This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “P-Block Elements – Phosphine”.
1. Which of the following is not true about phosphine?
a) Phosphine is a colorless gas
b) Phosphine has a rotten fish smell
c) Phosphine is inflammable
d) Phosphine is highly poisonous
Explanation: Phosphine is a colorless gar with rotten fish smell and is highly poisonous. In pure state, phosphine is non-flammable but becomes inflammable owing to the presence of P2H4 or White phosphorous (P4) vapours.
2. Phosphine acts as a Lewis acid.
Explanation: Phosphine acts as a Lewis base and reacts with acidic species to form phosphinium salts.
PH3 + HBr → PH4Br
Phosphine reacts with hydrogen bromide to form phosphinium salts which is a property of a base.
3. Phosphine is prepared from which of the following methods?
a) By reacting calcium phosphide with water or dilute HCl
b) By directly reacting phosphorous with hydrogen
c) By passing dry chlorine over P2H4
d) Heating while phosphorous with halides
Explanation: Phosphine is prepared by the reaction of calcium phosphide with water or dilute HCl.
With water: Ca3P2 + 6H2O → 2PH3 + 3Ca(OH) 2
With dilute HCl: Ca3P2 + 6HCl → 2PH3 + 3CaCl2.
4. Which of the following is a property of phosphine?
a) It is insoluble in water
b) It is inflammable
c) The solution of phosphine in water decomposes in the presence of light
d) It acts as a Lewis acid
Explanation: Phosphine is slightly soluble in water and is non-flammable in pure form. It is a Lewis base and the solution of phosphine in water decomposes in the presence of light giving red phosphorous and hydrogen gas.
5. Which of the following gas is used as Holmes signal?
a) Hydrogen per oxide
Explanation: When a mixture of calcium carbide and calcium phosphide placed in a container is made to react with water, it produces the gases phosphine and acetylene. This signal produced due to the burning gases is called Holmes signal.
6. What is the reaction involved in Holmes signal?
a) Ca3P2 + 6H2O → 2PH3 + 3Ca(OH) 2
b) Ca3P2 + 6H2O → 2PH3 → P2H4 + P4O6 and CaC2 + H2O → C2H2
c) Ca3P2 + 6HCl → 2PH3 + 3CaCl2
d) P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2
Explanation: The reactions involved in Holmes signal are:
Ca3P2 + 6H2O → 2PH3 → P2H4 + P4O6 and CaC2 + H2O → C2H2
Calcium carbide and calcium phosphide both react with water to give acetylene and phosphine respectively. These gases burn to give the signal.
7. What is the hybridization of phosphine?
a) sp2 hybridized
b) sp3 hybridized
c) sp hybridized
d) No hybridization
Explanation: The hybridization of phosphine seems like sp2 but in reality the molecule has no hybridization as it forms all bonds using its pure p orbitals. This can be proved from its bong angle data which shows that its bond angles are 93.5°.
8. What is the structure of phosphine?
a) Trigonal pyramidal
b) Trigonal bi-pyramidal
Explanation: Phosphine has a trigonal pyramidal structure with molecular symmetry. The length of the bond between phosphorous and hydrogen is 1.42 × 10-10 m and the bond angles are equal and are known to be 93.5°.
9. Which of the following compounds react with water to give phosphine?
a) Phosphorous trichloride
b) Phosphorous pentachloride
c) Black phosphorous
d) Aluminium phosphide
Explanation: Metal phosphides such as aluminium phosphide reacts with water to from phosphine and metal hydroxide.
Reaction: AlP + 3H2O → PH3 + Al(OH) 3.
10. Phosphine like ammonia has very high affinity for water.
Explanation: The ammonia molecule has the capability to form a dative bond due to the lone pair of electrons. There is a formation of H-bond due to strong dipole-dipole attraction between ammonia and water molecules where as in phosphine the H-bond is weak and the P-H bond is non-polar so, it is only slightly soluble in water but highly soluble in non-polar solvents.
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