Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The correct order of bond dissociation energy among \(\mathrm{N}_{2}, \mathrm{O}_{2}, \mathrm{O}_{2}^{-}\)is shown in which of the following arrangements? (a) \(\mathrm{N}_{2}>\mathrm{O}_{2}^{-}>\mathrm{O}_{2}\) (b) \(\mathrm{O}_{2}^{-}>\mathrm{O}_{2}>\mathrm{N}_{2}\) (c) \(\mathrm{N}_{2}>\mathrm{O}_{2}>\mathrm{O}_{2}^{-}\) (d) \(\mathrm{O}_{2}>\mathrm{O}_{2}^{-}>\mathrm{N}_{2}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{N}_2 > \mathrm{O}_2 > \mathrm{O}_2^-\)

Step 1: Understanding Bond Dissociation Energy

Bond dissociation energy is the energy required to break a bond in a molecule to form neutral isolated atoms. For diatomic molecules, it depends on the bond order.

Step 2: Analyzing Nitrogen Molecule ( \(\mathrm{N}_2\) )

For \(\mathrm{N}_2\), the bond order is 3, which is very high, resulting in a strong triple bond with a high bond dissociation energy. \(\mathrm{N}_2\) is known to have one of the highest bond dissociation energies.

Step 3: Analyzing Oxygen Molecule ( \(\mathrm{O}_2\) )

For \(\mathrm{O}_2\), the bond order is 2, which is lower than \(\mathrm{N}_2\), so it has lower bond dissociation energy compared to \(\mathrm{N}_2\).

Step 4: Analyzing Superoxide Anion ( \(\mathrm{O}_2^-\) )

The addition of an electron to \(\mathrm{O}_2\) to form \(\mathrm{O}_2^-\) results in a bond order of 1.5, reducing the bond strength further compared to neutral \(\mathrm{O}_2\). Therefore, \(\mathrm{O}_2^-\) has the lowest bond dissociation energy among the three.

Step 5: Comparing Bond Dissociation Energies

Based on the bond orders and the resulting strength of the bonds, the order of bond dissociation energy is \(\mathrm{N}_2 > \mathrm{O}_2 > \mathrm{O}_2^-\).

Key Concepts

Bond Order

Bond order is an essential concept when discussing the strength and stability of chemical bonds in molecules. It is defined as the number of chemical bonds between a pair of atoms. For instance, a bond order of 1 indicates a single bond, while a bond order of 2 indicates a double bond. This concept is crucial in determining a molecule's bond dissociation energy—the energy needed to break the bond between atoms.

• Higher bond order often implies a stronger, more stable bond with higher bond dissociation energy.
• Lower bond orders suggest weaker bonds with lower bond dissociation energy.

Understanding bond order is fundamental in predicting how a molecule will behave in chemical reactions and how much energy is required to break its bonds.

Triple Bond

A triple bond occurs when three pairs of electrons are shared between two atoms. This type of bond is notably strong, with higher bond dissociation energy compared to single or double bonds. A prime example is the nitrogen molecule (\(\mathrm{N}_2\)).

• In \(\mathrm{N}_2\), each nitrogen atom shares three electrons, resulting in a bond order of 3.
• This makes \(\mathrm{N}_2\) one of the most stable molecules with one of the highest bond dissociation energies.

Triple bonds are less common than single or double bonds, but they play a critical role in the properties of molecules such as nitrogen, which has essential implications in various chemical processes.

Diatomic Molecules

Diatomic molecules are those composed of two atoms, which may be the same or different elements. Familiar examples include \(\mathrm{N}_2\) and \(\mathrm{O}_2\), both of which are crucial components of Earth's atmosphere.

• These molecules often have simple structures and predictable chemical behavior.
• The bond order of diatomic molecules directly influences their bond dissociation energy.

In \(\mathrm{N}_2\), as a diatomic molecule with a triple bond, we find high structural stability, making it a less reactive gas. Conversely, the \(\mathrm{O}_2\) molecule, with a double bond, exhibits lower bond dissociation energy, which affects its ability to engage readily in oxidation reactions.

Superoxide Anion

The superoxide anion (\(\mathrm{O}_2^-\)) is a fascinating species arising from the addition of an electron to the oxygen molecule \(\mathrm{O}_2\). This additional electron interaction decreases the bond order compared to the neutral \(\mathrm{O}_2\) molecule.

• The bond order falls to 1.5, reflecting a weakened bond strength.
• This decrease in bond order results in a lower bond dissociation energy.

Superoxide anions play significant roles in biological systems and industrial processes. They can be involved in oxidative stress within cells or act as intermediates in various chemical reactions. Understanding their properties helps in comprehending their behavior in more complex chemical contexts.