Problem 34 - A Complete Resource Book in Chemistry for JEE Main

Question

Each of the following is true for white and red phosphorus except that they (a) can be oxidized by heating in air (b) can be converted into one another (c) are both soluble in \(\mathrm{CS}_{2}\) (d) consist of the same kind of atoms

Step-by-Step Solution

Answer

Option (c) is not true for both.

Step 1: Identify the Difference

We need to determine which of the listed statements is not true for both white and red phosphorus. The options suggest similarities in properties and reactions.

Step 2: Option A Evaluation

Both white and red phosphorus can be oxidized by heating in air. White phosphorus is more reactive and ignites at a lower temperature compared to red phosphorus, but both do share this property. Hence, (a) is true for both.

Step 3: Option B Evaluation

White phosphorus can be converted into red phosphorus by heating in the absence of air. Thus, this conversion is indeed possible, making option (b) true.

Step 4: Option C Evaluation

White phosphorus is soluble in carbon disulfide ( md{CS}_2), but red phosphorus is insoluble in it. This makes option (c) incorrect for red phosphorus and hence not true for both types.

Step 5: Option D Evaluation

White and red phosphorus consist of the same type of atoms, which are phosphorus atoms, and both are allotropic forms of phosphorus. Thus, (d) is true.

Key Concepts

White Phosphorus

White phosphorus is a fascinating form of the element phosphorus, known for its unique properties and high reactivity. It is composed of P$_4$ tetrahedra, where four phosphorus atoms are bound together in a pyramid shape.
Due to this structure, white phosphorus is very reactive, especially in air, because it readily oxidizes.

• It is waxy in appearance and glows faintly in the dark when exposed to oxygen - a phenomenon called chemiluminescence.
• Highly flammable and reactive, it ignites easily at temperatures around 30°C.
• Stored underwater or in inert gas to prevent premature ignition.

Its high reactivity makes it useful in various applications such as in incendiary devices and as a precursor for other phosphorus compounds. However, its handling requires care due to its potential hazards.

Red Phosphorus

Red phosphorus is another allotrope of phosphorus that is markedly different from its white counterpart. It is more stable and less reactive due to its polymeric structure.
Unlike the discrete P$_4$ units in white phosphorus, red phosphorus contains long chains of phosphorus atoms.

• Appears as a dark red powder and is much more stable at room temperature.
• Does not ignite spontaneously in air, making it safer to handle.
• Used in safety matches, fireworks, and smoke bombs due to its stability and lower reactivity.

Red phosphorus can be produced from white phosphorus through a heating process in an inert atmosphere, showcasing its notable difference in reactivity and structure from white phosphorus.

Solubility in Carbon Disulfide

Solubility is a key property that helps differentiate white phosphorus from red phosphorus.
White phosphorus is unique because it is soluble in carbon disulfide (CS$_2$), a property not shared by red phosphorus.

• This solubility is due to the molecular structure of white phosphorus, allowing it to dissolve readily in organic solvents like CS$_2$.
• Red phosphorus, with its polymeric and more stable structure, does not dissolve in carbon disulfide, making it distinguishable from its white counterpart.

Understanding this difference is crucial in chemical reactions and separation processes involving phosphorus allotropes.

Oxidation of Phosphorus

The oxidation behavior of phosphorus is an important characteristic that varies significantly between its different allotropes.
White phosphorus is highly reactive with oxygen, igniting spontaneously and requiring less energy to oxidize. This reactivity makes it useful but hazardous to handle. Upon oxidation in air, it forms phosphorus pentoxide (P$_4$O$_{10}$).
On the other hand, red phosphorus is less reactive and does not ignite easily. It can oxidize, but this is a slower process requiring higher temperatures for ignition.

• Oxidation leads to the formation of oxides, typically involving phosphorus pentoxide formation.
• This variance in reactivity between the allotropes showcases their different applications and necessary precautions during handling.

By understanding these oxidation behaviors, one can better handle and utilize these allotropes safely in various industrial and chemical contexts.