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Acids are substances that ionize in aqueous solutions to form hydrogen ions, thereby increasing the concentration of H+ ions.
Because hydrogen atom consists of a proton and an electron, H+ is simply a proton.
Thus, acids are often called proton donors.
Molecules of different acids can ionize to
form different numbers of H+ ions.
Both hydrochloric acid and nitric acid are
monoprotic acids, which yield one per
molecule of acid.
Sulfuric acid is a diprotic acid, one that
yields two H+ per molecule of acid.
Bases are substances that accept H+
ions.
Bases produce hydroxide ions when
they dissolve in water.
When dissolved in water, they
dissociate into their component ions,
introducing OH- ions into the solution.
ACID BASE
Sour taste Bitter taste
Neutralizes bases Neutralizes acids
Turns litmus paper blue to red Turns litmus paper red to blue
Soapy and slippery feeling
Indicator Color in strongly acidic
solution
pH at which color changes
Color in strongly alkaline
solution
Methyl orange Red 4 Yellow
Litmus Red 7 Blue
Phenolphthalein Colorless 9 Red
Screened Methyl Orange
Red 4 Green
Acids and bases that are strong
electrolytes (completely ionized in
solution) are called strong acids and
strong bases.
Those that are weak electrolytes (partly
ionized) are called weak acids and weak
bases.
STRONG ACIDS STRONG BASES
Hydrochloric acid Lithium hydroxide
Hydrobromic acid Sodium hydroxide
Hydroiodic acid Potassium hydroxide
Chloric acid Rubidium hydroxide
Perchloric acid Cesium hydroxide
Nitric acid Calcium hydroxide
Sulfuric acid Strontium hydroxide Barium hydroxide
To classify a soluble substance as a strong
electrolyte, weak electrolyte, or nonelectrolyte,
we simply use the following table:
Strong Electrolyte
Weak Electrolyte
Nonelectrolyte
Ionic All None None
Molecular Strong acids Weak acids
Weak bases All other compounds
If an acid is not listed, it is probably a weak electrolyte. NH3 is only a weak base that we consider.
When a solution of an acid and that of
a base are mixed, a neutralization
reaction occurs.
The products of the reaction have
none of the characteristics properties
of either the acidic and the basic
solutions.
By analogy to this reaction, the term salt
has come to mean any ionic compound
whose cation comes from a base and
whose anion comes from an acid.
A neutralization reaction between an acid
and a metal hydroxide produces water
and salt.
1. Arrhenius Acids and Bases
2. Bronsted-Lowry Acids and Bases
3. Lewis Acids and Bases
Swedish chemist Svante Arrhenius (1859-1927)
proposed a revolutionary way of defining and
thinking about acids and bases.
He said that acids are hydrogen-containing
compounds that ionize to yield hydrogen ions (H+)
in aqueous solutions.
He also said that bases are compounds that ionize
to yield hydroxide ions (OH-) in aqueous solutions.
Acids that contain one ionizable hydrogen, such as
nitric acid, are called monoprotic acids.
Acids that contain two ionizable hydrogens, such
as sulfuric acid, are called diprotic acids.
Acids that contain three ionizable hydrogens, such
as phosphoric acid, are called triprotic acids.
In 1923, the Danish chemist Johannes Bronsted
and the English chemist Thomas Lowry
independently proposed a new definition.
Defines an acid as a hydrogen-ion donor.
Defines a base as a hydrogen-ion acceptor.
A conjugate base is the particle that remains when
an acid has donated a hydrogen ion.
A conjugate base is the particle that remains when
an acid has donated a hydrogen ion.
Conjugate acids and bases are always paired with
a base or an acid, respectively.
A conjugate acid-base pair consists of two
substances related by the loss or gain of a single
hydrogen ion.
The third theory of acids and bases was proposed
by Gilbert Lewis.
Lewis focused on the donation or acceptance of a
pair of electrons during a reaction.
This concept is more general than either the
Arrhenius theory or the Bronsted-Lowry theory.
A Lewis acid is a substance that can accept a pair
of electrons to form a covalent bond.
A Lewis base is a substance that can donate a
pair of electrons to form a covalent bond.
A hydrogen ion (Bronsted-Lowry acid) can accept
apair of electrons in forming a bond.
A hydrogen ion, therefore, is also a Lewis acid.
A Bronsted-Lowry base, or a substance that
accepts a hydrogen ion, must have a pair of
electrons available and is also a Lewis base.
A widely used system for expressing [H+] is the pH
scale, proposed in 1909 by the Danish scientist
Soren Sorenson.
It ranges from 0-14, neutral solutions have a pH of
7.
A pH of 10 is strongly basic.
The pH of a solution is the negative logarithm of
the hydrogen-ion concentration.
The pH may be represented
mathematically using the following
equation:
pH = - log [H+]
Similarly, the pOH of a solution equals
the negative logarithm of the
hydroxide-ion concentration.
pOH = - log [OH-]
A neutral solution has a pOH of 7.
A solution with a pOH less than 7 is basic.
A solution with a pOH greater than 7 is acidic.
A simple relationship between pH and pOH makes
it easy to find either one when the other is known.
pH + pOH = 14
pH = 14 – pOH
pOH = 14 - pH
1 2 3 4 5 6 7 8 9 10 11 12 13 14
10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-1110-12 10-1310-14
NE
UT
RA
L
Increasing Basicity Increasing Acidity
What is the pH of a solution with a hydrogen-ion concentration of 1.0 x 10-10 M?
Find the pH of each solution: ◦ [H+] = 1.0 x 10 -4 M ◦ [H+] = 0.0010 M
◦ [H+] = 1.0 x 10 -9 M ◦ [H+] = 1.0 x 10 -12 M
◦ [H+] = 0.010 M
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