Chemistry of Phosphine

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1. Introduction & Physical Properties

• Colourless, highly poisonous gas with rotten fish/garlic odour
• Molecular formula: PH₃  Molar mass: 34 g/mol
• Boiling point: –87.7°C  Melting point: –133.8°C
• Less soluble in water than NH₃ (1 vol water dissolves ~0.26 vol PH₃)
• Does not form hydrogen bonds (P–H bond is almost non-polar)
• Pure phosphine is non-inflammable but impure (P₂H₄, P₄) catches fire spontaneously in air → “Will-o’-the-wisp”

2. Structure

• Trigonal pyramidal (sp³ hybridisation)
• Bond angle: 93.5° (much smaller than NH₃ 107° because phosphorus uses almost pure p-orbitals, lone pair in nearly pure s-orbital → less repulsion)
• Weakly polar molecule
• Very weak Lewis base (lone pair less available)

Most frequently asked comparison with NH₃: lower basicity, lower bond angle, no H-bonding, lower boiling point.

3. Laboratory Preparation

(i) From phosphonium iodide
PH₄I + NaOH → PH₃↑ + NaI + H₂O (best lab method, pure PH₃)

(ii) Hydrolysis of metal phosphides
Ca₃P₂ + 6H₂O → 3Ca(OH)₂ + 2PH₃↑
AlP + 3H₂O → Al(OH)₃ + PH₃↑

White phosphorus + conc. NaOH (in inert atmosphere) also gives PH₃
P₄ + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂

4. Industrial Preparation

• Not manufactured industrially on large scale (unlike NH₃)
• Obtained as by-product in manufacture of sodium hypophosphite

5. Chemical Properties (Very Important for Exams)

(a) Basic Nature (Very Weak)

PH₃ + H₂O ⇌ PH₄⁺ + OH⁻ (negligible, Kʙ ≈ 10⁻²⁸)
PH₃ + HI → PH₄I (phosphonium iodide, stable only with HI/HBr)

(b) Combustibility

4PH₃ + 8O₂ → P₄O₁₀ + 6H₂O (burns with greenish-white flame)
Impure PH₃ self-ignites due to P₂H₄ → used in Holme’s signal & smoke screens

(c) Reducing Nature (Strong Reducing Agent)

• Reduces CuSO₄ → Cu₃P₂ (black ppt)
  3CuSO₄ + 2PH₃ → Cu₃P₂ + 3H₂SO₄
• Reduces AgNO₃ → Ag (silver mirror)
• Reduces HNO₃, halogens, SO₂ etc.

(d) Reaction with Halogens

PH₃ + 4Cl₂ → PCl₅ + 3HCl (excess Cl₂)
PH₃ + 3Cl₂ → PH₃Cl₂ (phosphonium dichloride, limited Cl₂)

(e) Complex Formation

• Forms very few complexes (weak Lewis base)
• Example: Ni²⁺ + 4PH₃ → [Ni(PH₃)₄] (unstable)

6. Tests for Phosphine

• Rotten fish smell
• Burns with luminous flame
• Black ppt with AgNO₃ (Ag₃P)
• With CuSO₄ → black ppt of Cu₃P
• Phosphine glow test: PH₃ passed through hot tube → luminous rings due to deposition of red phosphorus

7. Uses

• Holme’s signal (sea distress signal – spontaneous ignition)
• Smoke screens (P₄O₁₀ absorbs moisture → dense smoke)
• In synthesis of organophosphorus compounds

Key Comparison: NH₃ vs PH₃ (Most Asked in Exams)

Property	NH₃	PH₃
Basicity	Moderately strong	Extremely weak
Bond angle	107°	93.5°
Boiling point	–33°C	–87.7°C
H-bonding	Yes	No
Stability of +1 state	Less stable	More stable (inert pair)
Reducing power	Weak	Strong
Phosphonium salts	Unstable	PH₄I stable

Practice Questions (High-Yield MCQs)

1. Phosphine is prepared in laboratory by heating

(A) P₄ + NaOH  (B) PH₄I + NaOH  (C) PCl₅ + H₂O  (D) H₃PO₄

Answer: (B) PH₄I + NaOH

2. The bond angle in PH₃ is

(A) 107°  (B) 104.5°  (C) 93.5°  (D) 90°

Answer: (C) 93.5°

3. Pure phosphine is

(A) Non-inflammable  (B) Self-inflammable  (C) Explosive  (D) Highly soluble in water

Answer: (A) Non-inflammable

4. Which is the best reducing agent?

(A) NH₃  (B) PH₃  (C) AsH₃  (D) SbH₃

Answer: (D) SbH₃ > BiH₃ (reducing power increases down the group)

5. Phosphine produces luminous rings when passed through a hot tube due to

(A) Oxidation  (B) Reduction  (C) Thermal decomposition  (D) Combustion

Answer: (C) Thermal decomposition to red phosphorus

6. Holme’s signal contains cylinders of

(A) Ca₃P₂ + CaC₂  (B) Ca₃P₂ only  (C) CaC₂ only  (D) P₄ only

Answer: (A) Ca₃P₂ + CaC₂ (produces PH₃ + C₂H₂)

7. Assertion: PH₃ has lower boiling point than NH₃.
Reason: PH₃ does not form hydrogen bonds.

Answer: Both A and R true and R is correct explanation of A

8. The gas produced when red P reacts with moist air is

(A) PH₃  (B) P₂O₅  (C) H₃PO₄ mist  (D) None

Answer: (A) PH₃ (cause of spontaneous combustion of P)