As a chemical expert with a background in organic and inorganic chemistry, I am well-versed in the intricacies of molecular structure and reactivity. Let's delve into the comparison between hydrofluoric acid (HF) and hydroiodic acid (HI), and why the former is often considered stronger than the latter, despite the common misconception that HI is the strongest hydrohalic acid.

Firstly, it is crucial to clarify what is meant by "stronger" in the context of acids. In chemistry, the strength of an acid is typically measured by its ability to donate protons (H+ ions) in solution. The stronger the acid, the more readily it donates protons. This is quantified by the acid dissociation constant (Ka), where a higher Ka value indicates a stronger acid.

Now, let's address the misconception mentioned in the prompt. Historically, it was believed that the strength of hydrohalic acids followed the trend: HI > HBr > HCl > HF, based on their ability to ionize in water. However, this is not entirely accurate. The strength of an acid is not solely determined by its tendency to ionize but also by the stability of the resulting conjugate base.

The statement that "Iodine is so much bigger than fluorine that the charge is more stable on this larger atom" is a reference to the concept of atomic size and its effect on the stability of the conjugate base. The larger the atom, the more diffuse the electron cloud, which can lead to greater stabilization of the negative charge in the conjugate base. This is why HI was initially thought to be the strongest hydrohalic acid.

However, this overlooks the significant role of electronegativity in acid strength. Fluorine is the most electronegative element, and its high electronegativity leads to a strong attraction for the shared electrons in the H-F bond. This results in a polar bond where the hydrogen atom is significantly protonic. When HF donates a proton, it forms the fluoride ion (F-), which is a very stable anion due to fluorine's high electronegativity and small size.

The stability of the conjugate base is a critical factor in determining acid strength. The more stable the conjugate base, the more readily the acid will donate a proton. In the case of HF, the fluoride ion is extremely stable, which makes HF a strong acid in terms of its ability to donate protons, despite its relatively low Ka value compared to HI.

Furthermore, the solvation of ions in water also plays a role in acid strength. Water is a highly polar solvent and can stabilize ions through solvation. The fluoride ion, being smaller and more electronegative, is more effectively solvated by water molecules than the iodide ion, which contributes to the stability of the conjugate base and the overall acid strength of HF.

In summary, while HI has a higher Ka value and is more readily ionized in water, the stability of the fluoride ion as the conjugate base of HF, along with the effects of electronegativity and solvation, contribute to HF being considered a stronger acid in terms of its ability to donate protons. This is a more nuanced view of acid strength that takes into account not just the initial ionization but also the subsequent stability of the resulting species in solution.

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Iodine is so much bigger than fluorine that the charge is more stable on this larger atom. Thus, the trends for acidity of the hydrohalic acids are as follows: HI is strongest, followed by HBr, HCl, and finally HF. ... That is, somehow the presence of these electronegative atoms must stabilize the conjugate base anion.read more >>