Which one of the following is formed (mainly) when red phosphorus is heated in a sealed tube at 803 K ?
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Remember the preparation conditions for the two forms of black phosphorus: \(\alpha\)-form comes from red phosphorus at high temperature, while the \(\beta\)-form comes from white phosphorus at high pressure.
Step 1: Understanding the Question:
This is a factual question about the allotropes of phosphorus. We need to identify the specific allotrope formed under the given conditions. Step 2: Detailed Explanation:
Phosphorus exists in several allotropic forms, the most common being white, red, and black phosphorus. Black phosphorus is the most thermodynamically stable form and exists in two modifications: \(\alpha\)-black and \(\beta\)-black phosphorus.
The methods of preparation distinguish them:
- White (or Yellow) Phosphorus: This is the least stable, most reactive form. It is prepared by heating phosphate rock with coke and sand.
- Red Phosphorus: This is formed by heating white phosphorus in an inert atmosphere at about 573 K. It is more stable and less reactive than white phosphorus.
- \(\alpha\)-Black Phosphorus: This form is obtained when red phosphorus is heated in a sealed tube at 803 K. It has an opaque, monoclinic or rhombohedral crystal structure.
- \(\beta\)-Black Phosphorus: This form is prepared by heating white phosphorus at 473 K under very high pressure (around 12,000 atmospheres). It has a layered structure similar to graphite.
Based on the given conditions (heating red phosphorus at 803 K), the product is \(\alpha\)-black phosphorus. Step 3: Final Answer:
Heating red phosphorus in a sealed tube at 803 K mainly forms \(\alpha\)-black phosphorus.
