Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following process is energy librated? (a) \(\mathrm{Cl} \longrightarrow \mathrm{Cl}^{+}+\mathrm{e}\) (b) \(\mathrm{HCl} \longrightarrow \mathrm{H}^{+}+\mathrm{Cl}^{-}\) (c) \(\mathrm{Cl}+\mathrm{e} \longrightarrow \mathrm{Cl}^{-}\) (d) \(\mathrm{O}^{-}+\mathrm{e} \longrightarrow \mathrm{O}^{2-}\)

Step-by-Step Solution

Answer

Option (c) \( \mathrm{Cl} + \mathrm{e} \longrightarrow \mathrm{Cl}^{-}\) liberates energy.

Step 1: Understand Energy Liberation

Energy is liberated in a process when it is released, typically in the form of heat or light, suggesting an exothermic reaction. This usually occurs during the formation of bonds or the addition of electrons.

Step 2: Analyze each Option

Let's consider each option:- **(a) \ \( \mathrm{Cl} \longrightarrow \mathrm{Cl}^{+}+\mathrm{e} \ \):** This process describes the removal of an electron from a neutral chlorine atom to form a positive ion. Removing an electron requires energy input; hence, this process absorbs energy.- **(b) \ \( \mathrm{HCl} \longrightarrow \mathrm{H}^{+}+\mathrm{Cl}^{-} \ \):** This reaction involves the dissociation of HCl into ions H⁺ and Cl⁻. Breaking the ionic bond is typically endothermic, so energy is absorbed.- **(c) \ \( \mathrm{Cl} + \mathrm{e} \longrightarrow \mathrm{Cl}^{-} \ \):** Here, an electron is added to a neutral chlorine atom forming a negative ion. This process usually releases energy, as the chlorine atom gains stability.- **(d) \ \( \mathrm{O}^{-} + \mathrm{e} \longrightarrow \mathrm{O}^{2-} \ \):** Adding an electron to a negatively charged ion like O⁻ to form O²⁻ typically requires energy input due to electron repulsion.

Step 3: Identify the Exothermic Process

From the analysis above, option (c) is the process where energy is released. When a chlorine atom gains an electron to become chloride ion (Cl⁻), energy is liberated because of the release of an enthalpy change, known as electron affinity.

Key Concepts

Exothermic Reaction

Exothermic reactions are chemical processes that release energy into the surrounding environment. This energy can be in the form of heat or, in some cases, light. In these reactions, the system releases more energy than it absorbs, leading to a decrease in the energy of the reacting molecules.
Some characteristics of exothermic reactions include:

• Heat is given off during the process.
• Products have lower potential energy compared to reactants.
• Temperature of the surroundings increases because energy is discharged to the environment.

An everyday example of an exothermic reaction is combustion, like burning wood or gasoline. In the context of the original problem, the process where a chlorine atom gains an electron and becomes a chloride ion is exothermic, as it releases energy. This energy release is connected to a concept called electron affinity.

Electron Affinity

Electron affinity refers to the energy change that occurs when an atom or molecule gains an electron. It measures the tendency of an atom to attract and hold onto an added electron. While the electrostatic attraction between the nucleus and the added electron results in a release of energy, electron affinity is crucial in understanding how atoms form negative ions.
Key points to remember:

• High electron affinity means the atom releases a significant amount of energy when it gains an electron.
• Atoms with high electron affinity are typically in the non-metal category.
• The released energy makes the atom more stable.

The original exercise about chlorine gaining an electron exemplifies high electron affinity. When chlorine becomes chloride ( Cl⁻ ), it liberates energy because of its strong attraction for the additional electron. Thus, electron affinity helps explain why energy release is prominent in specific reactions.

Enthalpy Change

Enthalpy change ( ΔH ) is a scientific term for the amount of heat absorbed or released during a chemical reaction at constant pressure. It's an essential factor in understanding whether a reaction is endothermic (absorbing heat) or exothermic (releasing heat). Knowing the enthalpy change helps us predict the energy dynamics of a given reaction.
Here's what you need to know about enthalpy change:

• Positive ΔH indicates an endothermic process where heat is absorbed.
• Negative ΔH indicates an exothermic process where heat is released.
• Measured in units like joules ( J ) or kilojoules ( kJ ) per mole.

In our original exercise, the chlorine atom acquiring an electron and releasing energy illustrates a negative ΔH . This signifies that the reaction is exothermic. Understanding enthalpy change aids in fully grasping how and why different processes either store or discharge energy.