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Acids and Bases

Chemistry
by

Stephanie Duverge

on 11 March 2016

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Transcript of Acids and Bases

Monoprotic and Polyprotic Acids
For most uses, scientists found the Arrhenius definition of acids and bases to be adequate. However, as scientists further investigated acid-base behavior, they found that some substances acted as acids or bases when they were not in a water solution.
Brønsted-Lowry
acid
is a molecule or ion that is a proton donor.
An acid that can donate only one proton (hydrogen ion) per molecule is known as a
monoprotic acid
.
Brønsted-Lowry Acids & Bases
Acid-Base Theories
In 1923, the Danish chemist J. N. Brønsted and the English chemist T. M. Lowry independently expanded the Arrhenius acid definition.
A Brønsted-Lowry
base
is a molecule or ion that is a proton acceptor.
In a Brønsted-Lowry acid-base reaction, protons are transferred from one reactant (the acid) to another (the base).
Water can act as an acid. Since its donating one proton.
Perchloric acid, HClO
Hydrochloric acid, HCl
Nitric acid, HNO
3
4
A polyprotic acid
is an acid that can donate more than one proton per

molecule.
Sulfuric acid, H SO
Phosphoric acid, H PO
2 4
3 4
Sulfuric acid is the type of polyprotic acid that can donate two protons per molecule, and it is therefore known as a diprotic acid.
H SO + H O
H O + HSO
2 4 2 3 4
HSO + H O
H O + SO
4 2 3 4
+ -
+ 2-
Phosphoric acid is the type of polyprotic acid known as a triprotic
acid—an acid able to donate three protons per molecule.
Lewis Acids & Bases
The Arrhenius and Brønsted-Lowry definitions describe most acids and bases. Both definitions assume that the acid contains or produces hydrogen ions. A third acid classification, based
on bonding and structure, includes, as acids, substances that do not contain hydrogen at all.
This definition of acids was introduced in 1923 by G. N. Lewis, the American chemist whose name was given to electron-dot structures.

Lewis’s definition emphasizes the role of electron pairs in acid-base reactions.

A Lewis acid is an atom, ion, or molecule that accepts an electron pair to form a covalent bond.
The Lewis definition is the broadest of the three acid definitions you have read about so far. It applies to any species that can accept an electron pair to form a covalent bond with another species.

A bare proton H is a Lewis acid in reactions in which it forms a covalent bond.
+
Brønsted-Lowry
Arrhenius
Acid-Base Definition
Lewis
Acid: H donor
Base: OH donor
+
-
Acid: H donor
Base: H acceptor
+
-
Acid: Electron pair acceptor
Base: Electron pair donor
Conjugated Acids & Bases
The species that remains after a Brønsted-Lowry acid has given up a proton is the
conjugate base
of that acid.
Acid-Base Reactions
The species that is formed when a Brønsted-Lowry base gains a proton is the
conjugate acid
of that base.
Strength of Conjugate Acids and Bases
The extent of the reaction between a Brønsted-Lowry acid and base
depends on the relative strengths of the acids and bases involved.
The stronger an acid is, the weaker its conjugate base; the stronger a base is, the weaker its conjugate acid.
HClO +H O H O + ClO
4(aq) 2 (l)---> 3 (aq) 4 (aq)
stronger acid stronger base weaker acid weaker base
+
CH COOH + H O H O + CH COO
weaker acid weaker base stronger acid stronger base
3 (aq) 2 (l) ---> 3 (aq) 3 (aq)
+ -
Note that in the reactions for both perchloric acid and acetic acid, the favored direction is toward the weaker acid and the weaker base.

This observation leads to a second important general conclusion: proton-transfer reactions favor the production of the weaker acid and the weaker base.
Any species that can react as
either an acid or a base is
described as amphoteric.
–OH in a Molecule
Molecular compounds containing -OH groups can be acidic or amphoteric.
The covalently bonded -OH group in an acid is referred to as a
hydroxyl group.
The behavior of a compound is affected by the number of oxygen atoms bonded to the atom connected to the -OH group.
The larger the number of such oxygen atoms is, the more acidic the compound is.
Neutralization Reactions
There are many common examples of acidic compounds reacting with basic compounds, each neutralizing the other.
In aqueous solutions,
neutralization
is the reaction of hydronium ions and hydroxide ions to form water
molecules.
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
An acid-base reaction occurs in aqueous solution between hydrochloric acid, a strong acid that completely ionizes to produce H3O+, and sodium hydroxide, a strong base that completely dissociates to produce OH .
A
salt
is an ionic compound composed of a cation from a base and an anion from an acid.
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