Question

Combustion of octane takes place in an automobile engine. The homogeneous equation of combustion is:
$C_8{H}_{18}\left(g\right)+\frac{25}{2}{O}_2\left(g\right)⟶8{CO}_2\left(g\right)+9H_{2}O\left(g\right)$
the signs of $\Delta H,\Delta S$ and $\Delta G$ for the reaction will be:

• A. $+ve,-ve,+ve$
• B. $-ve,+ve,-ve$
• C. $-ve,+ve,+ve$
• D. $+ve,+ve,-ve$

Answer 1

Answer: (B)

$-ve,+ve,-ve$

$C_8{H}_{18}\left(g\right)+\frac{25}{2}{O}_2\left(g\right)\to {8CO}_2\left(g\right)+9H_{2}O\left(g\right)$

For a combustion, heat is always liberated. i.e. it is an exothermic reaction.

$\therefore \Delta H>0i.e\Delta H=+ve$

Also, a combustion reaction is always spontaneous.

$\therefore \Delta G<0i.e.\Delta G=-ve$

The number of molecules in product is greater than the number of molecules in reactant.

$\therefore$ Entropy is +ve i.e. $\Delta S=+ve$