Problem 27 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The following acids have been arranged in the order of decreasing acid strength. Identify the correct order. \(\mathrm{ClOH}\) (I), \(\mathrm{BrOH}(\mathrm{II}), \mathrm{IOH}(\mathrm{III})\) (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}\) (b) \(\mathrm{II}>\mathrm{I}>\mathrm{III}\) (c) \(\mathrm{III}>\mathrm{II}>\mathrm{I}\) (d) \(\mathrm{I}>\mathrm{III}>\mathrm{II}\)

Step-by-Step Solution

Answer

The correct order is (a) \(\mathrm{I} > \mathrm{II} > \mathrm{III}\).

Step 1: Understanding the Problem

The problem is to determine the order of decreasing acid strength of the given molecules: \(\mathrm{ClOH}\), \(\mathrm{BrOH}\), and \(\mathrm{IOH}\). Stronger acids will be listed before weaker ones.

Step 2: Analyze Electronegativity

The acid strength of compounds like \(\mathrm{ClOH}\), \(\mathrm{BrOH}\), and \(\mathrm{IOH}\) depends on the electronegativity of the halogen. The more electronegative the halogen, the stronger the acid, because the O-H bond becomes more polar, making it easier to donate the hydrogen ion. Chlorine is more electronegative than bromine, which is more electronegative than iodine.

Step 3: Arranging by Electronegativity

Given that chlorine is the most electronegative, followed by bromine, and iodine being the least, the order of acid strength is expected to be \(\mathrm{ClOH > BrOH > IOH}\).

Step 4: Verify Given Options

Checking the given options, option (a) \(\mathrm{I > II > III}\) corresponds to \(\mathrm{ClOH > BrOH > IOH}\), which matches the electronegativity analysis. This is the correct order.

Key Concepts

Electronegativity

Electronegativity is a key concept in understanding why certain molecules behave as they do, especially in acids. It refers to the tendency of an atom to attract shared electrons in a chemical bond. The higher the electronegativity, the stronger the pull on electrons.

In the context of halogen acids such as \(\mathrm{ClOH}\), \(\mathrm{BrOH}\), and \(\mathrm{IOH}\), electronegativity is crucial because it influences how easily the O-H bond can release a hydrogen ion. Among the halogens, chlorine is the most electronegative, followed by bromine, and then iodine.

This means that in \(\mathrm{ClOH}\), the O-H bond is highly polarized, making it easier for chlorine to pull electron density away from hydrogen and thus disassociate easily. This results in a stronger acid compared to \(\mathrm{BrOH}\) and \(\mathrm{IOH}\).

• Chlorine, being more electronegative, creates a situation where the O-H bond is weaker, thus making \(\mathrm{ClOH}\) the strongest acid.
• Bromine's lower electronegativity means \(\mathrm{BrOH}\) is less acidic than \(\mathrm{ClOH}\) but more than \(\mathrm{IOH}\).
• Iodine, with the lowest electronegativity, forms the weakest acid due to the O-H bond being least polarized.

Halogen Acids

Halogen acids are a type of oxyacid where a halogen element is directly bonded to an oxygen atom, which is in turn bonded to a hydrogen atom. These acids are characterized by their general formula \(\mathrm{XOH}\), where \(\mathrm{X}\) represents a halogen.

In our specific examples, we have \(\mathrm{ClOH}\), \(\mathrm{BrOH}\), and \(\mathrm{IOH}\). These compounds illustrate the trend in acid strength among halogens, which directly correlates with their electronegativity.

Halogens strongly influence the acid since they affect how the oxygen-hydrogen bond behaves:

• The presence of a highly electronegative halogen leads to a stronger acid.
• Oxyacids like \(\mathrm{ClOH}\) are stronger than their counterparts \(\mathrm{BrOH}\) or \(\mathrm{IOH}\), mainly due to the halogen's ability to draw electron density away from the O-H bond.

Ultimately, understanding the nature of halogen acids helps in predicting their behavior and strength, which is important for a variety of chemical processes.

O-H Bond Polarity

O-H bond polarity is another fundamental aspect in determining acid strength. The polarity of an O-H bond is influenced by the atoms attached to the oxygen, which affects how well the hydrogen ion can dissociate.

The nature of the halogen directly affects the electron distribution along the O-H bond:

• In \(\mathrm{ClOH}\), the high electronegativity of chlorine makes the O-H bond highly polar, meaning that oxygen can pull more electron density away from hydrogen. This weakening of the bond facilitates the release of hydrogen ions, increasing the acid's strength.
• In \(\mathrm{BrOH}\), the O-H bond is less polar than in \(\mathrm{ClOH}\) due to bromine's lower electronegativity, which results in a moderate acid strength.
• Lastly, \(\mathrm{IOH}\) has the least polar O-H bond, as iodine's electronegativity is the weakest among the three. Consequently, it is the weakest acid due to the strong O-H bond that is less likely to release hydrogen ions.

By understanding O-H bond polarity, it becomes clear how acid strength is affected by the molecular structure and the nature of atoms involved.