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

(b) is incorrect as \(\text{F}_2\) has lower bond dissociation energy than \(\text{Cl}_2\).

Step 1: Evaluate Electronegativity Order

The electronegativity decreases down the group in the halogens. Therefore, the order \(\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2\) is correct because fluorine is the most electronegative element.

Step 2: Evaluate Bond Dissociation Energy Order

Bond dissociation energy generally decreases as the size of the halogen atoms increases down the group. However, an exception occurs for fluorine due to strong repulsions between lone pairs in \(\text{F}_2\), which is smaller and results in lower dissociation energy than \(\text{Cl}_2\). Thus, the given order \(\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2\) is incorrect.

Step 3: Evaluate Oxidizing Power Order

Oxidizing power decreases down the group. Fluorine is a stronger oxidizing agent than chlorine, followed by bromine and iodine, matching the order \(\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2\), which is correct.

Step 4: Evaluate Acidic Property of Hydrogen Halides

The acidic property of hydrogen halides in water increases from \(\text{HF}\) to \(\text{HI}\) due to the bond strength with hydrogen decreasing down the group. Thus, the order \(\text{HI} > \text{HBr} > \text{HCl} > \text{HF}\) is correct.

Key Concepts

Electronegativity

The concept of electronegativity plays a vital role in halogen chemistry. Electronegativity is the ability of an atom to attract shared electrons in a chemical bond. In the halogen group, electronegativity decreases as we move down from fluorine to iodine. This is because the atomic size increases down the group, reducing the nucleus's pull on the valence electrons.
Fluorine, being at the top of the group, is the most electronegative, making it extremely effective at attracting electrons. This property explains why the order

• F₂ > Cl₂ > Br₂ > I₂

accurately reflects the trend of decreasing electronegativity.

Bond Dissociation Energy

Bond dissociation energy refers to the energy required to break a particular bond in a molecule, resulting in separate atoms. For halogens, bond dissociation energy generally decreases from chlorine to iodine. This occurs because larger atoms have longer bond lengths, reducing the strength of the interaction between bonding atoms.
However, fluorine is an exception. Due to its small size, there is substantial repulsion between the electron pairs in

• F₂,

making its bond weaker than that of chlorine. Therefore, the expected order is not followed, as **Cl₂** actually has a higher bond dissociation energy than **F₂**. Hence, the sequence

• F₂ > Cl₂ > Br₂ > I₂

is incorrect for bond dissociation energies.

Oxidizing Power

The oxidizing power of an element indicates its ability to gain electrons and hence oxidize other substances. In halogens, the oxidizing power decreases down the group; this is a reversal of trends like electronegativity. Fluorine, at the top, is the strongest oxidizing agent.
It has a high affinity for electrons, much greater than any other halogen. Therefore, the order

• F₂ > Cl₂ > Br₂ > I₂

is correct and reflects this trend accurately. Strong oxidizing agents like fluorine readily take electrons from other materials, carrying out oxidation efficiently.

Acidic Properties of Hydrogen Halides

The acidic properties of hydrogen halides involve their ability to donate a proton (H⁺) in water, forming acidic solutions. The strength of these acids increases down the group, from hydrofluoric acid (HF) to hydroiodic acid (HI).
This is due to the decreasing bond strength between hydrogen and the halogen as their size increases. Weaker bonds mean that they can dissociate more easily in solution, releasing more protons and resulting in stronger acids. Thus, the order

• HI > HBr > HCl > HF

correctly depicts the increasing acidic strength of hydrogen halides as the halogen atom grows larger.