Problem 12 - A Complete Resource Book in Chemistry for JEE Main

Question

Abnormality high heat of formation and shortening of bond length are criteria of (a) hybridisation (b) resonance (c) electron delocalisation (d) ionization

Step-by-Step Solution

Answer

(b) resonance

Step 1: Understand Heat of Formation

The heat of formation is the change in enthalpy when one mole of a compound forms from its constituent elements in their standard states. If the heat of formation is abnormally high, it suggests extra stability in the molecule.

Step 2: Understand Bond Lengths

Bond length is the average distance between the nuclei of two bonded atoms. A shorter bond length usually indicates a stronger bond, which can be due to higher electron density between the two nuclei.

Step 3: Identification of Relevant Concepts

Consider how each option could relate to high heat of formation and shortening of bond length: (a) Hybridisation involves mixing atomic orbitals but does not inherently suggest high heat of formation or shorter bonds; (b) Resonance involves electron delocalisation across structures, often leading to lower energy and potentially shorter bond lengths; (c) Electron delocalisation can stabilize the molecule through resonance, affecting both heat of formation and bond length; (d) Ionization typically involves removing electrons, which does not directly relate to bond length or heat of formation.

Step 4: Match Criteria to Concepts

Resonance is the most relevant concept where electron delocalisation leads to stabilization, potentially causing high heat of formation and shortened bond lengths. This phenonenon does not apply to hybridisation or ionization processes.

Key Concepts

Heat of Formation

The heat of formation is an essential concept in chemistry, especially when analyzing the stability of a molecule. It refers to the change in enthalpy when one mole of a compound is formed from its individual elements, each in their standard states. This change in enthalpy reveals a lot about the compound's stability. For example, an abnormally high heat of formation suggests that the molecule is more stable than expected. When examining heat of formation, consider:

• It is a measure of energy released or absorbed during formation.
• High heat of formation often indicates a stable molecule with strong bonds.
• In resonance structures, where electron delocalisation occurs, the stabilization effects can contribute to an increased heat of formation.

Understanding the heat of formation can help explain why certain molecular structures are more prevalent or more reactive in chemical reactions.

Bond Length

Bond length is another crucial topic when discussing molecular stability and strength. It represents the average distance between the nuclei of two bonded atoms. A shorter bond length usually implies a stronger bond, which relates to the increase in electron density between the two nuclei. Several factors can influence bond length:

• The type of atoms involved and their electronegativity differences.
• The bond order, where a higher bond order (like double or triple bonds) results in shorter bond lengths.
• Resonance structures can cause bond lengths to shorten due to the delocalization of electrons, contributing to the overall stability of the molecule.

This concept underscores how electron distribution affects bond strength and, in turn, molecular stability.

Electron Delocalisation

Electron delocalisation is a fascinating phenomenon that significantly impacts molecular stability and behavior. It involves the spreading of electrons across several atoms, rather than being localized between just two atoms. This effect is often seen in resonance structures. Benefits of electron delocalisation include:

• Increased molecular stability due to the distribution of electron density.
• Ability to explain phenomena like increased heat of formation and decreased bond lengths.
• Lower energy states for molecules, often leading to more stable formations.

In the context of resonance, electron delocalisation allows certain molecules to exist in multiple forms, thus distributing energy evenly and avoiding potential instabilities. This process effectively lowers the potential energy of the molecule, which is why it relates directly to the concepts of high heat of formation and shortened bond lengths.