Question

Hydrazine (${\mathbf{N}}_2{\mathbf{H}}_4$ ) is used as a rocket fuel because it reacts very exothermically with oxygen to form nitrogen gas and water vapour. The heat released and the increase in the number of moles of gas provide thrust. Calculate the heat of the reaction.

Short answer

The heat of reaction is -1089kJ /mol of hydrazine

Step-by-step solution

Reaction of hydrazine

Heat of reaction

The heat of reaction is determined by calculating the difference in the energies of bond broken – bonds formed

$\begin{array}{c}\mathrm{Energy}\mathrm{required}\mathrm{to}\mathrm{break}\mathrm{bonds}=1\left(\mathrm{N}-\mathrm{N}\right)+4\left(\mathrm{N}-\mathrm{H}\right)\\ =160+4\left(391\right)\mathrm{kJ}\\ =1724\mathrm{kJ}\\ \mathrm{Energy}\mathrm{released}\mathrm{when}\mathrm{bonds}\mathrm{form}=1(\mathrm{N}\mathrm{N})+4\left(\mathrm{O}-\mathrm{H}\right)\\ =945+4\left(467\right)\\ =2813\mathrm{kJ}\\ \mathrm{Therefore}\mathrm{heat}\mathrm{of}\mathrm{reaction}\mathrm{is}=\left(1724-2813\right)\mathrm{kJ}/\mathrm{mol}\\ =-1089\mathrm{kJ}/\mathrm{mol}\mathrm{of}\mathrm{hydrazine}\end{array}$