Question

What will be the heat of formation of methane, if the heat of combustion of carbon is $-xkJ$, heat of formation of water is $-ykJ$ and heat of combustion of methane is $zkJ$?

• 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

$C+O_2⟶C{O}_2-xkJ$ ...(i)

$H_2+\frac{1}{2}{O}_2⟶H_{2}O-ykJ$ ...(ii)

$C{H}_4+2O_2⟶C{O}_2+2H_{2}O+zkJ$ ...(iii)

The required equation is

$C+2H_2⟶C{H}_4+Q$

Where Q is the heat of formation or the $C{H}_4$

To get the required equation, operate

$\left(i\right)+2\times \left(ii\right)-\left(iii\right)$

Thus, we get

$C+2H_2⟶C{H}_4+\left[\right(-x)+(-2y)-(-z\left)\right]$

Thus, heat of formation of methane is $(-x-2y+z)kJ$

Hence, option $D$ is correct