Question

Hydrazine, N2H4, is a colorless liquid used in rocket fuel. What is the enthalpy change for the process in which hydranzine is formed from its elements?

N2(g) + H2(g) => N2H4(l)

Use the following reactions and enthalpy changes:

N2H4 + O2 => 2H2O DeltaH = -622.2KJ
H2 + 1/2O2 => H2O DeltaH = -285.5KJ


So how would you do this?

Answer 1

Your first reaction is not quite complete:

N2H2 + O2 >> N2 + 2H2O deltaH=-622.2kJ/mol

H2+ 1/2 O2>>H2O deltaH=-285.5KJ

Well, if we double the second reaction, and subtract that from the first reaction

N2H2 -2H2 >> N2 deltaH=-622.2+2(285.5)
or
N2H2 >> N2+2H2 deltaH=above about-50kj/mol
reversing the reaction...
N2+H2>>N2N2 deltaH= - above. So, the heat of formation is a +value, an endothermic process.