Question

Given:

$\ce{ 2C(s) + 2O_2 (g) -> 2CO_2 (g) ; \Delta H = -787 kJ} $

$\ce{ H_2 (g) + \frac{1}{2} O_2(g) -> H_2O(\ell ) ; \Delta H = - 286 kJ}$

$\ce{C_2H_2 (g) + 2 \frac{1}{2} O_2 (g) \to 2CO_2 (g) + H_2O(\ell) ; \Delta H = - 1310 kJ } $

The heat of formation of acetylene is

• A. -1802 kJ
• B. +1802 kJ
• C. +237 kJ
• D. -800 kJ

Answer 1

Answer: (C)

$2 C (s)+2 O _{2}(g) \longrightarrow 2 CO _{2}(g) ; $
$ \Delta H =-787 \,kJ \ldots (i)$
$ H _{2}(g)+\frac{1}{2} O _{2}(g) \longrightarrow H _{2} O (l) ;$
$ \Delta H =-286\, kJ \ldots (ii)$
$ C _{2} H _{2}(g)+2 \frac{1}{2} O _{2}(g) \longrightarrow 2 CO _{2}(g)+ H _{2} O (l) $
$ \Delta H=-1310 \,kJ \ldots (iii) $
Add the Eqs. (i) and (ii) and subtract Eq. (iii) from them.
$ 2 C + H _{2} \longrightarrow C _{2} H _{2}-787-286+1310 $
$ 2 C + H _{2} \longrightarrow C _{2} H _{2}-1073+1310 $
$2 C + H _{2} \longrightarrow C _{2} H _{2}+237$
Hence, the heat of formation of acetylene is + $237 \,kJ$.