Question

The deciding factor on why HF is a weak acid and not a strong acid like the other hydrogen halides is entropy. What occurs when HF dissociates in water as compared to the other hydrogen halides?

Short answer

Because the value of $\mathit{\Delta }{\mathit{S}}^{\mathbf{o}}$for HF is negative, the value of $\mathit{\Delta }{\mathit{G}}^{\mathbf{o}}$for HF is positive and the value of${\mathit{K}}_{\mathbf{a}}$, for HF is less. Because the entropy of hydration resists the dissociation of HF in water, it is a weak acid.

Step-by-step solution

Step 1: Introduction to the Concept

The bond strength of a chemical bond is represented by bond energy. Bond enthalpy and bond dissociation energy are other terms for the same thing.

Step 2: Solution Explanation

${\mathit{K}}_{\mathbf{a}}$ is less than 1 for HF, while it is greater than 1 for the rest. In terms of $\mathit{\Delta }{\mathit{G}}^{\mathbf{o}}$ ,HF has a positive value, whilst the others have a negative value.

This is owing to the fact that the entropy values of all hydrogen halides acids differ.

For HF, the value of $\mathit{\Delta }{\mathit{S}}^{\mathbf{o}}$ is negative and also quite huge.

In water, HF breaks down into $H^{+}$ and $F^{-}$ions. Hydrogen bond forms between water molecules and $F^{-}$. As a result, there is a great degree of order amongst them. As a result, hydration entropy prevents HF from dissociating in water, making it a weak acid.

Because the entropy of hydration resists the dissociation of HF in water, HF is a weak acid.