Problem 192 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following substances has the highest proton affinity? (a) \(\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{H}_{2} \mathrm{~S}\) (c) \(\mathrm{NH}_{3}\) (d) \(\mathrm{PH}_{3}\)

Step-by-Step Solution

Answer

Ammonia, \\(\mathrm{NH}_{3}\\), has the highest proton affinity.

Step 1: Understanding Proton Affinity

Proton affinity is a measure of how strongly a molecule can hold onto a proton. A higher value indicates a stronger attraction to protons, meaning the molecule can easily form a bond with a proton.

Step 2: Comparing Proton Affinities

Typically, molecules with lone pairs on more electronegative atoms tend to have higher proton affinities. Oxygen and nitrogen are both more electronegative than sulfur and phosphorus. This suggests \(\mathrm{H}_{2}\mathrm{O}\) and \(\mathrm{NH}_{3}\) might have higher proton affinities.

Step 3: Evaluate Molecules with Lone Pairs

Both \(\mathrm{NH}_{3}\) and \(\mathrm{H}_2\mathrm{O}\) have lone pairs that can accept protons, but \(\mathrm{NH}_{3}\) is known for its strong affinity due to the high electron density around the nitrogen atom compared to oxygen.

Step 4: Conclusion - Select Highest Proton Affinity

Among the given options, \(\mathrm{NH}_{3}\) has the highest proton affinity due to nitrogen's ability to effectively share its lone pair with a proton when compared to the other molecules listed.

Key Concepts

Electronegativity

Electronegativity is a key concept in chemistry that describes an atom's ability to attract and hold onto electrons. When considering proton affinity, electronegativity plays a significant role. Atoms with higher electronegativity tend to pull electrons more strongly towards themselves.
For instance, in our original exercise, nitrogen and oxygen are more electronegative than sulfur and phosphorus. This directly influences the tendency of the molecule to attract protons, as the atoms with higher electronegativity will create a higher electron density around themselves. This increased electron density helps the molecules, such as \[\mathrm{NH}_{3}\] and \(\mathrm{H}_{2}\mathrm{O}\), to attract and hold onto protons more effectively. Therefore, understanding electronegativity can help predict and explain the behavior of molecules in various chemical reactions.

Lone Pairs

Lone pairs are pairs of unshared electrons found on atoms within a molecule. These lone pairs play a crucial role when we discuss the ability of a molecule to form bonds, especially hydrogen bonds involving protons. Lone pairs can readily accept protons, forming a coordinate covalent bond.
In the given choices, both \(\mathrm{NH}_{3}\) and \(\mathrm{H}_{2}\mathrm{O}\) have lone pairs that can donate to protons. However, the effectiveness with which they hold protons relate to the density and arrangement of these lone pairs. The lone pair on the nitrogen in ÷\(\mathrm{NH}_{3}\) is more electron-dense than that on oxygen in \(\mathrm{H}_{2}\mathrm{O}\), contributing to the higher proton affinity observed in ammonia.

Molecular Structure

The molecular structure refers to the 3D arrangement of atoms within a molecule. Understanding molecular structure is essential as it affects properties such as reactivity, polarity, and proton affinity. The spatial arrangement of atoms and lone pairs can influence how molecules interact with protons.
For example, the geometry of \(\mathrm{NH}_{3}\) is trigonal pyramidal, which affects how its lone pair is positioned relative to the surrounding atoms. This optimal alignment allows the lone pair to more effectively interact with protons, enhancing proton affinity. By contrast, the molecular geometries of \(\mathrm{H}_{2}\mathrm{~S}\) and \(\mathrm{PH}_{3}\) position their lone pairs less favorably due to larger atomic sizes and differing geometries.

JEE Main Chemistry

The concepts of electronegativity, lone pairs, and molecular structure are pivotal topics in the JEE Main Chemistry syllabus. In exams like JEE Main, students often face questions that involve evaluating chemical properties such as proton affinity. Understanding how electronegativity affects molecular behavior, or how lone pairs contribute to chemical bonding, is crucial.
By synthesizing these fundamentals, students can solve complex problems. Practicing questions concerning chemical reactivity, bond formation, and molecular interaction will bolster a student's capability to tackle various challenges on the exam. Indeed, the problem of identifying the molecule with the highest proton affinity is an excellent example of how JEE Main questions intertwine multiple chemistry concepts to assess a student's comprehensive understanding.