p-Block Elements: Speed-Solving Techniques (4)

Question

Arrange the following in increasing order of acidic character: $\text{N}_2\text{O}_5$, $\text{N}_2\text{O}_3$, $\text{NO}_2$, $\text{N}_2\text{O}$. Justify briefly.

Solution — Step by Step

Final Answer: N$_2$O $<$ N$_2$O$_3$ $<$ NO$_2$ $<$ N$_2$O$_5$.

Why This Works

For oxides of any element, higher oxidation state means the central atom pulls electrons more strongly. This polarizes the O–H bond in the corresponding hydroxide, making it easier to release H$^+$. So the acid is stronger, and we call the parent oxide more acidic.

The same trend explains: SO$_3$ (acidic) vs SO$_2$ (less acidic), Cl$_2$O$_7$ (very acidic) vs Cl$_2$O (mildly acidic), and so on.

Alternative Method

Estimate the strength of the corresponding oxoacid using Pauling’s rules: more O atoms not bonded to H means higher acidity. HNO$_3$ has $2$ such O atoms (very acidic), HNO$_2$ has $1$ (weak acid). Same ranking, different lens.