Question

Explain why HF is a weak acid, whereas HCl, HBr, and HI are all strong acids.

Short answer

HF is a weak acid due to its stronger bond compared to HCl, HBr, and HI, resulting from the increased electronegativity of fluorine. This stronger bond makes it less likely to donate a hydrogen ion in solution and therefore exhibits weaker acidity. Meanwhile, HCl, HBr, and HI are strong acids because their weaker bonds allow them to more readily donate hydrogen ions when dissolved in water.

Step-by-step solution

Compare bond strengths

First, let's compare the bond strengths of these four acids. The bond strength generally decreases down the periodic table within the same group due to the increase in size of the atoms. As a result, HCl, HBr, and HI have weaker bonds than HF. This makes it easier for them to donate a hydrogen ion (H+) when dissolved in water (i.e., they are more likely to ionize).

Electronegativity and bond polarity

Electronegativity is a measure of an atom's ability to attract electron density towards itself. Fluorine is the most electronegative element in the periodic table, with chlorine, bromine, and iodine being progressively less electronegative down the group. When a hydrogen atom is bonded to a highly electronegative atom such as fluorine, the bond becomes highly polar, with the hydrogen atom carrying a partial positive charge (δ+) and the fluorine atom carrying a partial negative charge (δ-). This high polarity makes it more difficult for the hydrogen atom to be donated as a proton, leading to weaker acidity.

Level of ionization in solution

When acids dissolve in water, they can ionize partially or completely. Strong acids like HCl, HBr, and HI ionize almost completely, meaning that most of the H+ ions are released into solution. This high level of ionization makes them strong acids. On the other hand, HF is a weak acid because it ionizes only partially, leaving a significant portion of the HF molecules undissociated in solution.

Conclusions

In summary, HF is considered a weak acid because its bond strength is stronger than that of HCl, HBr, and HI due to the increased electronegativity of fluorine. This means that it is less likely to donate a hydrogen ion in solution, resulting in weaker acidity. Meanwhile, HCl, HBr, and HI are all classified as strong acids because they have weaker bonds and more readily donate hydrogen ions when dissolved in water.