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) releases energy.

Step 1: Understand the Exercise

The exercise is asking us to determine in which of the given processes energy is released (liberated). This usually occurs in exothermic reactions or where stability is increased by gaining electrons.

Step 2: Analyze Each Option

For option (a) \( \mathrm{Cl} \rightarrow \mathrm{Cl}^{+} + \mathrm{e} \), a neutral chlorine atom loses an electron, which requires energy input and is an endothermic process. For option (b) \( \mathrm{HCl} \rightarrow \mathrm{H}^{+} + \mathrm{Cl}^{-} \), hydrogen chloride dissociates into ions; this process does involve energy but generally requires energy input, resulting in an endothermic reaction. For option (c) \( \mathrm{Cl} + \mathrm{e} \rightarrow \mathrm{Cl} \), a chlorine atom gains an electron to become a chloride ion, which usually releases energy, making it exothermic as it's a stabilization process.For option (d) \( \mathrm{O}^{-} + \mathrm{e} \rightarrow \mathrm{O}^{2} \), an oxygen ion becomes more negatively charged, which is energetically unfavorable and requires energy input.

Step 3: Determine the Correct Option

Option (c) represents a process where energy is released because gaining an electron to become a stable ion is an exothermic process. Chlorine has a high affinity for electrons, so when it gains an electron, it releases energy.

Key Concepts

Exothermic Reactions

Exothermic reactions are processes that release energy into the surroundings, usually in the form of heat. This release of energy occurs because the products are more stable (lower energy state) than the reactants. When molecules or atoms undergo changes that lead to the release of energy, spontaneously occurring reactions result.

In an exothermic reaction:

• The energy of the products is lower than that of the reactants.
• Heat is often released, warming the surroundings.
• Common examples include combustion and the formation of chemical bonds.

For example, when we analyze option (c) from the exercise—where a chlorine atom gains an electron ( ext{Cl} + ext{e} ightarrow ext{Cl}^-)—it is a clear instance of an exothermic reaction. The chlorine atom becomes more stable by gaining an electron, releasing energy to the surroundings.

Electron Affinity

Electron affinity refers to the amount of energy released when an atom in the gas phase accepts an electron to form a negatively charged ion. A measure of the tendency of an atom to gain electrons, electron affinity is an important factor in understanding why some reactions are exothermic.

The higher the electron affinity,

• The more energy is released when an electron is added.
• The greater the tendency of the atom to gain an electron.
• Atoms like chlorine with high electron affinities release significant energy when they gain electrons.

The exercise example of chlorine gaining an electron ( ext{Cl} + ext{e} ightarrow ext{Cl}^-)highlights this well. Chlorine has a high electron affinity thus naturally releases energy when it gains an electron, resulting in an exothermic process.

Ion Formation

Ion formation is a fundamental chemical process where an atom or molecule alters its electron configuration to form charged particles called ions. This process is crucial because it often leads to the formation of more stable structures.

There are two main types of ions:

• **Cations:** Positively charged ions formed by the loss of one or more electrons (e.g., ext{Na} ightarrow ext{Na}^+ + ext{e}).
• **Anions:** Negatively charged ions formed by the gain of one or more electrons (e.g., ext{Cl} + ext{e} ightarrow ext{Cl}^-).

In the context of the exercise, we see ion formation clearly illustrated through option (c), where chlorine forms a chloride ion by gaining an electron. This transition allows chlorine to achieve a more stable electron configuration, releasing energy in the case of negative ion formation. The concepts of electron affinity and exothermic reactions are directly involved in ion formation, explaining why energy is liberated in some processes.