Question

I am not to sure how to approach the problem...

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

Answer 1

Think of some of the energy released has been used to transform water(l) to water(g) and adjust accordingly.
Watch the stoichiometric coefficient of H2O.

Answer 2

got it, thank you