Why is HCl a stronger acid than H2S on the basis of electronegativity, but HF is a weaker acid than HCl? [duplicate]








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[Why can the acidic strength between H2S and HCl be explained on the basis of electronegativity but H2S and H2O can't?](/questions/107516/why-can-the-acidic-strength-between-h2s-and-hcl-be-explained-on-the-basis-of-ele)

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We know that $\ce{HCl}$ is a stronger acid than $\ce{H2S}$. This can be explained due to that fact that chlorine is more electronegative than sulfur, which means the hydrogen ion has a much easier time leaving when bonded to chlorine than sulfur. However, when looking at $\ce{HCl}$ and $\ce{HF}$, $\ce{HCl}$ is actually a stronger acid, even though $\ce{HF}$ is more electronegative and polar. Is there is flaw in my reasoning?




There are several factors involved in acidity in water. The most important is probably the X-H bond strength. This effectively explains the increasing acidity from HF to HI The bonds become weaker. It also explains the increasing acidity from H2O to H2S, H2Se and H2Te. A second factor is solvation of the anions. There probably is a sweet spot here for the surrounding water molecules to disperse the charge and minimize crowding. This might also involve the shape of the ion. Another consideration is the ability to hydrogen bond. The hydrogen bond will give a reprotonation reaction more opportunity and should lower the acidity of HF, H2O and possibly a bit for HCl.


All the factors should be considered in making comparisons especially when comparing between groups. Also the extent of chemical reactions is governed by the Second Law energetics


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