Heats of combustion of ${\mathrm{C}}_2{\mathrm{H}}_4$ , ${\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_6$ are –1409 KJ, –285 KJ and –1558 KJ. Heat of hydrogenation of ethylene is

The required reaction is:

${\text{C}}_2{\mathrm{H}}_4+{\mathrm{H}}_2\to {\mathrm{C}}_2{\mathrm{H}}_6$ 

The heat of hydrogenation = Heat of combustion $[{\mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{H}}_2-{\mathrm{C}}_2{\mathrm{H}}_6]$.

The heat of hydrogenation = $[-1409-285-(-1558\left)\right]$

The heat of hydrogenation = -136 KJ.

Therefore, the correct option is C.