In solid , each molecule has six other molecules as nearest neighbours. The enthalpy of sublimation — Thermodynamics and Thermochemistry Chemistry Question
Question
In solid $\text{NH}_3$, each $\text{NH}_3$ molecule has six other $\text{NH}_3$ molecules as nearest neighbours. The enthalpy of sublimation of $\text{NH}_3\text{(s)}$ at its melting point is 30.4 kJ/mol and estimated value of enthalpy of sublimation of $\text{NH}_3\text{(s)}$, where there were no hydrogen bonds, is 15.4 kJ/mol. What is the average strength of hydrogen bonds in solid $\text{NH}_3$ in kJ/mol?
💡 Solution & Explanation
The extra sublimation enthalpy due to hydrogen bonding is $30.4 - 15.4 = 15.0\text{ kJ/mol}$. Every $\text{NH}_3$ molecule is surrounded by 6 nearest neighbors, implying 6 hydrogen bonds. However, each bond is shared between two molecules, so the effective number of hydrogen bonds per molecule is $6 / 2 = 3$. Average strength per H-bond = $15.0 / 3 = 5\text{ kJ/mol}$.