Problem 4 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The process that is NOT endothermic in nature is : (a) \(\mathrm{Ar}(\mathrm{g})+\mathrm{e}^{-} \rightarrow \mathrm{Ar}^{-}(\mathrm{g})\) (b) \(\mathrm{H}(\mathrm{g})+\mathrm{e}^{-} \rightarrow \mathrm{H}^{-}(\mathrm{g})\) (c) \(\mathrm{O}^{-}(\mathrm{g})+\mathrm{e}^{-} \rightarrow \mathrm{O}^{2-}(\mathrm{g})\) (d) \(\mathrm{Na}(\mathrm{g}) \rightarrow \mathrm{Na}^{+}(\mathrm{g})+\mathrm{e}^{-}\)

Step-by-Step Solution

Answer

Option (d) is not endothermic; it's exothermic.

Step 1: Identifying Endothermic Processes

Endothermic processes absorb energy from the surroundings. In this context, a process is endothermic if it requires energy input to occur, such as the addition of electrons to atoms or ions against existing electron-electron repulsions.

Step 2: Analyzing Option (a)

The reaction \(\mathrm{Ar}(\mathrm{g})+\mathrm{e}^{-}\rightarrow\mathrm{Ar}^{-}(\mathrm{g})\) involves adding an electron to a noble gas atom. This process typically requires energy because noble gases have a complete valence shell, making it unfavorable to add an extra electron.

Step 3: Analyzing Option (b)

The reaction \(\mathrm{H}(\mathrm{g})+\mathrm{e}^{-}\rightarrow\mathrm{H}^{-}(\mathrm{g})\) involves adding an electron to hydrogen. This process is endothermic because adding an extra electron to a neutral hydrogen atom faces repulsion similar to those in noble gases.

Step 4: Analyzing Option (c)

The reaction \(\mathrm{O}^{-}(\mathrm{g})+\mathrm{e}^{-}\rightarrow\mathrm{O}^{2-}(\mathrm{g})\) involves adding an electron to a negatively charged ion. This process is endothermic as it requires energy to overcome the repulsion between the additional electron and the already negatively charged ion.

Step 5: Analyzing Option (d)

The reaction \(\mathrm{Na}(\mathrm{g})\rightarrow\mathrm{Na}^{+}(\mathrm{g})+\mathrm{e}^{-}\) involves the removal of an electron from a neutral sodium atom. This process releases energy because it results in a more stable ion configuration, making it exothermic rather than endothermic.

Key Concepts

Electron Affinity

Electron affinity is the amount of energy released when an electron is added to a neutral atom to form an anion. It is a measure of how much an atom wants to gain an electron.

• A positive electron affinity value indicates that energy is released when an electron is added.
• An element with a high electron affinity tends to gain electrons easily.

Most nonmetals have high electron affinities as they tend to form negative ions. For instance, adding an electron to oxygen releases energy, even though adding a second electron (as in turning \(\text{O}^{-}(\text{g})\) to \(\text{O}^{2-}(\text{g})\)) requires energy due to electron-electron repulsion, making the process endothermic.

Exothermic Reactions

An exothermic reaction is a chemical change that releases energy to the surroundings, usually in the form of heat. It makes the surroundings warmer.

• Characteristics include a decrease in the energy content of the system.
• Exothermic reactions often involve bond formation.

For example, the ionization of sodium (\(\text{Na}(\text{g}) \rightarrow \text{Na}^{+}(\text{g}) + \text{e}^{-}\)) is exothermic because losing an electron leads to a stable electronic configuration, releasing energy.

Noble Gases

Noble gases are elements in Group 18 of the periodic table, known for their full valence electron shells, making them particularly stable and chemically inert.

• They barely undergo chemical reactions due to their complete valence shells.
• Adding electrons to noble gas atoms is energetically unfavorable, hence often endothermic.

For instance, adding an electron to argon \(\mathrm{Ar}(\mathrm{g})+\mathrm{e}^{-} \rightarrow \mathrm{Ar}^{-}(\mathrm{g})\) requires energy input, as it disturbs the stable electronic configuration, characterizing it as an endothermic process.

Ionization Energy

Ionization energy is the energy required to remove an electron from a neutral atom in the gas phase, resulting in a cation.

• Elements with high ionization energies hold their electrons tightly.
• Metals have relatively low ionization energies as they easily lose electrons.

For example, the process of removing an electron from sodium \(\mathrm{Na}(\mathrm{g})\) actually releases energy (as a stable cation is formed) making it an exothermic process, contrary to the usual trend of endothermic ionization.