${C_{\left( {graphite} \right)}} + {O_{2\left( g \right)}} \to C{O_{2\left( g \right)}}\,;\Delta H = - 393.5\,kJ$
${H_{2\left( g \right)}} + 1/2{O_{2\left( g \right)}} \to {H_2}{O_{\left( l \right)}}\,;\,\Delta H = - 286.2\,kJ$
${C_2}{H_{4\left( g \right)}} + 3{O_{2\left( g \right)}} \to 2C{O_{2\left( g \right)}} + 2{H_2}{O_{\left( l \right)}}\,;\,\Delta H = - 1410.8\,kJ$
$ii)$ $H _2+\frac{1}{2} O _2 \rightarrow H _2 O \quad \Delta H _{i i }=-286.2$
$iii)$ $C _2 H _4+3 O _2 \longrightarrow 2 CO _2+2 H _2 O \quad \Delta H _{\text {iil }}=-1410.8$
$iv)$ $2 C +2 H _2 \longrightarrow C _2 H _4 \Delta H = (?)$
$iv)$ $=2 (i) - (ii) +2 (iii)$
$\therefore \Delta H =2 \Delta H _1+2 \Delta H _{ii}-2 \Delta H _{\text {iii}}$
$=2(-393.5)-2(-286.2)-(-1410.8)$
$=-787+572.4+1410.8$
$=-1359.4+1410.8$
$=+51.4\, kJ\, mol^{-1}$