Question

What will be the heat of formation of methane, if the heat of combustion of carbon is ' $-x$ ' kJ, heat of formation of water is ' $-y$ ' $kJ$ and heat of combustion of methane is ' $z$ ' kJ?

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

Answer 1

Answer: (D)

(I) $ C + O _{2} \longrightarrow CO _{2},\,\, \Delta H=-x \,kJ$
(II) $H _{2}+\frac{1}{2} O _{2} \longrightarrow H _{2} O , \,\, \Delta H=-y\, kJ$
(III) $CH _{4}+2 O _{2} \longrightarrow CO _{2}+2 H _{2} O$
$\Delta H=-z \,kJ$
Multiply Eq. (II) by 2, adding to (I) and then, subtract Eq. (III) from the sum
$C +2 H _{2} \longrightarrow CH _{4},\,\, \Delta H=(-x-2 y+z) kJ$