The forward reaction for: $\ce{HF<->H^+ +F^-}$ is entropically favourable; but energetically unfavorable: as there is too much electron density for $\ce{F^-}$ ion to cope up. That's a reason why $\ce{HF}$ is a weaker than $\ce{HI, HCl}$ etc.

But, can there be another explanation for weak acidity of $\ce{HF}$?

How about we introduce the idea of entropy to orbitals. Let me explain, in $\ce{HF}$ the elctron is in a molecular orbital of greater region than in $\ce{F^-}$. So, the valence electron pair is in an entropically more favorable state in $\ce{HF}$ than in $\ce{F^-}$: where it is contained in little space. That's why, $\ce{HF}$ is more favorable than separated $\ce{H^+}$ and $\ce{F^+}$.

Maybe, for other hydracids of same kind eg. $\ce{HI, HCl}$ this effect isn't so important. That's why they are stronger acids.

Share your thoughts.


1 Answer 1



The reason why a gas particle in a large volume has a large entropy is not because it has a lot of space to move around per se. A better explanation is that for a given energy, there are many accessible translational states (these states can be derived from the particle in a box model). If we assume that all of these translational states are equally likely to be populated, then the number of possible microstates for the system, $W$, is large and hence $S = k \ln W$ is large.

An electron in an orbital isn't whizzing about, and an orbital most certainly isn't a container of variable size for the electron to move around in. An orbital is simply one (electronic) state of an electron.

Therefore, it doesn't make sense to talk about the translational states of an electron that is moving about in an orbital. It also doesn't make sense to talk about the translational entropy of an electron as being larger in a larger orbital.


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