I'm an expert in the field of chemistry, and I'd be happy to explain why hydrofluoric acid (HF) is considered a weak acid compared to hydroiodic acid (HI).

Firstly, it's important to understand the concept of acidity in the context of the Bronsted-Lowry theory, which defines an acid as a substance that donates a proton (H+) to another substance. The strength of an acid is determined by its ability to donate this proton.

Now, let's delve into the factors that influence the strength of an acid:


1. Bond Strength: The strength of the bond between the hydrogen atom and the halogen (fluorine in HF, iodine in HI) is a critical factor. A strong bond is less likely to break and release the proton, which is why strong acids have weaker H-X bonds (where X is the halogen). In the case of HF, the H-F bond is quite strong due to the high electronegativity of fluorine and its small size, which allows for better overlap with the hydrogen's 1s orbital. This strong bond makes it more difficult for HF to donate a proton, classifying it as a weak acid.


2. Electronegativity: Electronegativity is the ability of an atom to attract electrons towards itself in a chemical bond. Fluorine, being the most electronegative element, pulls the electrons in the H-F bond towards itself, further stabilizing the bond and making it less likely to donate a proton.


3. Size of the Halogen: The size of the halogen also plays a role. Iodine, being larger than fluorine, has a less effective overlap with the hydrogen's 1s orbital, resulting in a weaker H-I bond. This weaker bond makes it easier for HI to donate a proton, which is why HI is a stronger acid than HF.


4. Inductive Effect: The inductive effect can also influence acidity. In the case of HF, the high electronegativity of fluorine exerts a strong electron-withdrawing inductive effect, which stabilizes the negative charge on the fluoride ion after the proton is donated. This stabilization makes the acid dissociation less favorable.


5. Polarizability: Polarizability refers to the ability of an atom to be distorted by an electric field. Iodine, being larger and more polarizable than fluorine, can be more easily distorted by the electric field of the hydrogen atom, leading to a weaker bond and a stronger acid.


6. Enthalpy of Dissociation: The enthalpy change associated with the dissociation of the acid into a proton and its conjugate base is another factor. A lower enthalpy change indicates that the dissociation process is more energetically favorable. For HF, the dissociation is less favorable due to the strong H-F bond, whereas for HI, the dissociation is more favorable due to the weaker H-I bond.

7.
Solvation: The solvation of the ions in water also affects acidity. In water, the fluoride ion is more strongly solvated than the iodide ion due to its higher charge density, which further stabilizes the fluoride ion and makes it less likely to form from HF.

In summary, while both HF and HI are acids that donate a proton, the strength of the H-X bond, the electronegativity of the halogen, the size and polarizability of the halogen, the inductive effect, the enthalpy of dissociation, and the solvation of the ions all contribute to HF being a weaker acid than HI. The strong H-F bond and the high electronegativity of fluorine make it difficult for HF to donate a proton, whereas the weaker H-I bond and the larger, more polarizable iodine make it easier for HI to donate a proton.

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HF is the weakest acid because of it's strong H-F bond. Fluorine being small in size overlaps better with 1s orbital of hydrogen leading to a strong bond.Hence can not give proton easily.Here bond strength overweighs the electronegativity of F . ... H-I bond is the weakest of all , so it is the strongest acid.read more >>