Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following orders is not in accordance with the property stated against it? (a) \(\mathrm{F}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}\); electronegativity (b) \(\mathrm{F}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2} ;\) bond dissociation energy (c) \(\mathrm{F}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}\); oxidizing power (d) \(\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}\); acidic property in water

Step-by-Step Solution

Answer

Option (b) is not in accordance.

Step 1: Understanding the Concept

Each option presents a property of the halogens (group 17 elements) and orders them according to that property. We need to identify which order deviates from the expected pattern based on known chemical principles.

Step 2: Analyzing Option (a)

The electronegativity of halogens typically decreases as you move down the group. Thus, the order \(\mathrm{F}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}\) is correct for electronegativity, as fluorine is the most electronegative element.

Step 3: Analyzing Option (b)

The bond dissociation energy is related to the strength of the bond between atoms. It typically decreases down the group after chlorine, making \(\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{F}_{2}>\mathrm{I}_{2}\) the correct order. Therefore, the given order for bond dissociation energy is incorrect.

Step 4: Analyzing Option (c)

The oxidizing power of halogens decreases as you move down the group. Therefore, \(\mathrm{F}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}\) is correct for oxidizing power, as fluorine is the strongest oxidizing agent.

Step 5: Analyzing Option (d)

For acidic property in water, the acidic strength generally increases as the electronegativity of the hydrogen halide decreases, thus \(\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}\) is the correct order for acidic property in water.

Step 6: Conclusion

After analyzing each option, option (b) is the one where the order given does not align with the expected chemical property associated with bond dissociation energy.

Key Concepts

Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. In the halogen group, this property plays a crucial role in their chemical behavior.
The general trend for electronegativity among halogen elements is that it decreases as you move down the group in the periodic table. This means that fluorine (\(F_2\)) is the most electronegative, followed by chlorine (\(Cl_2\)), bromine (\(Br_2\)), and iodine (\(I_2\)).
This trend is because as the atomic number increases, more electron shells are added, which results in a larger atomic radius. The increased distance between the nucleus and the valence electrons makes it harder for an atom to attract additional electrons.

• Fluorine is the most electronegative element in the entire periodic table.
• The high electronegativity of fluorine makes it a very reactive and strong oxidizing agent.
• Electronegativity influences properties such as bond polarity and molecule reactivity.

Bond Dissociation Energy

Bond dissociation energy (BDE) is a critical concept that refers to the energy required to break a bond in a molecule, resulting in the separation of atoms. In the context of halogens, BDE is indicative of bond strength.
For halogens, the bond dissociation energy typically starts high for chlorine and then decreases as you go down the group after chlorine.
Thus, the correct order is \(Cl_2 > Br_2 > F_2 > I_2.\) The reasoning includes:

• Fluorine, despite being highly electronegative, forms relatively weaker bonds due to repulsion between its small atoms.
• Chlorine forms stronger bonds than bromine and iodine due to optimal atom size and bond length that minimize energetic repulsion.
• As atomic size increases down the group, the bond energies decrease due to less overlap of the larger atomic orbitals.

Oxidizing Power

The oxidizing power of an element is its ability to accept electrons and thereby cause oxidation in other compounds. Halogens are well-known oxidizing agents due to their high electronegativity.
In the halogen group, oxidizing power decreases from fluorine to iodine. Therefore, the correct order is \(F_2 > Cl_2 > Br_2 > I_2\).
This is due to several reasons:

• Fluorine is the strongest oxidizer and can accept electrons readily because of its high electronegativity and small atomic size.
• As you move down the group, the size of the halogen atoms increases, making it harder for them to attract additional electrons, leading to decreased oxidizing power.
• The strong oxidizing nature of fluorine is evident in its ability to react with many substances, including noble gases.

Acidic Property in Water

The acidic property of halogen acids in water depends on their ability to release hydrogen ions (\(H^+\)) into the solution. The acidic strength of hydrogens halides in water is influenced by the bond's stability and the size of the halogen atom.
The order of acidity for halogen acids in water is \(HI > HBr > HCl > HF\).
This counterintuitive order can be explained by the following:

• Hydrogen fluoride (\(HF\)) has the strongest bond with hydrogen due to its high electronegativity, but this makes it less able to dissociate in water.
• The weaker bond of hydrogen iodide (\(HI\)), due to iodine's larger atomic size, breaks more readily, thus making \(HI\) a stronger acid in water.
• The size and bond strength balance shifts as we move from hydrogen iodide to hydrogen fluoride, impacting acidic strength in water.