Two rocket fuels below are determined by their high performance. N2H4(l) + O2(g) -> N2(g) + 2H2O(l) ΔH= -632 kJ H2 (g) + ½ O2(g) -> H2O(l) ΔH = -286 kJ If equal masses of hydrazine and hydrogen are used, which of the following has better performance?

Explanation

The performance of rocket fuel is measured by its enthalpy of combustion, which is the amount of heat released when one mole of the fuel is completely burned in excess oxygen to form its products at standard conditions. The enthalpy of the formation of hydrazine is -632 kJ/mol, and that of hydrogen gas is -286 kJ/mol. Since equal masses of hydrazine and hydrogen are used, we can compare their performances by calculating the amount of heat released per gram of fuel. >For hydrazine, the amount of heat released per gram of fuel is (-632 kJ/mol) / (32 g/mol) = -19.75 kJ/g. >For hydrogen gas, the amount of heat released per gram of fuel is (-286 kJ/mol) / (2 g/mol) = -143 kJ/g. Therefore, hydrogen gas has better performance as a rocket fuel than hydrazine since it releases more heat per gram of fuel.