1
Arrhenius Theory
Acids are substances that produce H+ ions when they dissolve in water.
Bases are substances that react with H+ ions to produce water.
Alkalis are bases which dissolve in water to produce OH— ions.
The stronger the dissociation, the stronger the acid.
Note, H+ ions exist as Hydronium H3O+ ions, associating with water
molecules by coordinate bonds.
2
Arrhenius Theory
Arrhenius theory limits itself to the concentration of H+ and OH— ions
determining the strength of acids and bases.
Hence, this theory is restricted to polar solvents like water.
However, this is also why Hydrochloric and Ethanoic acid show acidity.
Strong Acid:
Weak Acid:
HCl → H+ + Cl—
CH3CO2H
⇌ H+ + CH3CO2—
+
−
Bronsted-Lowry
Theory
3 HCl(g)
+ H2O(l) → H3O (aq) + Cl (aq)
According to this, an acid is a proton (H+) donor, and bases are proton
Hydrochloric
acid
is
an
acid
because
it
donates
a
proton
acceptors.
to water. This means that water is acting as a Brønsted–
In the following reaction HCl is an acid because it donates a proton to water.
Lowry base. The water is accepting a proton.
This means that water is acting as a Brønsted– Lowry base. The water is
accepting a proton.
+
H donated
+
–
HCl(g) + H2O(l) → H3O (aq) + Cl (aq)
acid
base
4
Bronsted-Lowry Theory
Also, a base has to be a substance with a lone pair, to coordinate with the
proton.
HCl(g)
H2O(l)
+
+
H3O+(aq)
proton
Cl–(aq)
+
H
H
O
Cl
H
+
The proton, H , is donated
by the hydrogen chloride
H
O
–
H
Cl
H
…forming the
positive hydroxonium ion…
…and leaving a
negative chloride ion.
e 8.12 An acid is a proton donor. Hydrogen chloride is the acid in this reaction. A base is a proton acceptor. Water is the base in this reaction.
+
mber that a proton is a hydrogen ion, H .
5
Bronsted-Lowry Theory
NH3(g)
+
NH4+(aq)
H2O(l)
+
OH–(aq)
+
H
H
H
H
N
O
H
H
+
The proton, H , is donated
by the water and accepted
by the ammonia…
H
N
–
H
O
H
H
…forming
the positive
ammonium ion…
…and the
negative
hydroxide ion.
e 8.13 Water is the proton donor (it is the acid); ammonia is the proton acceptor (it is the base).
6
Bronsted-Lowry Theory
A conjugate acid is a base that has accepted a proton.
A conjugate base is an acid that has donated a proton.
NH3 + H2O → NH4+ + OH—
+
Hence, the NH4 ion is said to be the conjugate acid of NH3, and likewise, NH3
+
is the conjugate base of NH4 ion.
H2O is the conjugate acid of the OH— ion, and the OH— ion is the conjugate
base of H2O.
NH
(g) + (aq)
H2O(l)
(aq)
+ 3OH
NH4 (aq) + OH (aq)
+
•
H
O
is
the
conjugate
base
of
the
acid
H
O
.
2
3
acid
base
Bronsted-Lowry
Theory
In a conjugate pair, the acid has one proton more.
7 reaction
d as this
does not go
Theofconjugate
pairs for the equilibrium between
ctant is linked to a product by the transfer
a
This
suggests
that
a
substance
can
both
be
an
acid
and
a
base
we call this pair a conjugate pair. Consider
theand water to form ammonium ions and
ammonia
depending
on
the
situation.
ng reaction:
hydroxide ions are:
conjugate pair
nts HCl(g)
are acids
which
+ Hand
O(l)
2
base
acid
wing acid
reactions:
+
NH3 + H3O
+
−
O2
HCOOH
+
ClO
2
2
conjugate pair
conjugate pair
+
–
H3O (aq) + Cl (aq)
base
g at the forward reaction:
the conjugate base of the acid HCl
NH3(g) + H2O(l)
base
acid
+
–
NH4 (aq) + OH (aq)
acid
conjugate pair
base
8
Strength Of Acids
The strength of acids is judged by their tendency to donate protons.
The strength of bases is judged by their tendency to accept protons.
HCl + H2O → H3O+ + Cl—
The above reaction proceeds to completion, therefore, HCl is a stronger acid
than H3O+, and H2O is a stronger base than Cl—.
9
Strength Of Acids
CH3CO2H + H2O ⇌ H3O+ + CH3CO2—
Th above reaction however is a reversible reaction, with the equilibrium
position well towards the left.
+
—
H3O is a stronger acid than CH3CO2H, and CH3CO2 is a stronger base than
H2O.
Hence, strong acids have weak conjugate bases, and vice versa.
10 Skill Check
Identify which reactants are acids and which are bases:
(a) NH4+ + H2O ⇌ NH3 + H3O+
(b) HCOOH + HClO2 ⇌
+
HCOOH2
+
—
ClO2
(c) HClO4 + CH3COOH ⇌ ClO4— + CH3COOH2+
(d) H2SO4 + HNO3 ⇌
—
HSO4
+
(e) H2S + H2O ⇌ HS− + H3O+
+
H2NO3