The average energy required to break a P - P bond in P 4 (s) into gaseous atoms is 53.2 kcal mol -1. The bond dissociation energy of H 2( g ) is 104.2 kcal mol -1 ; Δ H f 0 of PH 3( g ) from P 4( s ) is 5.5 kcal mol -1. The P - H bond energy in kcal mol -1 is [Neglect presence of van der Waals forces in P 4( s ) ]

Question

The average energy required to break a P - P bond in P 4 (s) into gaseous atoms is 53.2 kcal mol -1. The bond dissociation energy of H 2( g ) is 104.2 kcal mol -1 ; Δ H f 0 of PH 3( g ) from P 4( s ) is 5.5 kcal mol -1. The P - H bond energy in kcal mol -1 is [Neglect presence of van der Waals forces in P 4( s ) ] (A)  (B)  (C)  (D)

Tardigrade Admin · Accepted Answer

P 4( s ) arrow 4 P ( g ) ; Δ H =53.2 × 6 H 2( g ) arrow 2 H ( g ) ; Δ H =104.2 (1/4) P 4( s )+(3/2) H 2( g ) arrow PH 3( g ) ; Δ H =5.5 (1/4) × 6 × 53.2+(3/2) × 104.2-3 ϵ P - H =5.5 ⇒ ϵ P - H =76.866 i.e. 76.9 kcal mol -1

P4​(s)→4P(g);ΔH=53.2×6 H2​(g)→2H(g);ΔH=104.2 41​P4​(s)+23​H2​(g)→PH3​(g);ΔH=5.5 41​×6×53.2+23​×104.2−3ϵP−H​=5.5 ⇒ϵP−H​=76.866 i.e. 76.9kcalmol−1