Question

Among the following, the correct order of acid strength is:
A.$A{l_2}{O_3}$ < $Si{O_2}$ < $S{O_2}$ < ${P_2}{O_3}$
B.$Si{O_2}$ < $S{O_2}$ < $A{l_2}{O_3}$ < ${P_2}{O_3}$
C.$S{O_2}$ < ${P_2}{O_3}$ < $Si{O_2}$ < $A{l_2}{O_3}$
D.$A{l_2}{O_3}$< $Si{O_2}$ < ${P_2}{O_3}$ < $S{O_2}$

Answer 1

The acidic character depends on the electronegativity difference between the atoms. An atom that has a higher electronegativity will also have a higher oxidation state. Thus, it can be said to be more acidic.

Complete step by step answer:
If an atom in a bond between two atoms is more electronegative than the other then the shared pair of electrons between the two atoms will be drawn closer to the more electronegative atom. This can cause an increase in the oxidation state of the atom in case the bond is split. An increase in the oxidation state means that there will be an increase in the acidic character. It is well known that the acidity of an atom depends on a higher oxidation state.

If we take for example an $O - H$ bond that is attached to an electronegative atom then the electron density will shift away from the oxygen and towards the electronegative atom. This makes it possible for the hydrogen to become positively charged and the compound becomes acidic.

From the compounds given in the question, we can see which atom is more acidic than the others.
In $A{l_2}{O_3}$, we can see that the oxidation number of aluminium is $+ 3$. therefore, the acidity will be less compared to the others.
$Si{O_2}$ contains a silicon atom that has an oxidation number of $+ 4$.
${P_2}{O_3}$ has phosphorus which contains an oxidation number of $+ 3$ as well. But in this compound, the electronegativity difference between the atoms is higher when compared to the other two compounds.

Finally, $S{O_2}$ contains sulphur whose oxidation state is $+ 4$. But the electronegativity difference is very high compared to the previous three compounds.
Therefore, the order will be:
$A{l_2}{O_3} < Si{O_2} < {P_2}{O_3} < S{O_2}$

therefore, the correct answer will be option D.

Note: Acidity depends upon the electronegativity of atoms. The higher the oxidation state of the atom, the acidity of the atom. Acidity increases across the period from left to right as there is an increase in the oxidation number.