Question

White phosphorous is a tetra atomic solid $\left[P_4\right(s\left)\right]$ at room temperature and on strong heating in absence of oxygen, it polymerizes into red phosphorus as:

$\Delta H=-104\text{kJ/mol of}{P}_4$
The enthalpy of sublimation $\left[P_4\right(s)\to P_4(g\left)\right]$ white is $59\text{kJ/mol}$ and enthalpy of atomization is $316.25\text{kJ/mol}$ of $P\left(g\right)$.
Now find the average $P-P$ bond enthalpy in $P_4$ molecule is:

• A. $102\text{kJ}$
• B. $201\text{kJ}$
• C. $104\text{kJ}$
• D. $120\text{kJ}$

Answer 1

The correct option is B $201\text{kJ}$

Bond enthalpy is calculated in the gasesous phase.
$P_4\left(s\right)\to P_4\left(g\right)\Delta H=59\text{kJ}.....\left(1\right)$
$P_4\left(g\right)\to 4P\left(g\right)\Delta H=1265\text{kJ}....\left(2\right)$
Now substracting the equation (1) from equation (2), we get
$P_4\left(g\right)\to 4P\left(g\right)\Delta H=1206\text{kJ}$
Average $P-P$ bond enthalpy $=\frac{1206}{6}=201\text{kJ}$