The Arrhenius
Theory of
Acids
Two important theories have been developed to explain the properties of acids, bases, and salts. These
are the Arrhenius Theory and the Bronsted-Lowry Theory. Here, we’ll look at the Arrhenius theory.
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.
In 1887, a Swedish chemist, Svante August Arrhenius proposed that an acid is any substance
that releases hydrogen or H + ions in water.
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.
e.g. HCl(g)
An example is the gas hydrogen chloride, HCl
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.
e.g. HCl(g)
H Cl
This pair of e–s
is being shared
Looking at the Lewis structure of hydrogen chloride, we see that there is a covalent bond
between the hydrogen and chlorine atoms. They are sharing a pair of electrons.
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.
e.g. HCl(g)
A weak covalent bond
H Cl
However, the bond between hydrogen and chlorine is quite polar, and although it keeps the atoms
together in the gas phase, when HCl is added to water, this bond is too weak to keep H and Cl together.
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.
e.g. HCl(g)
H Cl
When HCl is added to water, (click) the molecule breaks up
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.
e.g. HCl(g)
H
Cl
The chlorine atom, being more electronegative than the hydrogen atom, takes both of the
shared electrons with it.
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.
e.g. HCl(g)
H+
Cl
Has lost 1 e–, so
its charge is +
Because the hydrogen atom has lost its electron, it acquires a positive charge
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.
e.g. HCl(g)
H+
Cl
Has lost 1 e–, so
its charge is +
Has gained 1 e–,
so its charge is –
And because chlorine has gained one electron, it acquires a negative charge. It is now called a
chloride ion, Cl minus.
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.


HCl (g)  H(aq)
 Cl (aq)
e.g. HCl(g)
This whole process can be summarized by the equation: HCl gas gives H+ aqueous plus Cl
minus aqueous. Aqueous means these ions are surrounded by water molecules.
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.


HCl (g)  H(aq)
 Cl (aq)
e.g. HCl(g)
H+ Cl–
H+
Cl–
H+
H+

H(aq)
Cl–
Cl–

Cl (aq)
So Arrhenius pictured an aqueous solution of HCl as a collection of H+ and Cl minus ions
floating around freely in the water.
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.


HCl (g)  H(aq)
 Cl (aq)
e.g. HCl(g)
H+ Cl–
H+
Cl–
H+
Cl–
+
H
Cl–
H2O(l)
The water molecules are not shown here in order to keep the diagram simple. Water is given a
blue colour in this diagram.
Arrhenius Theory
An ACID is any substance that releases hydrogen
(H+) ions in water.

HCl (g )  H(aq)
 Cl (aq )
e.g. HCl(g)
H+ Cl–
H+
Cl–
H+
Cl–
+
H
Cl–
H2O(l)
Because this is a water solution, the subscripts aq for aqueous are used for the hydrogen and
chloride ions in the equation.
Arrhenius Theory
A BASE is any substance that releases hydroxide
(OH–) ions in water.
According to Arrhenius theory, a base is any substance that releases hydroxide, or OH minus
ions in water.
Arrhenius Theory
A BASE is any substance that releases hydroxide
(OH–) ions in water.
e.g. NaOH(s)
An example is solid sodium hydroxide
Arrhenius Theory
A BASE is any substance that releases hydroxide
(OH–) ions in water.
e.g. NaOH(s)
Na+
OH–
Na+
OH–
OH–
Na+
OH–
Na+
Na+
OH–
Na+
OH–
OH–
Na+
OH–
Na+
Because NaOH is an ionic compound, solid NaOH is composed of a crystal lattice of sodium and
hydroxide ions. Here we show a 2-D representation of a few of the ions.
Na+
OH–
Na+
OH–
OH–
Na+
OH–
Na+
Na+
OH–
Na+
OH–
OH–
Na+
OH–
Na+
When a piece of solid NaOH is placed in water, the polar water molecules attract the ions (click)
and pull them away from the crystal.
Na+
OH–
Na+
OH–
Dissociation
Na+ OH– Na+
OH–
Na+
OH–
Na+
OH–
OH–
Na+
OH–
Na+
This process is called (click) dissociation
Arrhenius Theory
A BASE is any substance that releases hydroxide
(OH–) ions in water.
e.g.
NaOH  Na   OH 
(s)
(aq)
(aq)
Dissociation of NaOH
The dissociation of NaOH is shown by this equation, NaOH solid gives Na+ aqueous plus OH
minus aqueous.
Arrhenius Theory
A BASE is any substance that releases hydroxide
(OH–) ions in water. NaOH  Na   OH 
(s)
Na+
OH–
Na+
OH–
Na+
OH–
Na+
(aq)
(aq)


Na(aq)
 OH(aq)
OH–
So according to Arrhenius Theory an aqueous solution of the base sodium hydroxide, could be
viewed as free and mobile Na + and OH minus ions, surrounded by water molecules.
Arrhenius Theory
A SALT is


NaCl (s)  Na(aq)
 Cl (aq)
The other type of ionic compound Arrhenius defines is a salt. A salt is
Arrhenius Theory
A SALT is an ionic compound


NaCl (s)  Na(aq)
 Cl (aq)
An ionic compound
Arrhenius Theory
A SALT is an ionic compound that releases a
cation other than H+


NaCl (s)  Na(aq)
 Cl (aq)
That releases a cation other than H+
Arrhenius Theory
A SALT is an ionic compound that releases a
cation other than H+ and an anion other than OH–


in water. e.g. NaCl (s)  Na(aq)
 Cl (aq)
And an anion other than OH minus in water.
Arrhenius Theory
A SALT is an ionic compound that releases a
cation other than H+ and an anion other than OH–


in water. e.g. NaCl (s)  Na(aq)
 Cl (aq)
An example is the common salt, sodium chloride, NaCl
Arrhenius Theory
A SALT is an ionic compound that releases a
cation other than H+ and an anion other than OH–


in water. e.g. NaCl (s)  Na(aq)
 Cl (aq)
NaCl dissociates into Na+ ions
Arrhenius Theory
A SALT is an ionic compound that releases a
cation other than H+ and an anion other than OH–


in water. e.g. NaCl (s)  Na(aq)
 Cl (aq)
And Cl minus ions.
Arrhenius Theory
A SALT is an ionic compound that releases a
cation other than H+ and an anion other than OH–


in water. e.g. NaCl (s)  Na(aq)
 Cl (aq)
Na+
Cl–
Na+
Cl–
Na+
Cl–
Na+


Na(aq)
 Cl (aq)
Cl–
So a water solution of NaCl, would contain mobile Na+ and Cl minus ions.
Some Arrhenius acids are shown here:
Acids
HBr
HNO3
H2SO4
H3PO4
CH3COOH
A few Arrhenius acids are shown here.
Some Arrhenius acids are shown here:
Acids
HBr
HNO3
H2SO4
H3PO4
CH3COOH
Notice their formulas start with H. This is the hydrogen atom they lose as H + when they
dissociate in water.
Some Arrhenius acids are shown here:
Acids
HBr
HNO3
H2SO4
H3PO4
CH3COOH
In acetic acid, which is often written as CH3COOH, the H that the acid loses is written at the
end of the formula instead of the beginning. Organic acids are often written this way.
Some Arrhenius acids are shown here:
Acids
HBr
HNO3
H2SO4
H3PO4
CH3COOH
You just have to watch for the COOH at the end of the formula in order to recognize these as
organic acids.
Some Arrhenius bases are shown here:
Acids
Bases
HBr
KOH
HNO3
Ba(OH)2
H2SO4
NH4OH
H3PO4
LiOH
CH3COOH
Sr(OH)2
A few Arrhenius bases are shown here.
Some Arrhenius bases are shown here:
Acids
Bases
HBr
KOH
HNO3
Ba(OH)2
H2SO4
NH4OH
H3PO4
LiOH
CH3COOH
Sr(OH)2
Notice their formulas all end in OH. This OH is lost as a hydroxide, or OH minus ion when these
compounds dissociate in water.
Some Arrhenius bases are shown here:
Acids
Bases
HBr
KOH
HNO3
Ba(OH)2
H2SO4
NH4OH
H3PO4
LiOH
CH3COOH
Sr(OH)2
The cation in a base must be either a metal, or the ammonium ion, NH4 +.
Consider the compound CH3OH
Let’s have a look at the compound CH3OH.
Consider the compound CH3OH
methanol or
methyl alcohol
Its called methanol, or methyl alcohol.
Consider the compound CH3OH
At first, it may appear to be a base because it has OH at the end of its formula.
Consider the compound CH3OH
NOT A BASE
However, CH3OH is not a base.
Consider the compound CH3OH
H
H C O H
H
The structure of methanol can be represented like this.
Consider the compound CH3OH
H
H C O H
H
strong
covalent bond
The bond between the carbon and oxygen is a strong covalent bond, and is not easily broken.
Consider the compound CH3OH
H
OH– is not
released
H C O H
H
strong
covalent bond
Therefore, an OH minus ion is NOT released by CH3OH in water. And neither are any other
ions. All the covalent bonds in methanol are too strong to be broken by water.
Consider the compound CH3OH
H
H C O H
H
CH3OH is a molecular
compound, and does NOT
dissociate into ions in water.
Methanol is a molecular compound, and does not dissociate into ions in water.
Compounds in which OH is attached to a
carbon atom are NOT bases.
So we can summarize by saying that compounds in which OH is attached to a carbon atom are
NOT bases.
Compounds in which OH is attached to a
carbon atom are NOT bases.
Examples:
• CH3OH (methanol)
• CH3CH2OH (ethanol)
• CH3CH2CH2OH (1-propanol)
Some examples of these are alcohols like methanol, ethanol, or 1-propanol
Compounds in which end in “COOH” are
organic acids, and are NOT bases either.
And remember, compounds whose formula ends in COOH are organic acids, and therefore are
not bases either.
It should also be noted that a compound
called phenol (C6H5OH) dissolves in water,
but it is NOT a base.
O
H
Phenol (C6H5OH)
It should also be noted that a compound called phenol (C6H5OH) dissolves in water, but it is
NOT a base.
O
H+
In water, a few phenol molecules will lose this H as an H+ ion.
O
H+
Hydrogen ion
Phenoxy ion (C6H5O–)
And the negative charge on the resulting phenoxy ion (C6H5O–), is stabilized by the presence of
the benzene ring.
O
H+
Hydrogen ion
Phenoxy ion (C6H5O–)
The presence of free hydrogen ions
O
H+
Hydrogen ion
Phenoxy ion (C6H5O–)
Means this is an acid.
O
H+
Hydrogen ion
Phenoxy ion (C6H5O–)
And because it produces hydrogen ions rather than hydroxide ions, we can state that phenol
with the formula C6H5OH is NOT a base.
Some Arrhenius salts are shown here:
Acids
Bases
Salts
HBr
KOH
NaNO3
HNO3
Ba(OH)2
KBr
H2SO4
NH4OH
NH4Cl
H3PO4
LiOH
Li2SO4
CH3COOH
Sr(OH)2
NaCH3COO
In this column of the table, we see some salts. Notice, they are all ionic compounds that neither
start with H, nor end in OH.
Some Arrhenius salts are shown here:
Acids
Bases
Salts
HBr
KOH
NaNO3
HNO3
Ba(OH)2
KBr
H2SO4
NH4OH
NH4Cl
H3PO4
LiOH
Li2SO4
CH3COOH
Sr(OH)2
NaCH3COO
The last one, NaCH3COO looks like it’s organic. However, since it starts with Na, we know that
it must be a salt.
We can find both Na+ and CH3COO minus on the ion table.
NaCH3COO is ionic
So we know that NaCH3COO, called sodium ethanoate, or more commonly sodium acetate, is
ionic. And because it doesn’t start with H, or end in OH,
NaCH3COO is an
ionic salt
It is an ionic salt.