The enthalpy of combustion (in kcal/mol) of liquid N2H4 is — Thermodynamics and Thermochemistry Chemistry Question

💡 Solution & Explanation

The combustion of liquid hydrazine is: N2H4(l) + $O_2$(g) → $N_2$(g) + 2$H_2O$(l). Combine the reactions using Hess's law:<br>Reverse reaction (a): $H_2O$(l) + N2H4(l) → $N_2O$(g) + 3$H_2$(g); ΔH = +75.56 kcal.<br>Reverse reaction (d) multiplied by 2: 2$H_2$(g) + $O_2$(g) → 2$H_2O$(l); ΔH = -136.64 kcal.<br>Subtracting and adding other steps cancels $N_2O$ and $H_2$, yielding ΔH = -148.84 kcal/mol.