution.
BRONSTED–LOWRY ACID–BASE THEORY
Although it is widely used, Arrhenius acid–base theory has
some shortcomings.
• It is restricted to aqueous solution, and it does not explain
why compounds like ammonia (NH3), which do not contain
hydroxide ion, produce a basic water solution.
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A Bronsted–Lowry acid
i s a substance that can donate a proton (H+ ion) to some
other substance.
A Bronsted–Lowry base
is a substance that can accept a proton (H+ ion) from some
other substance.
In short, a Brønsted–Lowry acid is a proton donor (or
hydrogen ion donor), and a Brønsted–Lowry base is a proton
acceptor (or hydrogen ion acceptor).
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Brønsted–Lowry acid–base theory also includes the concept
that hydrogen ions in an aqueous solution do not exist in the
free state but, rather, react with water to form hydronium
ions.
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Simple Bronsted–Lowry acid–base reaction. When gaseous
hydrogen chloride dissolves in water, it forms hydroc
hloric acid.
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It is not necessary that a water molecule be one of the
reactants in a Bronsted–Lowry acid–base reaction; the
reaction does not have to take place in the liquid state.
Bronsted– Lowry acid–base theory can be used to describe
gas-phase reactions.
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Conjugate Acid–Base Pairs
The equilibrium mixture for a Bronsted–Lowry acid–base
reaction always has two acids and two bases present.
A conjugate acid–base pair is two species, one an acid and one
a base, that differ from each other through the loss or gain of a
proton (H+ ion).
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The two conjugate acid–base pairs are HF and F, and H3O+ and
H2O.
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• A conjugate acid
is the species formed when a proton (H+ ion) is added to a
Bronsted–Lowry base. The H3O+ ion is the conjugate acid of a
H2O molecule.
• A conjugate base
is the species that remains when a proton (H+ ion) is removed
from a Bronsted–Lowry acid. The H2O molecule is the conjugate
base of the H3O+ ion.
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• AMPHIPROTIC SUBSTANCES
An amphiprotic substance is a substance that can either lose or
accept a proton and thus can function as either a Bronsted–Lowry
acid or a Bronsted– Lowry base. Water is the most common
amphiprotic substance. Water functions as a base in the first of the
following two reactions and as an acid in the second.
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MONO-, DI-, AND TRIPROTIC ACIDS
• A monoprotic acid is an acid that supplies one proton (H+
ion) per molecule during an acid–base reaction.
• A diprotic acid is an acid that supplies two protons (H+ ions)
per molecule during an acid–base reaction. The transfer of
protons for a diprotic acid always occurs in steps. For H2CO3,
the two steps are
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• A triprotic acid is an acid that supplies three protons (H+ ions)
per molecule during an acid–base reaction. The three protontransfer steps for Phosphoric acid are
• A polyprotic acid is an acid that supplies two or more protons
(H+ ions) during an acid–base reaction.
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STRENGTHS OF ACIDS AND BASES
• A strong acid is an acid that transfers 100%, or very nearly
100%, of its protons (H+ ions) to water in an aqueous solution.
• A weak acid is an acid that transfers only a small percentage of
its protons (H+ ions) to water in an aqueous solution. The
majority of acids are weak rather than strong. Only seven
commonly acids are strong. Their chemical formulas and names
are given in the following.
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SALTS
• A salt is an ionic compound containing a metal or polyatomic ion
as the positive ion and a nonmetal or polyatomic ion (except
hydroxide) as the negative ion.
ACID–BASE NEUTRALIZATION CHEMICAL REACTIONS
• Neutralization chemical reaction is the chemical reaction between
an acid and a hydroxide base in which a salt and water are the
products.
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• THE pH CONCEPT
• The pH scale is a scale of small numbers that is used to
specify molar hydronium ion concentration in an aqueous
solution. The calculation of pH scale values involves the use
of logarithms. The pH is the negative logarithm of an
aqueous solution’s molar hydronium ion concentration.
Expressed mathematically, the definition of pH is
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Relationships among pH values, [H3O+], and [OH-] at 24oC.
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BUFFERS
• A buffer is an aqueous solution containing substances that prevent
major changes in solution pH when small amounts of acid or base
are added to it.
• Buffers contain two active chemical species:
(1) A substance to react with and remove added base, and
(2) A substance to react with and remove added acid. Typically, a
buffer system is composed of a weak acid and its conjugate base —
that is, a conjugate acid–base pair.
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