Hydrazine is used as a rocket fuel because its reaction with oxygen is extremely exothermic:

Question

N2H4(liquid) + O2(g)  N2(g) + 2H2O(liquid) ΔH(reaction) = -615 kJ mol-1
What is the enthalpy of this reaction if water were produced in the gaseous rather than the liquid form? Note that 40.7 kJ mol-1 is the molar enthalpy of vaporization of water.
A) -534 kJ mol-1
B) -656 kJ mol-1
C) -696 kJ mol-1
D) -574 kJ mol-1
E) -615 kJ mol-1

Answer 1

Some of the heat generated would have been diverted to convert 2H2O(l) to 2H2O(g),
so heat generated would be reduced by 2*40.7=81.4 kJ/mol.
Can you take it from here?

Answer 2

E) -615 kJ mol-1