The standard molar enthalpies of formation of trinitrotoluene(l), and are 65, –395 and –285 kJ/mol, — Thermodynamics and Thermochemistry Chemistry Question
Question
The standard molar enthalpies of formation of trinitrotoluene(l), $\text{CO}_2\text{(g)}$ and $\text{H}_2\text{O(l)}$ are 65, –395 and –285 kJ/mol, respectively. The density of trinitrotoluene is 1.816 g/ml. Trinitrotoluene can be used as rocket fuel, with the gases resulting from its combustion streaming out of the rocket to give the required thrust. What is the enthalpy density for the combustion reaction of trinitrotoluene?
💡 Solution & Explanation
TNT ($\text{C}_7\text{H}_5\text{N}_3\text{O}_6$) molar mass is $227\text{ g/mol}$. Reaction: $\text{TNT(l)} + 21/4\text{O}_2\text{(g)} \to 7\text{CO}_2\text{(g)} + 2.5\text{H}_2\text{O(l)} + 1.5\text{N}_2\text{(g)}$. $\Delta H_c = 7(-395) + 2.5(-285) - 65 = -3542.5\text{ kJ/mol}$. Enthalpy density = $(\Delta H_c / M) \times d = (-3542.5 / 227) \times 1.816 = -15.6 \times 1.816 = -28.34\text{ kJ/mL}$, which equals $-28.34\text{ MJ/L}$.