Question

Ethylene on combustion gives carbon dioxide and water. Its heat of combustion is $1410.0kJ.mo{l}^{-1}$. If the heat of formation of $C{O}_2$ and $H_{2}O$ are $393.3kJ$ and $286.2kJ$, respectively. Calculate the heat of formation of ethylene.

• A. ${\Delta }_{f}H=51.0kJmo{l}^{-1}$
• B. ${\Delta }_{f}H=102.0kJmo{l}^{-1}$
• C. ${\Delta }_{f}H=25.0kJmo{l}^{-1}$
• D. None of these

Answer 1

Answer: (A)

${\Delta }_{f}H=51.0kJmo{l}^{-1}$

As given,

$C_2{H}_2+5/2O_2\to 2C{O}_2+H_{2}O+1410kJ/mol$ ...........................i

$C+O_2\to C{O}_2+393.3kJ$....................................ii

$H_2+1/2O_2\to H_{2}O+286.2kJ$........................iii

required is

$2C+H_2\to C_2{H}_2+\Delta H$.......................iv

from given equations

$\Delta H=1410-393.3\times 2-286.2\times 2=51kJ/mol$