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 \(\mathrm{O}(\mathrm{g})+\mathrm{e}^{-} \longrightarrow \mathrm{O}^{-}(\mathrm{g}) ; \Delta \mathrm{H}^{\circ}=-142 \mathrm{~kJ} \mathrm{~mol}^{-1}\) \(\mathrm{O}^{-}(\mathrm{g})+\mathrm{e}^{-} \longrightarrow \mathrm{O}^{2-}(\mathrm{g}) ; \Delta \mathrm{H}^{0}=844 \mathrm{~kJ} \mathrm{~mol}^{-1}\) 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: Understanding the process

The formation of the oxide ion \(\mathrm{O}^{2-} \) from oxygen gas involves two steps: first adding one electron to an oxygen atom, which is exothermic, releasing energy \(-142 \; \mathrm{kJ}\, \mathrm{mol}^{-1}\). Then adding another electron to form \(\mathrm{O}^{2-} \), which is endothermic, requiring energy \(844 \; \mathrm{kJ}\, \mathrm{mol}^{-1}\). The first step is favorable due to oxygen's high electron affinity, but the second is not, requiring heat or effort to proceed.

Step 2: Analyzing the given options

Each option provides a reason why the second step might be endothermic. We examine them based on the physical and chemical principles governing the electron addition: (a) Electronegativity involves attracting electrons within covalent bonds rather than to an atom. (b) High electron affinity makes the first electron easy to add, not the second. (c) A larger size might not imply energy demand unless steric or shielding factors make further electrons less favorable. (d) Resistance to further electron addition due to electron-electron repulsion or achieving stability.

Step 3: Explaining endothermic second step

The addition of the first electron to oxygen is favorable due to high electron affinity. However, once an electron is added to form \(\mathrm{O}^{-} \), the ion repels additional electrons. The negative charge on \(\mathrm{O}^{-} \) resists accepting another electron because adding another electron introduces repulsive interactions that make it less favorable energetically.

Step 4: Choosing the correct answer

Based on the analysis, option (d) describes the situation best: \(\mathrm{O}^{-} \) ion has a negative charge, making it energetically unfavorable or resisting to add another electron because of electron-electron repulsions and additional energy requirements.

Key Concepts

Exothermic process

When we talk about an exothermic process, we refer to a chemical reaction or physical change that releases energy, usually in the form of heat, to its surroundings. In the case of oxide ion formation, the first step of adding one electron to an oxygen atom, \(\mathrm{O}(\mathrm{g})+\mathrm{e}^{-}\rightarrow\mathrm{O}^{-}(\mathrm{g})\), is an exothermic process. Here, it releases \(-142 \, \mathrm{kJ} \, \mathrm{mol}^{-1}\) of energy.
This step is favorable due to oxygen's high electron affinity. - Energy released in an exothermic process can often be due to the strong attraction between the incoming electron and the nucleus of the oxygen atom, pulling the electron closer and stabilizing the resulting ion.- The change in energy helps reduce the overall energy state of the system, making it more stable.
Exothermic reactions are often spontaneous, as they decrease the system's enthalpy, meaning less energy is needed to maintain the system. For students, remember that exothermic processes indicate a system releasing more energy than it consumes.

Endothermic process

An endothermic process contrasts with an exothermic one by absorbing energy from its surroundings. In the formation of the oxide ion \(\mathrm{O}^{2-}\), the second step involves adding another electron to the already negatively charged \(\mathrm{O}^{-}\), \(\mathrm{O}^{-}(\mathrm{g})+\mathrm{e}^{-}\rightarrow\mathrm{O}^{2-}(\mathrm{g})\), and it requires \(844 \, \mathrm{kJ} \, \mathrm{mol}^{-1}\) of energy.
This step is endothermic because adding an electron to a negatively charged ion requires overcoming the repulsive force between the like charges.- In a classroom setting, think of endothermic processes as energy investments; you need to "pay" energy into the system to achieve the desired outcome.- Endothermic processes are not naturally favorable as they increase the energy state of a system.
This concept can explain why some reactions need an external energy source, like heat, to proceed.

Electron affinity

Electron affinity is a measure of an atom's ability to attract and bind an extra electron. It's like how much an atom 'wants' an electron. For oxygen, the first step in forming \(\mathrm{O}^{-}\), when an electron is added, is exothermic because of its high electron affinity.
High electron affinity means the atom releases energy when it gains an electron, stabilizing the newly formed ion.- Electron affinity typically decreases as you move down a group in the periodic table because of increased atomic radius.- Higher electron affinity is often found in nonmetals, which naturally seek to gain electrons to fulfill their electron shells.
When studying, remember that having a high electron affinity for the first electron addition doesn't guarantee favorable further additions, as seen in the endothermic nature of forming \(\mathrm{O}^{2-}\).

Electron-electron repulsion

Once an electron is added to an oxygen atom forming \(\mathrm{O}^{-}\), any additional electrons face resistance due to electron-electron repulsion. This occurs because like charges repel each other. The process of adding another electron to form \(\mathrm{O}^{2-}\) must overcome the existing charge's repulsive force.
This repulsion increases the amount of energy needed to attach a second negative electron, explaining why the step is endothermic and requires more energy.- Electron-electron repulsion is a classic concept in chemistry describing why certain atoms or ions resist further electron addition.- Consider electron shielding when discussing this—inner layers of electrons partially block outer layers from feeling the full positive nuclear charge, potentially affecting electron repulsion.
For students trying to grasp the oxide ion formation mechanism, understanding electron-electron repulsion is crucial as it dictates the energy changes across steps.