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

Question

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

Step-by-Step Solution

Answer

HI has the most acidic character.

Step 1: Understand Acid Strength

The acidity of hydrogen halides (HX, where X = F, Cl, Br, I) increases down the group in the periodic table. This is because the H-X bond strength decreases down the group, making it easier for the molecule to donate a proton, hence increasing its acidic nature.

Step 2: Analyze Bond Strength

Consider the bond strength of each H-X bond. - HF has the strongest H-F bond due to the small size of fluorine. - HCl, HBr, and HI have progressively weaker H-X bonds as the size of the halogen increases. This weaker bond makes proton donation easier.

Step 3: Apply Acid Strength Trend

Apply the trend of increasing acidity with decreasing bond strength: HI > HBr > HCl > HF. This indicates that HI, with the weakest bond among the options, will have the greatest ability to donate a proton.

Key Concepts

Hydrogen Halides

Hydrogen halides are binary compounds consisting of hydrogen and one of the halogen elements: fluorine, chlorine, bromine, or iodine.
These compounds are typically represented as HX, where X stands for the halogen.
Each hydrogen halide has unique properties, but they share common characteristics due to the presence of hydrogen bonded to a halogen.

• **Acidity**: Hydrogen halides tend to become more acidic as you move down the group in the periodic table. This means compounds like HI are typically stronger acids than HF.
• **Natural Occurrence**: Fluorides and chlorides are often found in minerals, while bromides and iodides are more commonly found in seawater.
• **Reactivity**: These compounds are reactive due to their willingness to donate a proton (hydrogen ion) in reactions, contributing to their acidic nature.

Understanding these properties helps in predicting how these substances behave in chemical reactions, especially in terms of acid-base interactions.

Periodic Table Trends

The periodic table is organized in a way that reveals patterns in the chemical behavior of elements.
For hydrogen halides, an important trend observed is how the acidity of these compounds changes as we move down the halogen group.

• **Group Trend**: Moving from fluorine (top) to iodine (bottom), the acids become stronger. This trend is due to the decreasing strength of the hydrogen-halogen bond and the increasing size of the halogen atoms.
• **Reactivity and Size**: As halogen atoms increase in size, the bond length to hydrogen increases. Longer bonds are generally weaker, making it easier for the compound to release a proton.
• **Effects on Acidity**: Thus, the trend in acidity correlates with bond weakening. This is why HI is more acidic than HF.

Knowing these periodic trends can significantly aid in predicting the behavior of similar compounds across different reactions.

Bond Strength

The strength of the bond between hydrogen and the halogen atom in hydrogen halides plays a crucial role in determining their acidity.
As the size of the halogen increases from fluorine to iodine, the bond length increases, thereby weakening the bond.

• **Shorter Bonds Are Stronger**: In HF, the bond is short and strong due to the small size of fluorine, making it less acidic because the proton is not easily released.
• **Longer Bonds Are Weaker**: In HI, the bond is longer and weaker, primarily due to iodine's larger atomic size. This weak bond strength facilitates the release of a proton, thus increasing acidity.
• **Comparative Analysis**: Comparing all hydrogen halides, the trend is as follows: HF > HCl > HBr > HI in terms of bond strength, which inversely affects acidity (acidity increases as bond strength decreases).

Understanding bond strength among these molecules is pivotal for grasping why and how they interact in chemical environments, especially in processes involving proton transfer.