Question

What wiII be the heat of formation of methane, if the heat of combustion of carbon is ${}^{\prime }-x^{\prime }kJ$, heat of formation of water is ${}^{\prime }-y^{\prime }kJ$ and heat of combustion of methane is ${}^{\prime }{z}^{\prime }kJ$?

• A. $(-x-y+z)kJ$
• B. $(-z-x+2y)kJ$
• C. $(-x-2y-z)kJ$
• D. $(-x-2y+z)kJ$

Answer 1

The correct option is D $(-x-2y+z)kJ$

From given data, we have
$C+O_2\to C{O}_2-xkJ......\left(i\right)$
$H_2+\frac{1}{2}{O}_2\to H_{2}O-ykJ......\left(ii\right)$
$C{H}_4+2O_2\to C{O}_2+2H_{2}O+zkJ.....\left(iii\right)$
The required equation is
$C+2H_2\to C{H}_4+Q$

where Q is heat of combustion of $C{H}_4$
To get the required equation, operate
$\left(i\right)+2\times \left(ii\right)-\left(iii\right)$
$C+2H_2\to C{H}_4+\left[\right(-x)+(-2y)-(-z\left)\right]$
Thus, heat of formation of methane is
$(-x-2y+z)kJ$