Question

The enthalpies of formation of $C{O}_{2\left(g\right)},H_2{O}_{\left(l\right)}$ and $C_2{H}_{4\left(g\right)}$ are respectively $-393.5,-286$ and $+52.3kJmo{l}^{-1}$. The enthalpy change for the combustion of $C_2{H}_{4\left(g\right)}$ is :

• A. $-1306kJmo{l}^{-1}$
• B. $-1411.3kJmo{l}^{-1}$
• C. $-509.7kJmo{l}^{-1}$
• D. $-731.5kJmo{l}^{-1}$

Answer 1

Answer: (B)

$-1411.3kJmo{l}^{-1}$

$C_2{H}_4+3O_2\to 2C{O}_2+2H_{2}O$

$\Delta H=2\times {\Delta }_{F}H\left(C{O}_2\right)+2{\Delta }_{F}H\left(H_{2}O\right)-3\times {\Delta }_{F}H\left(O_2\right)-{\Delta }_{F}H\left(C_2{H}_4\right)$

$\Delta H=2\times (-393.5)+2\times (-286)-0$$-\left(52.3\right)$

$\Delta H=-1411.3$

$\Delta H$ combustion is always - ve.

$\Delta H=-1411.3kJmo{l}^{-1}$