Problem 116 - A Complete Resource Book in Chemistry for JEE Main

Question

The process (es) requiring the absorption of energy is/ are : (a) \(\mathrm{Cl} \rightarrow \mathrm{Cl}^{-}\) (b) \(\mathrm{O}^{-} \rightarrow \mathrm{O}^{2-}\) (c) \(\mathrm{Fe}^{3+} \rightarrow \mathrm{Fe}^{2+}\) (d) \(\mathrm{Ar} \rightarrow \mathrm{Ar}\)

Step-by-Step Solution

Answer

The process \( \text{O}^{-} \rightarrow \text{O}^{2-} \) requires energy absorption.

Step 1: Understanding Each Process

Let's examine each process. Process (a): \( \text{Cl} \rightarrow \text{Cl}^{-} \) represents the addition of an electron to chlorine, which generally releases energy. Process (b): \( \text{O}^{-} \rightarrow \text{O}^{2-} \) represents the addition of an electron to \( \text{O}^{-} \), which requires energy since negative ion repels the incoming electron. Process (c): \( \text{Fe}^{3+} \rightarrow \text{Fe}^{2+} \) represents the gain of an electron by \( \text{Fe}^{3+} \). This typically releases energy (reduction is exothermic). Process (d): \( \text{Ar} \rightarrow \text{Ar} \) doesn't change anything, thus no energy change.

Step 2: Identifying Energy Absorption

In step (b), \( \text{O}^{-} \rightarrow \text{O}^{2-} \), an electron is being added to already negative \( \text{O}^{-} \). This additional electron is being added against electrostatic repulsion and energy input is necessary. This is the only process that requires the absorption of energy. Processes (a) and (c) typically release energy, while (d) involves no electron transfer at all.

Step 3: Conclusion

The process \( \text{O}^{-} \rightarrow \text{O}^{2-} \) is the only one that requires absorption of energy due to the need to overcome electron repulsion in adding an electron to an already negatively charged ion.

Key Concepts

Electron Affinity

Electron affinity is a measure of the energy change when an electron is added to a neutral atom in the gas phase to form a negative ion. It can help us understand how much energy is released or absorbed when the atom gains an electron. Generally, when an atom, like chlorine, gains an electron, energy is released, indicating a negative electron affinity.

However, not all electron gain goes smoothly without energy input. Specifically, when adding an electron to an already negatively charged ion, such as when converting t\(\mathrm{O}^{-}\rightarrow\mathrm{O}^{2-}\), energy is required to overcome repulsion. This is because the incoming electron is repelled by the existing negative charge, leading to an endothermic process where energy absorption occurs. Typically, the first electron affinity is exothermic while subsequent electron affinities are often endothermic, requiring energy to proceed.

Electrostatic Repulsion

Electrostatic repulsion occurs when like charges push away from each other. It's a fundamental concept that influences many atomic and molecular interactions, particularly during processes where charged particles are involved. In the context of chemical reactions, when you try to add an extra electron to a negatively charged ion, it doesn't happen easily because of electrostatic repulsion.

Think of it like trying to push two magnets with the same poles together. They naturally resist being close to each other. Similarly, in the case of converting \( \mathrm{O}^{-}\) to \( \mathrm{O}^{2-} \), the negatively charged \( \mathrm{O}^{-}\) ion strongly repels the incoming electron. Hence, additional energy is required to force the electron into the orbit around the oxygen ion. This energy input is why this particular electron addition process requires absorption of energy.

Oxidation States

Oxidation states, or oxidation numbers, are used to keep track of electrons during chemical reactions. They are especially useful for identifying the transfer of electrons between atoms. In simplest terms, the oxidation state of an element is a conceptual charge for its atoms, assuming complete transfer of electrons in reactions.

For example, in the process \(\mathrm{Fe^{3+} \rightarrow Fe^{2+}}\), the iron ion gains an electron, reducing its oxidation state from +3 to +2. Converting \(\mathrm{Fe}^{3+}\) to \(\mathrm{Fe}^{2+}\) typically releases energy because the iron ion undergoes a reduction process, gaining an electron. Therefore, processes that alter oxidation states are crucial for understanding redox reactions and energy changes in chemical processes.

Identification of oxidation states before and after a reaction helps predict whether energy will be absorbed or released, as evident in various chemical behaviors and transformations.