Relative acidity

[Clarice Chui 2C]

What are the properties that determine how strong an acid is? Why is HI a stronger acid than HF?

[Alfred Barrion 2H]

Bond length is a component of relative acidity. Since the bond length in HI is longer and weaker compared to the short and strong bond in HF, HI is able to dissociate much easier in a solution compared to HF.

[Matthew ILG 1L]

The strength of an acid depends on two things: The strength of the A-H bond, and the stability of the resulting anion. Bond strength is related to the length of the bond, and because Iodine has a much larger atomic radius than Fluorine, HI has a much longer, and therefore weaker, bond. The hydrogen is removed fairly easily, making HI a stronger acid.

[105311039]

Im pretty sure that has to do with the size difference between F and I. HI is a much bigger molecule than HF which leads to it being more acidic.

[Eesha Sohail 1D]

At first I thought HF would be a stronger acid because of the electronegativity of the F atom. Where do you look at the size/polarizability of the anion, and where does electronegativity matter more?

[Milisuryani Santoso 1L]

Essentially, because F is so electronegative, it has an extremely strong hold on H's electron, which thus requires more energy to break. Because HI has a longer bond + I is less electronegative, its easier to break and thus dissociates more than HF.

[DHavo_1E]

The strength of an acid depends on two things: The strength of the A-H bond, and the stability of the resulting anion. Bond strength is related to the length of the bond, and because Iodine has a much larger atomic radius than Fluorine, HI has a much longer, and therefore weaker, bond. The hydrogen is removed fairly easily, making HI a stronger acid.

Hi,

Can you explain what you mean by the stability of the resulting anion? What determines whether it is stable? Thank you!

[derinceltik1K]

Stability is achieved by withdrawing electron from the negatively charged atom. In the HCl and HI examples;

Between Cl-O, Cl has an inductive effect (same thing as electronegativity), pulling O's electrons thus stabilizing the negatively charged O.
With I-O, I does not have as high of an inductive effect as Cl and cannot withdraw electron density from negatively charged O, resulting in a less stable molecule .

You can think of it as trying to reduce the Formal Charge around atoms when drawing Lewis Structures. The most stable form is the one with closest Formal Charge to 0. In the acid example, Cl is doing the withdrawing of electron density around O.