why N2 does not have an enthalpy change of formation?

Question

N2H4 + N2O4 --> 3N2 + 4H2O

or the marking scheme is wrong ?

Answer 1

By DEFINITION, all elements in their standard states, like diatomic gases like N2, C solid in a specified state, P in a specified state , ( see tables on enthalpy) have an enthalpy of ZERO.

Answer 2

It has been a long time since I took thermo but I vaguely remember something about the formation of an element having no enthalpy change (maybe only if it is formed in the gaseous state). This is probably mentioned in your book somewhere. I hope this is of use to you.