Modified Theory of Acids and Bases
chapter 6
lesson 5
Objective
You will be able to;
· explain why Arrhenius’ theory is not adequate
for certain observations.
· use the modified theory of acids and bases to
write dissociation equations.
Is ammonia acid, base or neutral?
Some substances, such as ammonia, are bases
even though they do not have hydroxide as part
of their formula.
These substances react with water to form the
hydroxide as follows.
NH3(g) + H2O (l) → NH4+ (aq) + OH-(aq)
Notice how the ammonia accepts the proton!
This was one of the reasons that the theory
Arrhenius put forward needed to be changed.
Recall that Arrhenius’ model of acids called for the
production of a hydrogen ion. Research has shown that
no such ions are detectable in acids. Instead the ion
reacts with water to form a new species called the
hydronium ion.
H20(l) + H+(aq) → H3O+(aq)
Recall that water is extremely polar. This means
that it would most likely be actively involved in
the dissociation or ionization process. We must
include water’s action in this process.
The ionization of hyrogen chloride then is as
follows.
HCl (g) + H2O(l) → H3O+ (aq) + Cl-(aq)
Notice how the acid donated a proton to the
water!
Modified Definition (Brønsted-Lowery)
of Acids and Bases
Acids are proton donors
Bases are proton acceptors
Oxides
Non-metallic oxides, such as carbon dioxide, form
acids in water.
CO2(g) + H2O(l) → H2CO3(aq)
H2CO3 (aq) + H2O(l) → H3O+(aq) + HCO3-(aq)
Metallic oxides, such as sodium oxide, form bases in
water.
Na O + H O → 2Na+
+ 2 OH2 (s)
2 (l)
(aq)
(aq)
(accepts proton here)
Assignment
Read pages 216 - 217
Do p. 217 #1,3,4,5