Question

(a) Carbon dioxide is nonpolar while water is polar. What conclusion do you draw about their structures from these?
(b) Classify the following compounds into acidic, basic and amphoteric oxides.
${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}$, ${\text{C}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{7}}}$

Answer 1

The overall polarity of a molecule depends upon the structure of the molecule and placement of the polar bonds. The cumulative resultant of dipoles of all bonds decides the overall polarity of the molecule. Oxides are classified according to the pH of the products formed by their reaction to water. Mostly, metals form basic oxides and non-metals form acidic oxides.

Complete step by step answer:
(a) We know that carbon has four valence electrons. In carbon dioxide, carbon is bonded to two oxygen atoms and forms a double bond with each atom. As a result, carbon is ${\text{sp}}$ hybridized and carbon dioxide is a linearly shaped molecule with carbon in the middle and two oxygen atoms on each side.
We know it for a fact that oxygen is more electronegative than carbon. So the polarity of each carbon oxygen double bond is towards the oxygen atom. Due to the linear shape of carbon dioxide, the dipole moments of both the carbon oxygen bonds cancel out each other making the molecule non-polar.
We know that in a water molecule, an oxygen atom is bonded to two hydrogen atoms. Apart from the two bond pairs, oxygen also has two unshared pairs of electrons. As a result, oxygen is ${\text{s}}{{\text{p}}^{\text{3}}}$ hybridized and is a bent molecule with oxygen in the middle.
We know it for a fact that oxygen is more electronegative than hydrogen. So the polarity of each hydrogen oxide bond is towards the oxygen atom. The dipole moments of both the bonds are in the same direction and thus add up making the molecule polar.

(b) Aluminium oxide,${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ , is an amphoteric oxide.
It contains oxide ions and reacts with acids the same way alkali and alkaline earth metal oxides do. In addition to this, it also displays basic properties and reacts with bases like sodium hydroxide.
Dichlorine heptoxide, ${\text{C}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{7}}}$, is an acidic oxide.
It is the anhydride of perchloric acid and yields the acid in water making the solution acidic.

Note:
Apart from the basic character shown by metal oxides, aluminium oxide also shows acidic character because the electronegativity difference between aluminium and oxygen is small. Thus the metal holds on to the oxygen atom more strongly and breaking of the bond is not very easy.