Question

Write the balanced chemical reaction (showing appropriate symbols and states) for the chemical reaction with enthalpy change equal to and defined by the quantity dHformation [NH3(g)]

N2(g) + 3H2(g) --> 2NH3(g)

Is that all the question is asking for?

Answer 1

No but I'm having a little trouble understanding the problem. Look up the dHo formation for NH3. I'll call that x. IF THE problem is asking for dH rxn it is
N2(g) + 3H2(g) ==> 2NH3(g) + 2x kJ

BUT IF THE problem wants dHo formation for NH3/mol it is
1/2 N2(g) + 3/2 H2(g) ==> NH3 + x

That "equal to and defined by the quantity" makes me think it is the latter but I'll leave it to you to decipher.