The enthalpy of formation of ethane $(\mathrm{C}_2 \mathrm{H}_6)$ from ethylene by addition of hydrogen where the bond-energies of $\mathrm{C}-\mathrm{H}, \mathrm{C}-\mathrm{C}, \mathrm{C}=\mathrm{C}, \mathrm{H}-\mathrm{H}$ are $414 \mathrm{~kJ}, 347 \mathrm{~kJ}, 615 \mathrm{~kJ}$ and $435 \mathrm{~kJ}$ respectively is $-$ __________ $\mathrm{kJ}$