Problem 79 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Give reasons for the following : (i) The experimentally determined \(\mathrm{N}-\mathrm{F}\) bond length in \(\mathrm{NF}_{3}\) is greater than the sum of the single covalent bond radii of \(\mathrm{N}\) and \(\mathrm{F}\). [1995 - 2 Marks] (ii) Ammonium chloride is acidic in liquid ammonia solvent. [1991 - 1 Mark] (iii) Phosphine has lower boiling point than ammonia. [1989-1 Mark] (iv) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is a dibasic acid. [1989-1 Mark] (v) Orthophosphorus acid is not tribasic acid. [1987-1 Mark] (vi) A bottle of liquor ammonia should be cooled before opening the stopper. [1983-1 Mark] (vii) Orthophosphoric acid, \(\mathrm{H}_{3} \mathrm{PO}_{4}\), is tribasic, but phosphorous acid, \(\mathrm{H}_{3} \mathrm{PO}_{3}\), is dibasic. [1982 - 1 Mark]

Step-by-Step Solution

Answer

(i) Lone pair repulsion and bond character; (ii) Dissociates as an acid in ammonia; (iii) Ammonia's hydrogen bonding; (iv) Can donate two protons; (v) Only two ionizable hydrogens; (vi) High vapor pressure; (vii) Differing ionizable hydrogen atoms.

Step 1: NF3 Bond Length Explanation

The bond length in a molecule depends on various factors including atomic size, electronegativity, and the bond type. For \( \mathrm{NF}_{3} \), the excess length of the \( \mathrm{N}-\mathrm{F} \) bond over the sum of the single covalent bond radii is primarily due to the involvement of lone pair repulsion and the partial double bond character from resonance, which impacts the bond length.

Step 2: Acidity of Ammonium Chloride in Liquid Ammonia

In liquid ammonia, \( \text{NH}_4\text{Cl} \) behaves as an acid because it is capable of donating a proton (\( \text{NH}_4^+ \) ion) to ammonia, forming \( \text{NH}_3 \) and \( \text{NH}_2^- \). This reaction demonstrates its acidic nature in a non-aqueous solvent like liquid ammonia.

Step 3: Boiling Point Comparison Between Phosphine and Ammonia

Ammonia \((\text{NH}_3)\) has higher boiling point than phosphine \((\text{PH}_3)\) due to the presence of hydrogen bonding. Ammonia can form strong hydrogen bonds due to high electronegativity and small size of nitrogen, whereas phosphine molecules are held together by weaker van der Waals forces.

Step 4: Dibasic Nature of \( \mathrm{H}_3\mathrm{PO}_3 \)

\( \text{H}_3\text{PO}_3 \), phosphorous acid, is dibasic as it can donate two protons. The H atoms that are attached to oxygen are ionizable, contributing to its acidic nature; the third H is directly bonded to phosphorus and non-ionizable under normal conditions.

Step 5: Orthophosphorus Acid Non-Tribasic

Orthophosphorus acid \((\mathrm{H}_3\mathrm{PO}_3 )\) is not tribasic because only two of its hydrogen atoms are ionizable (attached to oxygen). The third hydrogen, attached directly to phosphorus, isn't labile to form ions, confirming its dibasic property.

Step 6: Cooling of Liquor Ammonia

A bottle of liquor ammonia should be cooled because ammonia gas has high vapor pressure at room temperature. Cooling decreases the vapor pressure, preventing rapid release and reducing the risk of spills or exposure.

Step 7: Chemical Nature Differences Between \( \mathrm{H}_3\mathrm{PO}_4 \) and \( \mathrm{H}_3\mathrm{PO}_3 \)

Orthophosphoric acid \((\mathrm{H}_3\mathrm{PO}_4)\) is tribasic as all three hydrogen atoms are bonded to the oxygen atoms and can ionize; however, in phosphorous acid \((\mathrm{H}_3\mathrm{PO}_3)\), only two hydrogens are bonded to oxygen, making the acid dibasic.

Key Concepts

Chemical Bonding

Chemical bonding is fundamental to understanding the structure of molecules and how they interact. The bond length, which is the distance between the nuclei of two bonded atoms, can be influenced by multiple factors. In the case of nitrogen trifluoride (\( F_3 \)), the bond length between nitrogen and fluorine is found to be longer than expected when adding up the covalent radii of the two atoms.
This length discrepancy arises mainly due to lone pair repulsions on nitrogen, which can increase the size of the molecule. Additionally, resonance can impart partial double-bond character on the \( F_3 \) structure, altering the expected bond length. Understanding these effects provides insight into how molecular structures deviate from simplistic models based on single and double bond predictions. By visualizing these forces, students can better comprehend why molecules like \( F_3 \) behave unexpectedly when applying straightforward covalent radius arithmetic.

Acids and Bases

Acids and bases are characterized by their ability to donate or accept protons (H\(^+\)) in chemical reactions. When ammonium chloride (\( H_4Cl \)) is dissolved in liquid ammonia, it acts as an acid. In this non-aqueous solvent, \( H_4^+ \) ions donate protons to ammonia (\( NH_3 \)) molecules, producing \( H_3 \) and \( H_2^- \). This proton donation is a classic behavior of an acid, even though the medium is not water but another polar solvent.
Understanding acid-base interactions in different solvents is essential because their behavior can vary significantly from one medium to another. Liquid ammonia provides a unique environment where compounds can display different acidic or basic properties than they would in water. This variability challenges students to think beyond traditional acid-base contexts usually observed in aqueous solutions.

Molecular Structure

The molecular structure of phosphine (\( H_3 \)) and ammonia (\( H_3 \)) highlights the role of intermolecular forces in determining physical properties such as boiling points. Ammonia has a higher boiling point than phosphine because of the hydrogen bonds formed between ammonia molecules. The greater electronegativity and smaller size of nitrogen compared to phosphorus allow for the formation of these strong hydrogen bonds.
Phosphine, on the other hand, relies on weaker van der Waals forces due to the absence of such hydrogen bonding. This difference vividly illustrates how structure directly influences chemical properties and behaviors, such as boiling points.

• Ammonia forms hydrogen bonds: strong and directional
• Phosphine relies on van der Waals interactions: weaker, non-directional

By evaluating these differences, students gain a deeper understanding of how molecular structure parameters, like electronegativity and size, play critical roles in shaping material properties.