Problem 149 - A Complete Resource Book in Chemistry for JEE Main

Question

The formation of the oxide ion \(\mathrm{O}^{2-}(\mathrm{g})\) require first an exothermic and then an endothermic step as shown below \(\mathrm{O}(\mathrm{g})+\mathrm{e}^{-} \longrightarrow \mathrm{O}^{-}(\mathrm{g}) ; \Delta \mathrm{H}^{0}=-142 \mathrm{~kJ} \mathrm{~mol}^{-1}\) \(\mathrm{O}^{-}(\mathrm{g})+\mathrm{e}^{-} \longrightarrow \mathrm{O}^{2-}(\mathrm{g}) ; \Delta \mathrm{H}^{0}=844 \mathrm{~kJ} \mathrm{~mol}^{-1}\) This is because (a) oxygen is more electronegative (b) oxygen has high electron affinity (c) \(\mathrm{O}^{-}\)ion has comparatively larger size than oxygen atom (d) \(\mathrm{O}^{-}\)ion will tend to resist the addition of another electron

Step-by-Step Solution

Answer

(d) \( \mathrm{O}^{-} \) ion will tend to resist the addition of another electron.

Step 1: Analyze the Two Reactions

The problem provides two reactions. The first is \( \mathrm{O}(\mathrm{g})+\mathrm{e}^{-} \rightarrow \mathrm{O}^{-}(\mathrm{g}) \) with \( \Delta \mathrm{H}^{0}=-142 \mathrm{~kJ} \). This exothermic step indicates that the oxygen atom easily gains an electron due to its electronegativity. The second reaction is \( \mathrm{O}^{-}(\mathrm{g})+\mathrm{e}^{-} \rightarrow \mathrm{O}^{2-}(\mathrm{g}) \) with \( \Delta \mathrm{H}^{0}=844 \mathrm{~kJ} \), an endothermic process showing resistance to gaining another electron.

Step 2: Compare the Stability of Ions

Upon forming \( \mathrm{O}^{-} \), the oxygen atom becomes more stable, achieving a filled p-shell. Adding another electron to form \( \mathrm{O}^{2-} \) requires significant energy, as the \( \mathrm{O}^{-} \) already has a higher electron density and will repel the additional electron.

Step 3: Deduce the Reason for the Endothermic Step

Since the formation of \( \mathrm{O}^{2-} \) is endothermic, the \( \mathrm{O}^{-} \) ion resists gaining an additional electron. This is due to the increased electron-electron repulsion and not because of increased electronegativity or electron affinity, which are intrinsic properties of the neutral atom.

Key Concepts

Electron Affinity

Electron affinity refers to the energy change that occurs when an electron is added to a neutral atom to form a negative ion. It's an important concept when studying how atoms react with electrons. For many atoms, adding an electron releases energy and is an exothermic process. This is true for elements like halogens, which have high electron affinities.

In the context of oxygen, its first electron affinity is exothermic. This means that when a neutral oxygen atom in its gaseous form accepts an electron to become an oxide ion \( \mathrm{O}^{-}\), energy is released. This occurs because oxygen's high electronegativity eagerly attracts electrons, stabilizing the atom.

• High electron affinity = more energy released.
• Exothermic first step: \( \mathrm{O} + \mathrm{e}^{-} \rightarrow \mathrm{O}^{-} \).
• Increased stability due to energy release.

Understanding electron affinity helps us predict how elements will behave in reactions involving electron gain.

Endothermic and Exothermic Reactions

The terms endothermic and exothermic describe energy changes during chemical reactions. An exothermic reaction releases energy, typically as heat, because the final products are lower in energy than the reactants.

In contrast, an endothermic reaction absorbs energy to proceed. This means that the reactants absorb energy to transform into products, resulting in substances that are higher in energy than the initial reactants. In the formation of the oxide ion \( \mathrm{O}^{2-} \), we observe this dual nature:

• \( \mathrm{O} + \mathrm{e}^{-} \rightarrow \mathrm{O}^{-} \): Exothermic, releases \(-142 \mathrm{~kJ/mol} \).
• \( \mathrm{O}^{-} + \mathrm{e}^{-} \rightarrow \mathrm{O}^{2-} \): Endothermic, requires \(844 \mathrm{~kJ/mol} \).

These reactions demonstrate oxygen’s behavior: the initial gain of an electron is spontaneous and advantageous, while adding a second electron requires additional energy input due to electron repulsion.

Ion Stability

Ion stability is a critical concept when discussing the formation and behavior of ions. Stability in ions is largely determined by electron configurations and the energy associated with gaining or losing electrons. When a neutral atom gains an electron, it can become more stable if it achieves a filled or half-filled electron shell.

For the oxygen atom, forming the \( \mathrm{O}^{-} \) ion increases stability because it fills a p-orbital, lowering energy. However, when attempting to form \( \mathrm{O}^{2-} \), the situation is different. The added electron increases electron-electron repulsion due to the charge already present on \( \mathrm{O}^{-} \).

• \( \mathrm{O}^{-} \) ion: Relatively more stable with filled p-orbital.
• \( \mathrm{O}^{2-} \) ion: Less stability due to increased repulsion.
• High energy cost for additional electron: \( \mathrm{O}^{2-} \) formation is endothermic.

Understanding ion stability provides insight into why certain ions form more readily than others and the energetic challenges involved in achieving higher negative charges.