Question

The molar heat of combustion of $C_2{H}_2$, carbon and hydrogen are $-1300$ kJ, $-393.5$ kJ and $-282.6$ kJ respectively. The enthalpy of $C_2{H}_2$ in kJ mol${}^{-1}$ is:

• A. $620.3$
• B. $-226.8$
• C. $+226.8$
• D. $1980$

Answer 1

The correct option is C $+226.8$

(i) $C+O_2\to C{O}_2;\Delta H=-393.5$
(ii) $H_2+\frac{1}{2}{O}_2\to H_{2}O;\Delta H=-282.6$
(iii) $C_2{H}_2+\frac{5}{2}{O}_2\to 2C{O}_2+H_{2}O;\Delta H=-1300KJ$
Reaction: 2 x (i) + (ii) - (iii) will result into:
$2C+H_2\to C_2{H}_2$
$\Delta H=2\times (-393.5)+(-282.6)+1300$ $=226.8$