p-Block Elements: Application Problems (1)

Question

Arrange the following in order of increasing acidic strength of their hydrides: $H_2O$, $H_2S$, $H_2Se$, $H_2Te$. Justify the trend using bond strength and electronegativity arguments.

Solution — Step by Step

Why This Works

For Group 16 hydrides, the dominant factor is $H-X$ bond strength, not electronegativity. As bond length grows, the bond becomes weaker and the $H^+$ leaves more readily. This is the same trend as for HCl < HBr < HI in halogens: down the group, hydrides become stronger acids.

The “electronegativity → acidity” rule applies across a period, not down a group. CH₄ < NH₃ < H₂O < HF in acidity matches electronegativity. Down a group, bond strength dominates.

Alternative Method

Use $\Delta G_{\text{ionisation}} = \Delta H_{\text{bond}} +$ (other terms). Since $\Delta H_{\text{bond}}$ falls dramatically down the group, and the other terms (hydration, etc.) move less, $\Delta G$ falls and $K_a$ rises.

Common Mistake