This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “P-Block Elements – Phosphorus Halides”.
1. Phosphorus trichloride reacts violently with water.
Explanation: Phosphorus trichloride reacts violently with water to produce phosphorus acid (H3PO3) and hydrochloric acid.
PCl3 + 3H2O → H3PO3 + 3HCl.
2. Which of the following reacts with PCl3 to form PCl5?
Explanation: Phosphorus trichloride (PCl3) reacts with chlorine gas (Cl2) to form phosphorus pentachloride (PCl5).
PCl3(g) + Cl2(g) → PCl5(g).
3. Which of the following statement regarding PCl3 is false?
a) PCl3 forms metal chlorides on heating with finely divided metals
b) PCl3 does not react with organic compounds
c) PCl3 is a colourless pungent smelling liquid
d) PCl3 boils at 347 K
Explanation: Phosphorus trichloride is widely used as an important reagent in organic chemistry for replacing hydroxyl groups by chlorine atoms in organic reactions. For example, ethanol reacts with phosphorus trichloride to form chloroethane and phosphorous acid.
3CH3CH2OH + PCl3 → 3CH3CH2Cl + H3PO3.
4. Phosphorus trichloride does not react with Grignard reagents.
Explanation: Phosphorus trichloride reacts with Grignard reagents to form substituted phosphines. For example, PCl3 reacts with phenylmagnesium chloride to produce triphenylphosphate and magnesium chloride.
PCl3 + 3C6H5MgCl → P(C6H5)3 + 3MgCl2.
5. Which of the following is most stable?
Explanation: Phosphorus pentachloride is the most stable. As we move down a group in the periodic table, the stability of the pentahalides decreases due to inert pair effect. So, the correct order of stability is PCl5 > SbCl5 > AsCl5 > BiCl5.
6. What is the hybridization of phosphorus in PCl5?
Explanation: In PCl5, phosphorus undergoes sp3d hybridization and has trigonal bipyramidal geometry in gaseous and liquid states. It has three equatorial, P-Cl bonds and two axial, P-Cl bonds. Since two axial P-Cl bonds are repelled by three bond pairs while three equatorial bonds are repelled by two bond pairs, therefore, axial bonds are longer than equatorial bonds.
7. How does PCl5 exist in its solid state?
Explanation: Phosphorus pentachloride is a pale yellow crystalline solid with a characteristic pungent smell. In the solid state, it exists as an ionic solid, [PCl4]+[PCl6]– in which the cation, [PCl4]+ is tetrahedral while the anion, [PCl6]– is octahedral.
8. What is the liquid product formed on decomposition of PCl5?
d) No liquid product is formed
Explanation: Phosphorus pentachloride, when heated, sublimes but decomposes on stronger heating into phosphorus trichloride (a colourless pungent-smelling liquid) and chlorine gas.
PCl5 ⇌ PCl3 + Cl2.
9. Which of the following is not formed on hydrolysis of PCl5?
Explanation: In moist air, phosphorus pentachloride undergoes hydrolysis to first form POCl3 and then finally phosphoric acid.
PCl3 + H2O → POCl3 + 2HCl
POCl3 + 3H2O → H3PO4 + 3HCl
However, with excess of water, PCl5 reacts violently to form H3PO4 and HCl.
PCl5 + 4H2O (excess) → H3PO4 + 5HCl.
10. What type of agent is PCl5 when it reacts with H2?
a) Reducing agent
b) Oxidising agent
c) Both reducing and oxidising
d) Neither reducing nor oxidising
Explanation: PCl5 reacts with H2 to form PCl3.
PCl5 + H2 → PCl3 + 2HCl
In this reaction, PCl5 acts as an oxidising agent and thus oxidises H2 to HCl.
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