Ch.22 - The Main Group Elements

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.22 - The Main Group Elements

Problem 22.109

Chapter 22, Problem 22.109

Describe the structures of the white and red allotropes of phosphorus, and explain why white phosphorus is so reactive.

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White phosphorus consists of P4 tetrahedra, where each phosphorus atom is bonded to three others, forming a tetrahedral shape. This structure is highly strained due to the 60-degree bond angles, which are much smaller than the typical 109.5-degree angles in a tetrahedral geometry.

Red phosphorus, on the other hand, has a more complex, polymeric structure. It consists of chains of phosphorus atoms, where each atom is bonded to three others, forming a network. This structure is more stable and less reactive than white phosphorus.

The high reactivity of white phosphorus is primarily due to the strain in its P4 tetrahedral structure. The small bond angles create significant angle strain, making the molecule eager to react and relieve this strain.

White phosphorus is also more reactive because it is less stable thermodynamically compared to red phosphorus. The conversion from white to red phosphorus releases energy, indicating that red phosphorus is the more stable form.

In summary, the reactivity of white phosphorus is due to its strained tetrahedral structure and its higher energy state compared to the more stable, polymeric red phosphorus.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Allotropes of Phosphorus

Allotropes are different structural forms of the same element, where atoms are bonded together in distinct ways. Phosphorus has several allotropes, with the most notable being white phosphorus (P4) and red phosphorus. White phosphorus consists of tetrahedral P4 molecules, while red phosphorus has a network structure with polymeric chains, resulting in different physical and chemical properties.

Reactivity of White Phosphorus

White phosphorus is highly reactive due to its molecular structure and the presence of strained P-P bonds in the tetrahedral P4 molecule. This strain makes it energetically favorable for white phosphorus to react with other substances, particularly oxygen, leading to combustion and the formation of phosphorus oxides. Its reactivity is also influenced by its ability to form free radicals, which can initiate further reactions.

Phosphorus Oxides

Phosphorus oxides are compounds formed when phosphorus reacts with oxygen. The most common oxides are phosphorus trioxide (P2O3) and phosphorus pentoxide (P2O5). Understanding these oxides is crucial for comprehending the reactivity of white phosphorus, as its combustion leads to the formation of these products, which have significant implications in both chemical reactions and environmental chemistry.

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