Question Error Report

Thank you for reporting, we will resolve it shortly

Back to Question

Q. 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 $104.2 \,kcal\, mol ^{-1} ; \Delta H _{ f }^{0}$ 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 $P _{4}( s )$ ]

Thermodynamics

Solution:

$P _{4}( s ) \rightarrow 4 P ( g ) ; \Delta H =53.2 \times 6$
$H _{2}( g ) \rightarrow 2 H ( g ) ; \Delta H =104.2$
$\frac{1}{4} P _{4}( s )+\frac{3}{2} H _{2}( g ) \rightarrow PH _{3}( g ) ; \Delta H =5.5$
$\frac{1}{4} \times 6 \times 53.2+\frac{3}{2} \times 104.2-3 \epsilon_{ P - H }=5.5$
$\Rightarrow \epsilon_{ P - H }=76.866$ i.e. $76.9\, kcal\, mol ^{-1}$