Question

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

Short answer

The process \\(\mathrm{Cl} + \mathrm{e} \rightarrow \mathrm{Cl}^{-}\\) liberates energy.

Step-by-step solution

Identify Energy Liberation

The process where energy is liberated typically involves a change where potential energy decreases, often associated with electron gain (exothermic) rather than loss (endothermic).

Evaluate Each Process

(a) The process \ \(\mathrm{Cl} \rightarrow \mathrm{Cl}^{+}+\mathrm{e}\ \) involves losing an electron, which requires energy input, hence endothermic.(b) The process \ \(\mathrm{HCl} \rightarrow \mathrm{H}^{+}+\mathrm{Cl}^{-}\ \) requires energy to break the bond, thus endothermic.(c) The process \ \(\mathrm{Cl} + \mathrm{e} \rightarrow \mathrm{Cl}^{-}\ \) involves electron gain, which generally releases energy, hence exothermic.(d) The process \ \(\mathrm{O}^{-} + \mathrm{e} \rightarrow \mathrm{O}^{2-}\ \) can release or require energy depending on the electron affinity of the second electron, but typically it requires energy, hence endothermic.

Conclusion: Correct Process

The process that liberates energy is \ \(\mathrm{Cl} + \mathrm{e} \rightarrow \mathrm{Cl}^{-}\ \), as electron gain for a neutral atom usually results in energy release.

Key concepts

Energy Changes in Reactions

During a chemical reaction, energy can either be absorbed or released. This is a fundamental aspect of chemistry, as the breaking and forming of bonds involve energy changes. When bonds are broken, energy is generally required to overcome the forces holding the atoms together. Conversely, when bonds form, energy is often released as atoms come together to form a stable configuration.

These energy changes are at the core of determining whether a reaction is exothermic or endothermic. It's important to remember:

• An **exothermic reaction** releases energy, often in the form of heat or light, as bonds form.
• An **endothermic reaction** absorbs energy from its surroundings, usually causing a temperature drop in the external environment as bonds are broken.

Understanding these concepts is essential in predicting how a reaction will proceed and its potential effects on the surrounding environment.

Electron Affinity

Electron affinity is the energy change that occurs when an atom or molecule gains an electron. It essentially measures how much an atom wants to acquire an additional electron, influencing its chemical reactivity and bonding behavior.

In an exothermic process, when an atom gains an electron, it typically releases energy, indicating high electron affinity. This is because adding an electron makes the atom more stable. A classic example is the reaction involving chlorine, where an electron is added to form \(\mathrm{Cl}^{-}\), releasing energy:

• The more negative the electron affinity, the greater the energy released during the electron gain process.
• Elements like halogens have high electron affinities as they require only one electron to achieve a stable, closed-shell configuration.

Having a firm grasp of these concepts allows students to predict and explain why certain reactions involve energy release and others require energy input.

Endothermic and Exothermic Reactions

Reactions are categorized based on whether they absorb or release energy. This categorization helps in understanding the energy dynamics involved in chemical processes.

**Endothermic Reactions**:

• These reactions require energy input, typically in the form of heat, to proceed.
• An example is the breakdown of \(\mathrm{HCl}\) into \(\mathrm{H}^{+}\) and \(\mathrm{Cl}^{-}\), where energy is consumed to break the chemical bonds.

**Exothermic Reactions**:

• These processes release energy when they occur.
• A well-known example is the addition of an electron to chlorine to form \(\mathrm{Cl}^{-}\), which releases energy, making it an exothermic process.

Being able to differentiate between these two types of reactions helps students predict reaction behavior, especially in new or unfamiliar chemical processes.