Question

Calculate the heat of combustion of $1$ mole of $C_2{H}_4\left(g\right)$ to form ${CO}_2\left(g\right)$ and $H_{2}O\left(g\right)$ at $398K$ and $1$ atmosphere, given that the heats of formation of ${CO}_2\left(g\right),H_{2}O\left(g\right)$ and $C_2{H}_4\left(g\right)$ are $-94.1,-57.8$ and $+12.5$ $kcal$ ${mol}^{-1}$ respectively.

• A. $316.3kcal$
• B. $-316.3kcal$
• C. $216.3kcal$
• D. $-216.3kcal$

Answer 1

The correct option is B $-316.3kcal$

$C_2{H}_4+3O_2\to 2C{O}_2+2H_{2}O$

Now,

Heat of combustion of $C_2{H}_4=2\times$ heat of formation of $C{O}_2+2\times$ heat of formation of water - heat of formation of $C_2{H}_4$

Heat of combustionof $C_2{H}_4=2\times (-94.1)+2\times (-57.8)-12.5kcalmo{l}^{-1}$

Heat of combustion of $C_2{H}_4=-316.3kcal/mol$