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U-3
Usually, it is not possible to confidently predict which compounds are ionic and which are molecular (i.e.covalent)
simply by looking at their formulas. The nature of the bonding within the compound can frequently be assessed by
measuring its conductivity in water. Ionic compounds, such as NaCl generate strongly conducting solutions by
dissociating current-carrying cations and anions into solution.
NaCl
(s)
H2O
Na
+
(aq)
+ Cl
-
(aq)
By virtue of the fact that they ionize completely in water to give conducting solutions, ionic compounds are
referred to as electrolytes, (from the Greek lutos to loosen).
In sharp contrast, covalent compounds, such as sucrose, do not release ions when they dissolve in water and, thus,
they do not produce conducting solutions. In electrochemical parlance,
non-dissociating, covalent compounds are referred to as non-electrolytes.
In addition to their strongly contrasting physical properties, covalent and ionic compounds display radically different
reaction chemistries. This difference is particularly evident in reactions involving oxygen. Oxygen forms
covalent oxides with p-block elements and ionic oxides with highly electropositive metals. Most of the metal
oxides are called basic oxides because they react with water to form hydroxide solutions. For instance, Na2O
will dissociate as follows when added to water.
Na2O
(s)
H2O
2 Na
+
(aq)
+ O²
-
(aq)
Immediately upon its release, the oxide dianion is hydrolyzed to OH
-
(aq)
O²
-
(aq)
+ H2O
(l)
2 OH
-
(aq)
The equilibrium constant is very large and thus of all the O²
-
is consumed in the reaction:
Non-metal oxides are termed acidic oxides because the react with water to form acids:
CO2
(aq)
+ H2O
(aq)
H2CO3
(aq)
H2O
H3O
+
(aq)
+ HCO3
-
(aq)
The carbonic acid (H2CO3) in the equilibrium¹ above is formed by nucleophilic attack of a water molecule at the
electron deficient carbon atom in CO2.
1
Since it is a weak acid, only a small amount of the H2CO3 reacts with water to form hydronium ions and bicarbonate ions.
