Problem 113 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following statements about halogens is incorrect? (1) The electron affinities are in order \(\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}\) (2) \(\mathrm{F}_{2}\) has lower bond dissociation energy than \(\mathrm{Cl}_{2}\) (3) \(\mathrm{HF}\) is the strongest hydrothalic acid (4) Van der Waals forces are the weakest in iodine (a) 1,2 and 4 (b) 1,3 and 4 (c) 1,2 and 3 (d) 2,3 and 4

Step-by-Step Solution

Answer

Option (b) 1, 3, and 4 are incorrect.

Step 1: Understand the Problem

We need to identify which of the given statements about halogens are incorrect. Examining each statement based on what is known about halogens will help us determine this.

Step 2: Analyze Statement 1

Statement 1 claims that the electron affinities of halogens are in the order \(\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}\). However, chlorine actually has a higher electron affinity than fluorine due to less electron repulsion in the outer shell, so this order is incorrect. The order should be \(\mathrm{Cl}>\mathrm{F}>\mathrm{Br}>\mathrm{I}\).

Step 3: Analyze Statement 2

Statement 2 says \(\mathrm{F}_2\) has lower bond dissociation energy than \(\mathrm{Cl}_2\). This is correct because the lone pairs in \(\mathrm{F}_2\) are close enough to repel each other, weakening the bond, whereas \(\mathrm{Cl}_2\) does not have this as pronounced to the same degree.

Step 4: Analyze Statement 3

Statement 3 states \(\mathrm{HF}\) is the strongest hydrophalic acid. However, due to the strong hydrogen-fluorine bond, \(\mathrm{HF}\) does not dissociate completely in water, making it a weak acid compared to other hydrohalic acids like HCl, which dissociate more completely.

Step 5: Analyze Statement 4

Statement 4 claims that van der Waals forces are the weakest in iodine. This is incorrect because iodine, being the largest halogen, actually has the strongest van der Waals forces due to its larger electron cloud and higher polarizability compared to the other halogens.

Step 6: Determine Incorrect Statements and Answer

From our analysis, Statements 1, 3, and 4 are incorrect. This matches option (b) from the multiple-choice answers.

Key Concepts

Electron Affinity

Electron affinity is a crucial concept to understand when studying halogens, which are elements found in Group 17 of the periodic table. It refers to the energy change that occurs when an electron is added to a neutral atom in the gaseous state. Halogens are known for having high electron affinities because they are one electron short of achieving a stable, noble gas electron configuration. However, the statement "the order of electron affinities for halogens is \( \mathrm{F} > \mathrm{Cl} > \mathrm{Br} > \mathrm{I} \)" is incorrect. In reality, chlorine has the highest electron affinity, even greater than fluorine.

• This is due to smaller electron repulsions in chlorine's outer shell compared to fluorine's, where the compact size leads to increased electron-electron repulsion.
• The correct order is: \( \mathrm{Cl} > \mathrm{F} > \mathrm{Br} > \mathrm{I} \).

Understanding electron affinity helps explain why halogens react so vigorously with other elements, primarily through gaining an electron to form halide ions. This property highlights the reactivity pattern of halogens and their applications, like forming salts and various organic compounds.

Bond Dissociation Energy

Bond dissociation energy (BDE) is the energy required to break a bond in a molecule, separating it into individual atoms. In the context of halogens, BDE is significant for understanding the stability of diatomic halogen molecules. For example, in the exercise, Statement 2 highlights that \( \mathrm{F}_2 \) has lower BDE compared to \( \mathrm{Cl}_2 \). This is true because:

• The small size of fluorine atoms results in lone pairs that are closer together, leading to increased repulsion. This weakens the bond.
• Chlorine atoms, being larger, have less repulsion between lone pairs, resulting in stronger bonding in \( \mathrm{Cl}_2 \).

Bond dissociation energies help chemists predict reaction trends and understand molecular stability within halogens. They also influence reactivity, as molecules with lower BDE tend to react more readily with other substances due to weaker bonds that are easier to break.

Hydrohalic Acids

Hydrohalic acids are binary compounds formed by hydrogen and halogens. Though \( \mathrm{HF} \) (hydrofluoric acid) might seem like a strong acid due to the high electronegativity of fluorine, it is actually a weak acid. The statement that \( \mathrm{HF} \) is the strongest hydrohalic acid is not true.

• \( \mathrm{HF} \) does not dissociate completely in water because of the strong hydrogen-fluorine bond, which does not easily break.
• In contrast, \( \mathrm{HCl} \), \( \mathrm{HBr} \), and \( \mathrm{HI} \) are stronger acids because they dissociate more completely in water, releasing more hydrogen ions.

Understanding hydrohalic acids is essential for anticipating their behavior in chemical reactions and their usage in various industries. Stronger acids are more reactive, which makes them valuable in industrial processes, like etching glass (for \( \mathrm{HF} \)) or manufacturing (using \( \mathrm{HCl} \)). These acids play a critical role in both laboratory studies and practical applications.