Question

The heat of formations for $ CO_{2}(g), H_{2}O (l) $ and $ CH_{4}(g) $ are $ -400 \, kJ\, mol^{-1} $ , $ -280\, KJ\, mol^{-1} $ and $ -70 \, KJ\, mol^{-1} $ respectively. The heat of combustion of $ CH_{4} $ in $ kJ \, mol^{-1} $ is

• A. $ 890 $
• B. $ -160 $
• C. $ -890 $
• D. $ -90 $

Answer 1

Answer: (C)

$\Delta H_{reaction} =\sum\Delta H_{f} product -\sum\Delta H_{f}$ reactant
$\Delta H_{f}$ of a single element like $O_{2}, H_{2}, Cl_{2}$ is considered equal to zero
$CH_{4}+2O_{2} \to CO_{2}+2H_{2}O$
$\Delta H_{Reaction} =\Delta H_{f} \left(CO_{2}\right)+2\Delta H_{f} \cdot \left(H_{2}O\right)$
$-\Delta H_{f} \left(CH_{4}\right)-2\Delta H_{f} \left(O\right)_{2}$
$=-400+2\left(-280\right)-\left(-70\right)-0$
$=-890\,KJ \, mol^{-1}$