Chemistry Multiple Choice Questions on “P-Block Elements – Phosphorus – Allotropic Forms”.
1. Which allotrope of phosphorus is the most stable?
a) White phosphorus
b) Red phosphorus
c) Black phosphorus
d) Phosphine
Answer: c
Clarification: Black phosphorus is thermodynamically, the most stable allotrope of phosphorus and does not burn in air even up to 673 K. It has a sharp melting point of 860 K. Like graphite, it is fairly a good conductor of electricity.
2. Which allotrope of phosphorus is the most reactive?
a) White phosphorus
b) Metal phosphorus
c) Red phosphorus
d) Beta-black phosphorus
Answer: a
Clarification: The three allotropic forms of phosphorus differ widely in their chemical reactivity in which white phosphorus is the most reactive while black and red phosphorus are less reactive. White phosphorus is made up of discrete P4 tetrahedra which are subjected to very high angular strain as the angles is 60 degrees. This high angular strain makes white phosphorus unstable and highly reactive.
3. Red phosphorus is kept under water to protect it from air.
a) True
b) False
Answer: b
Clarification: White phosphorus is a highly reactive clement and must be stored underwater for safekeeping to prevent it from catching fire spontaneously in the air. In water, white phosphorus reacts with oxygen within hours or days. In water with low oxygen, white phosphorus may degrade to a highly toxic compound called phosphine, which eventually evaporates to the air and is changed to less harmful chemicals.
4. From which type of phosphorus is alpha -black phosphorus formed?
a) Phosphide
b) White phosphorus
c) Black phosphorus
d) Red phosphorus
Answer: d
Clarification: Alpha-black phosphorus is the most stable allotrope of black phosphorus. Alpha-black phosphorus is produced from red phosphorus. When red phosphorus is heated in a sealed tube at 803 K, it forms alpha-black phosphorus.
5. Beta-black phosphorus is prepared by heating white phosphorus.
a) True
b) False
Answer: a
Clarification: Yes, Beta-black phosphorus is prepared by heating white phosphorus at 473 K under high pressure(4000-12000 atm) in an inert atmosphere. It has layered structure in which each phosphorus atom is covalently bonded to three neighbouring phosphorus atoms.
6. Like white phosphorus, which phosphorus also exists as P4?
a) Black phosphorus
b) Red phosphorus
c) Phosphine
d) Beta-black phosphorus
Answer: b
Clarification: Like white phosphorus, red phosphorus also exists as P4 tetrahedra but these are joined together through covalent bonds to give a polymeric structure. Because of polymeric structure, its melting point (883 K) is much higher than that of white phosphorus(317 K).
7. Which allotrope of phosphorus does not catch fire easily?
a) White phosphorus
b) Alpha- black phosphorus
c) Beta- black phosphorus
d) Red phosphorus
Answer: d
Clarification: Red phosphorus is a relatively stable allotrope of phosphorus at room temperature. Its ignition temperature(543 K) is much higher than that of white phosphorus(303 K). As a result, it does catch fire easily.
8. White phosphorus can be reconverted to red phosphorus.
a) True
b) False
Answer: a
Clarification: Yes, red phosphorus sublimes on heating giving vapours which are the same as by white phosphorus. When these vapours are condensed, white phosphorus is obtained. This gives us simple method of reconverting red phosphorus into white phosphorus.
9. Which allotrope of phosphorus is also called yellow phosphorus?
a) Black phosphorus
b) Red phosphorus
c) Beta- black phosphorus
d) White phosphorus
Answer: d
Clarification: White phosphorus on exposure to light, it turns yellow. It glows greenish in the dark (when exposed to oxygen) and is highly flammable and pyrophoric (self-igniting) upon contact with air. Therefore it is called yellow phosphorus.
10. What is the allotrope of phosphorus in which P-atom completes its octet?
a) Alpha-black phosphorus
b) Beta-black phosphorus
c) White phosphorus
d) Red phosphorus
Answer: c
Clarification: White phosphorus exists as P4 units. The four sp3 hybridized phosphorus atoms lie at the corners of a regular tetrahedron with an angle of 60 degree. Each phosphorus atom is linked to three other P-atoms by covalent bonds so that each p-atom completes it octet.