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

Energy is liberated in process (c): \( \mathrm{Cl} + \mathrm{e} \longrightarrow \mathrm{Cl}^{-} \).

Step 1: Understanding the Concept of Energy Liberation

Energy is liberated in a process when it releases energy to the surroundings. Typically, this occurs in exothermic reactions, where energy is given off as substances reach a more stable state.

Step 2: Analyzing Process (a)

Process (a): \( \mathrm{Cl} \longrightarrow \mathrm{Cl}^{+}+\mathrm{e} \) This is the ionization of chlorine, requiring energy to remove an electron from a neutral chlorine atom, making it endothermic.

Step 3: Analyzing Process (b)

Process (b): \( \mathrm{HCl} \longrightarrow \mathrm{H}^{+}+\mathrm{Cl}^{-} \) This involves the dissociation of hydrogen chloride into ions, which typically requires energy, giving it an endothermic nature.

Step 4: Analyzing Process (c)

Process (c): \( \mathrm{Cl} + \mathrm{e} \longrightarrow \mathrm{Cl}^{-} \) An electron is added to a chlorine atom, forming a chloride ion, a process that releases energy because the ion formed is more stable than the isolated atom. Hence, this is exothermic.

Step 5: Analyzing Process (d)

Process (d): \( \mathrm{O}^{-} + \mathrm{e} \longrightarrow \mathrm{O}^{2-} \) Adding an electron to an already negatively-charged \( \mathrm{O}^{-} \) ion requires additional energy to overcome the repulsion, so this process is endothermic.

Key Concepts

Exothermic Reaction

In an exothermic reaction, energy is released into the surroundings, usually as heat. This occurs when the total energy of the products formed during a chemical reaction is lower than the total energy of the reactants.

Consider a scenario where you light a match. The heat and light you see are results of an exothermic reaction.

These reactions often feel warm or hot, because they release energy. Chemical bonds are formed during these processes, transitioning substances to a more stable state.

Electron Affinity

Electron affinity refers to the energy change when an atom gains an extra electron and forms a negative ion. When an electron is added to an atom, energy can be released (exothermic) or required (endothermic), depending on how the energy levels of the atom and the electron interact.

Chlorine, for example, has a high electron affinity. It strongly releases energy when gaining an electron, because the chlorine atom becomes much more stable as a chloride ion.

It's important to remember that, generally, non-metals have higher electron affinities compared to metals. This is because non-metals are closer to having a full valence shell of electrons.

Ionization Energy

Ionization energy is the energy required to remove an electron from a neutral atom. This process is typically endothermic, meaning it requires energy to proceed.

When you ionize an atom, you are pulling an electron away from the attraction of the positive nucleus. This process helps explain why ionization energies are essential for understanding reactivity.

• First ionization energy: energy needed to remove the first electron.
• Second ionization energy: energy needed to remove a second electron from a cation.

As you remove more electrons, each subsequent electron requires more energy to remove, because you're fighting stronger nuclear attraction.

Chemical Processes

Chemical processes refer to the transformations that substances undergo during chemical reactions. These processes involve breaking and forming chemical bonds.

Whether a reaction releases or requires energy depends on the balance of these processes.

• Exothermic chemical processes release energy, often resulting in a temperature increase in the surroundings.
• Endothermic chemical processes absorb energy, leading to a temperature decrease.

Understanding chemical processes is crucial in navigating how substances change and interact in different reactions.