Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

The electronic affinity values (in \(\mathrm{kJ} \mathrm{mol}^{-1}\) ) of three halogens \(\mathrm{X}, \mathrm{Y}\) and \(\mathrm{Z}\) are respectively \(-349,-333\) and \(-325\). Then \(\mathrm{X}, \mathrm{Y}\) and \(\mathrm{Z}\) respectively are (a) \(\mathrm{F}_{2}, \mathrm{Cl}_{2}\) and \(\mathrm{Br}_{2}\) (b) \(\mathrm{Cl}_{2}, \mathrm{~F}_{2}\) and \(\mathrm{Br}_{2}\) (c) \(\mathrm{Cl}_{2}, \mathrm{Br}_{2}\) and \(\mathrm{F}_{2}\) (d) \(\mathrm{Br}_{2}^{2}, \mathrm{Cl}_{2}\) and \(\mathrm{F}_{2}\)

Step-by-Step Solution

Answer

The correct answer is (b) \(\mathrm{Cl}_2, \mathrm{F}_2, \mathrm{Br}_2\).

Step 1: Understanding Electron Affinity

Electron affinity is the amount of energy released when an electron is added to a neutral atom in the gaseous state. A more negative value indicates a higher electron affinity because more energy is released.

Step 2: Identifying the Order of Electron Affinities

The three electron affinities given are \(-349 \ \mathrm{kJ/mol}\), \(-333 \ \mathrm{kJ/mol}\), and \(-325 \ \mathrm{kJ/mol}\). The question implies that these values correspond to different halogens in descending order of electron affinity.

Step 3: Recognizing Typical Halogen Electron Affinities

Fluorine (F_2), chlorine (Cl_2), and bromine (Br_2) are common halogens with notable electron affinities. Chlorine typically has the highest electron affinity, followed by fluorine, and then bromine.

Step 4: Assigning Electron Affinity Values

From highest to lowest electron affinity for the halogens: \(Cl_2\) is \(-349 \ \mathrm{kJ/mol}\), \(F_2\) is \(-333 \ \mathrm{kJ/mol}\), and \(Br_2\) is \(-325 \ \mathrm{kJ/mol}\).

Step 5: Matching the Values to the Options

Now, we compare this assignment to the given options: (a) \(F_2, Cl_2, Br_2\); (b) \(Cl_2, F_2, Br_2\); (c) \(Cl_2, Br_2, F_2\); (d) \(Br_2, Cl_2, F_2\). Option (b) matches our order of highest to lowest electron affinity: \(Cl_2, F_2, Br_2\).

Key Concepts

Halogens

Halogens are a group of elements found in Group 17 of the periodic table. These elements consist of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Halogens are known for their high reactivity, particularly with alkali metals and alkaline earth metals, due to their high desire to gain an extra electron and form stable halide ions.

The reactivity of halogens decreases down the group. Fluorine is the most reactive, followed by chlorine, bromine, and so on. This reactivity is attributed to their electronegativity and electron affinity. In practical applications, halogens are used in disinfection, in the creation of halogen lamps, and as key building blocks in the synthesis of organic compounds. Each halogen plays a significant role in biological processes and industrial applications.

• Commonly form salts when combined with metals
• Exist naturally in molecular form (e.g., F₂, Cl₂)
• Have distinct and strong colors and odors

Energy Released

When we talk about electron affinity, we're discussing the energy released when an electron is added to a neutral atom in its gaseous state. A more negative value indicates a greater amount of energy released, signifying a stronger attraction between the incoming electron and the atom. This is because the energy released compensates for the energy required to bring the electron near the nucleus.

In simpler terms, the more negative the electron affinity value, the more an atom wants to gain that additional electron, thus making it more energetically favorable. This energy release is pivotal in the formation of chemical bonds, influencing the stability and reactivity of an element. In the context of halogens, this energy reflects their high reactivity and ability to form stable ionic compounds.

• Exothermic process (releases energy)
• Shows a "desire" for gaining an electron
• A factor in forming ionic bonds

Electron Affinity Order

The order of electron affinity among halogens typically follows a specific trend. In general, chlorine has the highest electron affinity among the common halogens, with fluorine coming in second, followed by bromine. This order is not strictly according to the periodic table positions but changes because of factors such as atomic size and the electron cloud's effective nuclear charge.

Chlorine's higher electron affinity compared to fluorine is somewhat counterintuitive given that fluorine is more electronegative. However, fluorine's smaller atomic size causes increased electron-electron repulsion when an electron is added, slightly reducing the energy released during this process. Therefore, in electron affinity order, the descending sequence for the halogens typically studied is:
1. Chlorine (Cl) 2. Fluorine (F) 3. Bromine (Br)

• Reflects reactivity and chemical behavior
• Important for understanding chemical trends
• Influences bond formation

Gaseous State

The gaseous state of an element is when it exists as a gas, meaning its atoms or molecules are widely spaced and moving freely. In discussions of electron affinity, it's crucial because electron affinity values are measured in this state. This ensures that interactions between atoms are minimized, allowing for a pure measurement of the energy change associated with gaining an electron.

This state is essential for understanding intrinsic properties since it isolates the behavior of single atoms without the influence of bonding or other interactions. For halogens originally not found in their gaseous state under standard conditions, they are converted to this state for the precision required in scientific measurements.

• Ensures consistency in measuring electron affinity
• Allows analysis of individual atomic properties
• Standard condition for comparing elements