Using the bond enthalpies N$\equiv$N $= 946$, H-H $= 436$ and N-H $= 389$, all in kJ/mol, estimate $\Delta_r H$ for $\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightarrow 2\text{NH}_3\text{(g)}$.