Problem 126 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is the most thermodynamically stable allotropic from of phosphorus? (a) White (b) Black (c) Yellow (d) Red

Step-by-Step Solution

Answer

The most thermodynamically stable allotropic form of phosphorus is black phosphorus.

Step 1: Understand Allotropes of Phosphorus

Phosphorus has several allotropes, each having different structures and properties. The most common allotropes are white, red, black, and yellow phosphorus. This problem requires knowledge of which of these forms is the most stable under standard conditions.

Step 2: Review Properties of Each Allotrope

White phosphorus is highly reactive and less stable. Red phosphorus is more stable than white, but not the most stable among all. Yellow phosphorus is another name for white phosphorus under certain conditions, so it shares the same instability. Black phosphorus is the most stable because of its layered, graphite-like structure that accounts for minimal reactivity and energy.

Step 3: Determine Stability

Based on the stability and reactivity characteristics of each allotropic form: Black phosphorus is the most thermodynamically stable due to its structural properties, which resemble graphite, making it less reactive and energetically favorable compared to other allotropes.

Key Concepts

Black Phosphorus

Black phosphorus is a fascinating allotrope of phosphorus, distinct for its stability and unique structure. Unlike the highly reactive white phosphorus or even the more stable red phosphorus, black phosphorus stands out because of its layered arrangement. This allotrope can be seen as having a structure reminiscent of graphite, the form that carbon takes in pencil lead.
The black phosphorus layers are held together by weak van der Waals forces, allowing them to slide over one another easily. This layered nature not only contributes to its stability but also gives black phosphorus some intriguing physical properties, such as semiconducting behavior which is appealing in the field of electronics.

• Black phosphorus has a dark appearance, differing from the white or red counterparts.
• Its layered structure makes it less reactive, contributing to its stability.

This stability also means it is less prone to spontaneously combust or degrade when compared to other forms of phosphorus. Therefore, despite the existence of multiple phosphorus allotropes, black phosphorus is the preferred choice in applications requiring durability and minimal reactivity.

Thermodynamic Stability

Thermodynamic stability is a concept that helps us understand why certain forms of a substance exist more commonly under set conditions. For phosphorus, this means determining which allotrope is more energetically favorable, i.e., which one has the lowest energy state at standard conditions.
Black phosphorus ranks as the most thermodynamically stable allotrope of phosphorus. This is due to its lower energy configuration compared to the other forms. In general, a thermodynamically stable substance doesn't readily react with its environment or decompose over time.

• Black phosphorus, with its graphite-like layered structure, has reduced surface energy making it more stable.
• Lesser reactivity of black phosphorus with elements in the air contributes to its stability.

Understanding thermodynamics can also be pivotal in chemistry and material science when predicting substances' behavior under various conditions and their potential applications.

Graphite-like Structure

The term "graphite-like structure" refers to the layered arrangement found in both graphite and black phosphorus. In black phosphorus, this structure results in a zigzag pattern within each layer, reminiscent of how carbon atoms are bonded in graphite. This similarity is what gives black phosphorus its name and some of its physico-chemical properties.
Why is this structure significant? It's because the graphite-like structure contributes greatly to the stability and minimal reactivity of black phosphorus. The weak forces between the layers facilitate sliding, leading to mechanical flexibility while maintaining structural integrity.

• The graphite-like structure allows black phosphorus to act similarly to other materials considered for electronic uses.
• It ensures black phosphorus remains stable by minimizing the active surface area exposed to potential reactive elements.

In essence, the graphite-like structure endows black phosphorus with properties that are both scientifically intriguing and practically beneficial.