Problem 53 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following hasthe mostacidic character? (a) HF (b) \(\mathrm{HCl}\) (c) \(\mathrm{HBr}\) (d) HI

Step-by-Step Solution

Answer

HI is the most acidic because it has the weakest H-X bond, allowing it to dissociate most easily.

Step 1: Understanding Acidic Character

To determine which of the given hydrogen halides (HF, \( \mathrm{HCl} \), \( \mathrm{HBr} \), and HI) is the most acidic, we need to understand that the acidic character of these molecules is influenced by the strength of the H-X bond. A weaker H-X bond indicates a stronger acid, as it more easily dissociates to release H\(^+\) ions.

Step 2: Bond Strength Trends

The bond strength generally decreases as we move down the group in the halogen column (from F to I) in the periodic table. This is because the size of the halogen increases, leading to less effective overlap of orbitals and thus weaker H-X bonds.

Step 3: Applying the Trend

Among HF, \( \mathrm{HCl} \), \( \mathrm{HBr} \), and HI, HF has the strongest bond due to the small size of fluorine and HI has the weakest bond due to iodine's large size. This means HI will dissociate most easily, making it the most acidic.

Step 4: Conclusion

Based on bond strength and the ability to dissociate, HI is expected to have the most acidic character due to its weak bond strength and ready dissociation.

Key Concepts

Hydrogen Halides

Hydrogen halides are a group of compounds formed by hydrogen and the halogens from Group 17 of the periodic table. These elements include fluorine, chlorine, bromine, and iodine, forming the corresponding hydrogen halides: HF, HCl, HBr, and HI. Each of these hydrogen halides has unique properties that are influenced by the halogen involved. Fluorine is the smallest and most electronegative of the halogens, while iodine is larger and less electronegative.
When we look at hydrogen halides in terms of their acidic properties, they act as strong acids in solutions. HI, for example, completely ionizes in water, releasing hydrogen ions ( ^+) readily.
The concept of acidity here is tied largely to the behavior of these compounds in water. The extent to which these molecules release hydrogen ions determines their strength as acids.

Bond Strength

The strength of the bond between hydrogen and the halogen atom greatly affects the acidity of hydrogen halides. A weaker H-X bond implies that the bond can be more readily broken, facilitating the release of H ^+ ions, and thus making it more acidic.
One key factor influencing bond strength is the size of the halogen atom:

• Fluorine, being the smallest, forms the strongest bond with hydrogen due to efficient orbital overlap.
• In contrast, iodine, which is larger, forms a weaker bond due to less effective overlap.

As we move down the halogen group in the periodic table, the bond strength decreases. This is why HI, with the weakest bond, is the most acidic among the hydrogen halides.

Dissociation

Dissociation is the process by which a compound splits into smaller components, such as ions. In the context of acids, it specifically refers to the ability of a compound to release H ^+ ions into a solution. The ease of dissociation directly correlates with the strength of the acid.
For hydrogen halides, this dissociation is more straightforward when the H-X bond is weak.
Here's a summary of dissociation trends in hydrogen halides:

• HI, due to its weak bond, dissociates more easily, releasing H ^+ in solution, which enhances its acidic character.
• HF, with a strong bond, exhibits less dissociation, meaning it is the least acidic of the group.

Thus, the relationship between bond strength and dissociation is crucial in determining the acidic strength of hydrogen halides.