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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?

Ian Bush
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    Higher electronegativity actually usually means more difficulty to extract a proton from an acid. $\ce{HBr}$ and $\ce{HI}$ are both stronger acids than $\ce{HCl}$, and both halogen atoms are less electronegative than chlorine. Conjugate base stability is a better criterion, as more stable conjugate bases prefer to stay dissociated (like $\ce{Cl-}$,$\ce{Br-}$, and $\ce{I-}$) while unstable conjugate bases (like $\ce{F-}$) prefer to stay associated. Fluorine atoms are too small to effectively carry a negative charge, while chlorine, bromine, and iodine are big enough to do so. – Sam202 Dec 05 '22 at 02:12
  • As to why HI is a stronger acid than HF, you might want to look here. – user55119 Dec 05 '22 at 03:14
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    F- is unstable? Is that why fluorine is one of the strongest oxidizing agents known? – jimchmst Dec 05 '22 at 03:58
  • @jimchmst It's unstable relative to the bigger halide ions in terms of acid-base equilibria and acid strength. The oxidizing capability of $\ce{F-}$ is irrelevant in this case because an acid dissociating in aqueous solution involves a transfer of protons, not electrons. – Sam202 Dec 05 '22 at 04:26
  • Why can the acidic strength between H2S and HCl be explained on the basis of electronegativity but H2S and H2O can't? Note the proper spelling of HCl, too. – andselisk Dec 05 '22 at 05:41

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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

jimchmst
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