Calculate the enthalpy change when ammonia gas will be formed

Answer 1

You're probably looking for something like this:

1/2N₂+3/2H₂→NH₃ ΔH_f of NH₃=?

ΔH_rxn=(ΔH_f(NH3))-(1/2ΔH_f(N2)+3/2ΔH_f(H2))

Use a thermodynamic data chart with standard enthalpies of formation, and substitute those into the expression.