N2H4(g) + H2(g) 2 NH3(g) H1 = –1876 kJ
3 H2(g) + N2(g) 2 NH3(g) H2 = –922 kJ
The H f for the formation of hydrazine: 2 H2(g) + N2(g) N2H4(g) will be ? kJ/mol.
Turn equation 1 around and change the sign of H1.
Add to equation 2, cancel like molecules appearing on both sides, and add the two delta Hs. H1(with sign changed) + H2 = Hf
thank you