Question

The average energy required to break a P – P bond in P4 (s) into gaseous atoms is 53.2 kcal mol–1. The bond dissociation energy of H2(g) is 104.2 kcal mol–1; $\Delta$Hf0 of PH3(g) from P4(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 P4(s)]

• A. 85.2
• B. 57.6
• C. 76.9
• D. 63.3

Answer 1

Answer: (C)

76.9

Answer 2

P4(s) $\rightarrow$ 4P(g) $\Delta$H = 53.2 × 6

H2(g) $\rightarrow$2H(g) $\Delta$H = 104.2

$\frac{1}{4}$P4(s) + $\frac{3}{2}$ H2(g) $\rightarrow$ PH3(g) $\Delta$H = 5.5

$\frac{1}{4}$× 6 × 53.2 + $\frac{3}{2}$ × 104.2 – 3$\in$P–H = 5.5

$\Rightarrow$ $\in$P–H = 76.866 i.e. 76.9 kcal mol–1