Question

Of the hydrogen halides, only \(\mathrm{HF}\) is a weak acid. Give a possible explanation.

Short answer

HF is a weak acid among the hydrogen halides because of its high bond strength and the high electronegativity of fluorine. The strong H-F bond and fluorine's strong pull on bonding electrons make HF less likely to donate a proton in solution, resulting in its weak acidity. On the other hand, HCl, HBr, and HI have weaker bonds and are more likely to donate protons, classifying them as strong acids.

Step-by-step solution

Understanding the properties of hydrogen halides

Hydrogen halides are binary compounds formed from hydrogen and a halogen (Group 17 element). They include HF, HCl, HBr, and HI. Generally, these compounds are acidic in nature when dissolved in water.

Understanding strong and weak acids

Acids can be classified as strong or weak based on their ability to dissociate into their constituent ions in water. A strong acid completely dissociates into its constituent ions, while a weak acid only partially dissociates. In other words, a strong acid has a stronger tendency to donate a proton (H+), whereas a weak acid has a lesser tendency to donate a proton.

Comparing the bond strength of hydrogen halides

The strength of an acid is mainly determined by the bond strength between hydrogen and the halogen. The bond strength decreases as we go down the periodic table because the size of the halogen atom increases, making it easier to break the bond. In the case of hydrogen halides, the bond strength decreases in the order HF > HCl > HBr > HI.

Explaining the weaker acidity of HF

Although HF has the highest bond strength among the hydrogen halides, it is a weak acid. The reason lies in the fact that the fluorine atom has a very high electronegativity, which means it strongly attracts the bonding electrons, making the H-F bond more polar. As a result, HF is more reluctant to lose a proton (H+) due to the strong pull from the electronegative fluorine atom. Therefore, HF only partially donates a proton (H+) in the solution, making it a weak acid.

Comparing the acidity of other hydrogen halides

Now let's consider the other hydrogen halides. The bond strength decreases as we go down the group, so HCl, HBr, and HI have weaker bonds than HF. This means they can more easily lose a proton (H+) in solution, making them strong acids. Since their bond strengths are weaker relative to HF, the electron pull from the halogen atom does not prevent their proton donation as much as it does in HF. Therefore, HCl, HBr, and HI are strong acids. In conclusion, HF is a weak acid among the hydrogen halides because its bond strength is higher than the other hydrogen halides, and the high electronegativity of fluorine makes it less likely to donate a proton in solution.

Key concepts

Hydrogen Halides

Hydrogen halides are chemical compounds consisting of hydrogen and a halogen element from Group 17 of the periodic table. These halogens include fluorine ( F ), chlorine ( Cl ), bromine ( Br ), and iodine ( I ). When combined with hydrogen, they form the hydrogen halides: hydrofluoric acid ( HF ), hydrochloric acid ( HCl ), hydrobromic acid ( HBr ), and hydroiodic acid ( HI ). These compounds are commonly known for their acidic properties in aqueous solutions.
Hydrogen halides behave differently in water due to the varying strengths of the hydrogen-halogen bonds. The way these acids dissociate in water significantly influences their acidity.
In a solution:

• HF is known to be a weak acid.
• Other hydrogen halides like HCl, HBr, and HI are strong acids.

The difference in bond strength and the properties of the halogen atoms contribute to these varying acidic behaviors.

Acid Dissociation

Acid dissociation refers to the extent to which an acid releases its hydrogen ion ( H^+ ) in an aqueous solution. In simple terms, it measures how readily an acid donates a proton. Acids can fall into two broad categories: strong acids, which fully dissociate, and weak acids, which only partially dissociate.
For example:

• A strong acid like HCl completely releases protons, resulting in high concentrations of hydrogen ions in the solution.
• A weak acid like HF releases fewer hydrogen ions because it partially dissociates.

The degree of dissociation is crucial in explaining differences between acidic strengths among hydrogen halides. This behavior is primarily influenced by the interplay between bond strength and electronegativity of the halogens involved.

Bond Strength

Bond strength in hydrogen halides significantly affects their acidic character. It is the energy required to break the bond between hydrogen and the halogen atom. In the case of hydrogen halides, bond strength follows the order HF > HCl > HBr > HI .
This trend indicates that as you move down the group in the periodic table, the bond strength decreases. This is because the size of halogen atoms increases, making these bonds easier to break. Consequently, acids with weaker bonds tend to donate protons more readily, thus acting as strong acids in aqueous solutions.
Among the hydrogen halides, HF exhibits the strongest bond with hydrogen, contributing to its weak acidic behavior, while HI , having the weakest bond, behaves as a strong acid. Since bond strength is a decisive factor, understanding this concept helps explain why HF is less acidic despite its high electronegativity.

Electronegativity

Electronegativity measures an atom's ability to attract shared electrons in a bond. In hydrogen halides, electronegativity plays a pivotal role.
Fluorine, being the most electronegative element, strongly attracts the bonding electrons, enhancing the polarity of the H-F bond. This strong attraction makes it more challenging for HF to release a proton ( H^+ ), resulting in it being a weak acid. The strong pull from the fluorine atom holds on tightly, so it does not dissociate readily into H^+ ions in solution.
Other halogens like chlorine, bromine, and iodine are less electronegative than fluorine, allowing them to release the hydrogen ion more easily when in solution. Thus, HCl , HBr , and HI are strong acids. This difference highlights how electronegativity directly impacts the acid dissociation and overall acidic strength of hydrogen halides.