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

Process (b) \( \mathrm{O}^{-} \rightarrow \mathrm{O}^{2-} \) requires energy absorption.

Step 1: Understanding the Process

The problem asks us to identify which processes require the absorption of energy. For a process to need energy absorption, it typically involves the addition of an electron to an already negative ion or removal of an electron from a stable atom or positive ion.

Step 2: Analyzing Each Process

Let's consider each given process: (a) \( \mathrm{Cl} \rightarrow \mathrm{Cl}^{-} \): This process involves adding an electron to a neutral chlorine atom, which typically releases energy (since Cl has a high electron affinity).(b) \( \mathrm{O}^{-} \rightarrow \mathrm{O}^{2-} \): This process involves adding an electron to an already negative ion \( \mathrm{O}^{-} \), which requires additional energy due to added electron repulsion.(c) \( \mathrm{Fe}^{3+} \rightarrow \mathrm{Fe}^{2+} \): This process involves gaining an electron, thus energy is typically released as \( \mathrm{Fe}^{3+} \) becomes more stable as \( \mathrm{Fe}^{2+} \).(d) \( \mathrm{Ar} \rightarrow \mathrm{Ar} \): No electron change, no energy absorption needed.

Step 3: Conclusion of Process Energy Requirement

Out of the given processes, (b) \( \mathrm{O}^{-} \rightarrow \mathrm{O}^{2-} \) involves overcoming the electron-electron repulsion to add an electron to an existing negative ion, leading to energy absorption. Therefore, (b) is the process requiring energy absorption.

Key Concepts

Electron Affinity

In the world of chemistry, electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gas phase. This addition of electrons can release or require energy, depending on the atom involved. For elements that have very high electron affinity, like chlorine, energy is usually released when an electron is added.
High electron affinity means an atom is more capable of accepting an electron and integrating it into its electron cloud. Chlorine, for example, releases energy when a neutral atom gains an electron, as it reaches a more stable electronic configuration.
However, some atoms require energy to add an electron. This happens when there is increased repulsion between electrons, as seen in atoms that are already negatively charged.

Electron Repulsion

Electron repulsion is a concept that describes the forces among electrons in an atom. When a negative ion already contains an extra electron, adding another one can cause repulsion amongst the electrons.
Think of electrons like magnets: similar charge will repel each other. Thus, it takes energy to add more electrons to a negatively charged entity, such as turning \( \mathrm{O}^{-} \rightarrow \mathrm{O}^{2-} \).
This repulsion requires overcoming energy barriers, making the process of adding an electron energetically demanding. It is akin to forcing two magnets with the same pole to touch – effort must be exerted to achieve this, resulting in energy absorption during the process.

Ionic Stability

Ionic stability is about how comfortable or inert an ion is due to its electronic configuration. A stable ion doesn't want to accept or lose electrons without some external influence, often in the form of energy.
For instance, \( \mathrm{Fe}^{3+} \rightarrow \mathrm{Fe}^{2+} \) involves gaining an electron, which actually makes the ion more stable. This process emits energy, as evidenced by the stability that \( \mathrm{Fe}^{2+} \) offers over \( \mathrm{Fe}^{3+} \).
Ions tend to move towards stability by reaching electronic configurations that satisfy energy minima, matching the configurations of noble gases or achieving a balanced electrostatic environment.

Chemical Processes

Chemical processes involve a variety of reactions that can absorb or release energy. Each process has its unique characteristics, often dictated by the interactions of electrons.
When assessing chemical reactions, we look at the flow of electrons and changes in energy states to determine the process's nature. In the case of \( \mathrm{Ar} \rightarrow \mathrm{Ar} \), no electron movement occurs, indicating a lack of energy exchange. Understanding chemical processes requires recognizing that energy change is a primary indicator of what occurs during these interactions. Recognizing these patterns helps predict how different compounds react and the energy dynamics involved.