Introduction to Acids and Bases
The earliest definition was given by Arrhenius:
•An acid contains a hydrogen atom and dissolves
in water to form a hydrogen ion, H+.
HCl(g)
H+(aq)
+
Cl−(aq)
acid
•A base contains hydroxide and dissolves in water
to form −OH.
NaOH(s)
Na+(aq)
+
−OH(aq)
base
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Introduction to Acids and Bases
•The Arrhenius definition correctly predicts the
behavior of many acids and bases.
•However, this definition is limited and sometimes
inaccurate.
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Introduction to Acids and Bases
The Brønsted–Lowry definition is more widely used:
•A Brønsted–Lowry acid is a proton (H+) donor.
•A Brønsted–Lowry base is a proton (H+) acceptor.
This proton is donated.
HCl(g) + H2O(l)
H3O+(aq) + Cl−(aq)
•HCl is a Brønsted–Lowry acid because it donates
a proton to the solvent water.
•H2O is a Brønsted–Lowry base because it accepts
a proton from HCl.
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Introduction to Acids and Bases
Brønsted–Lowry Bases
•A Brønsted–Lowry base is a proton acceptor,
so it must be able to form a bond to a proton.
•A base must contain a lone pair of electrons that
can be used to form a new bond to the proton.
This e− pair forms a new
bond to a H from H2O.
H
N
H
+
H
Brønsted–Lowry
base
H2O(l)
+
H
H
N
H
+
−OH(aq)
H
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A Lewis acid is defined to be any
species that accepts lone pair
electrons.
A Lewis base is any species that
donates lone pair electrons.
Thus, H+ is a Lewis acid, since it can
accept a lone pair, while OH− and NH3
are Lewis bases, both of which donate
a lone pair.
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Dissociation of Water
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The pH Scale
Calculating pH
pH = −log [H3O+]
The lower the pH, the higher the concentration of
H3O+.
•Acidic solution:
pH < 7  [H3O+] > 1 x 10−7
•Neutral solution:
pH = 7  [H3O+] = 1 x 10−7
•Basic solution:
pH > 7  [H3O+] < 1 x 10−7
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