Question

The heat of combustion of ethanol into carbon dioxide and water is $-327 $ kcal at constant pressure. The heat evolved (in cal) at constant volume and $27^{\circ} C$ (if all gases behave ideally) is $\left( R =2\right.$ cal $\left. mol ^{-1} K ^{-1}\right)$

Answer 1

$C _{2} H _{5} OH _{(\ell)}+3 O _{2( g )} \longrightarrow 2 CO _{2( g )}+3 H _{2} O _{(\ell)}$

$\Delta n _{ g }=2-3=-1$

$\Delta_{ c } H =\Delta_{ c } U +\left(\Delta n _{ g }\right) RT$

$\Delta_{ c } H =\Delta_{ c } U - RT$

$\Delta_{ c } U =\Delta_{ c } H + RT$

$\quad=-327 \times 10^{3}+2 \times 300$

$=-326400 cal .$

$\therefore $ Heat evolved

$=326400$ cal.