Problem 147 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the oxyacids of the type HXO \(_{3}\), the weakest acid would be (a) \(\mathrm{HBrO}_{3}\) (b) \(\mathrm{HIO}_{3}\) (c) \(\mathrm{HClO}_{3}\) (d) al the three will be of same strength

Step-by-Step Solution

Answer

HIO_3 is the weakest acid.

Step 1: Understanding Oxyacid Strength

To determine the strength of the oxyacid of the form HXO_3, we examine the electronegativity of the central atom X. Typically, the greater the electronegativity, the stronger the acid. Therefore, we need to compare the electronegativity of Br, I, and Cl.

Step 2: Comparing Electronegativity

Chlorine (Cl) is more electronegative than Bromine (Br), which in turn is more electronegative than Iodine (I). Thus, Cl > Br > I in terms of electronegativity.

Step 3: Relating Electronegativity to Acid Strength

Since greater electronegativity of the central atom typically increases the acidic strength of the oxyacid due to a stronger electron-withdrawing effect, \[\text{HClO}_3 > \text{HBrO}_3 > \text{HIO}_3 \text{ in acidic strength.}\]

Step 4: Identifying the Weakest Acid

Given that the strength of the acids decreases with decreasing electronegativity, the weakest acid among \[\text{HClO}_3, \text{HBrO}_3, \text{and } \text{HIO}_3\] is \[\text{HIO}_3.\]

Key Concepts

Electronegativity

Electronegativity is a measure of how strongly an atom can attract or pull electrons towards itself in a bond. This concept plays a crucial role in determining the strength of oxyacids. In general, the more electronegative the central atom of an oxyacid, the stronger the acid will be. This is because higher electronegativity enhances the electron-withdrawing ability, which makes it easier for the molecule to lose a hydrogen ion (H\(^+\)) in solution, increasing the acid's ability to donate protons.

In comparing the electronegativities of different elements, it is useful to remember the periodic trend: electronegativity increases across a period from left to right and decreases down a group. In the context of our exercise, Chlorine (Cl) is more electronegative than Bromine (Br), which in turn is more electronegative than Iodine (I). Therefore, in the series of oxyacids given, the order of electronegativity for the central atoms is: Cl > Br > I.

Comparison of Acids

When comparing the strengths of different acids, especially oxyacids like HXO\(_3\), our primary focus is on the central atom's ability to withdraw electrons.

Let's examine the following acids:

• **HClO\(_3\):** Chlorine, being very electronegative, can effectively withdraw electrons, making HClO\(_3\) a strong acid.
• **HBrO\(_3\):** Bromine is less electronegative than Chlorine but more than Iodine. Thus, HBrO\(_3\) has intermediate acidic strength.
• **HIO\(_3\):** Iodine, with the lowest electronegativity among the three, is the weakest electron withdrawer, making HIO\(_3\) the weakest acid in this group.

Each acid's ability to donate protons is key to understanding its strength. Hence, stronger electron-withdrawing nature (due to higher electronegativity of the central atom) predicts greater acid strength.

Central Atom Influence

The central atom in an oxyacid greatly influences the acid's properties. Its electronegativity determines how efficiently the acid can donate a proton. The more electronegative the central atom, the better it is at stabilizing the negative charge that forms when the acid dissociates.

For instance, in the acids at hand:

• Chlorine in HClO\(_3\): Very effective at attracting electrons, leading to strong acid behavior.
• Bromine in HBrO\(_3\): Less effective than chlorine but better than iodine, thus exhibiting moderate strength.
• Iodine in HIO\(_3\): Least effective, resulting in weak acid characteristics.

The ability of the central atom to distribute the negative charge affects how easily the acid releases hydrogen ions in solution, directly influencing the acid's strength.

Acidic Strength Order

Understanding the order of acidic strength involves analyzing the ability of acids to dissociate and donate protons. For oxyacids, this is largely determined by the central atom's electronegativity. In the present exercise, the order of oxyacid strength is dictated by the electronegativity of the central atoms, leading to the conclusion: - **HClO\(_3\)** is the strongest acid because Chlorine is the most electronegative.
- **HBrO\(_3\)** follows next, as Bromine is less electronegative than Chlorine but more than Iodine.
- **HIO\(_3\)** is the weakest because Iodine has the lowest electronegativity.

In summary, the acidic strength of oxyacids can be arranged as \[ ext{HClO}_3 > ext{HBrO}_3 > ext{HIO}_3 \] This understanding helps predict that the ability of an oxyacid to release hydrogen ions in water is enhanced as the electronegativity of the central atom increases.