Question

The heat of formation of ethane is $-19.46Kcal$. Bond energies of $H--H$, $C--H$, and $C--C$ bonds are $104.18$, $99.0$ and $80.0Kcal$, respectively. If the heat of atomisation of graphite is x then find out the value of x.

• A. $x=171Kcal$
• B. $x=-171Kcal$
• C. $x=0Kcal$
• D. None of these

Answer 1

The correct option is A $x=171Kcal$

Sol. Given: The balanced chemical reaction for the formation of ethane is
$2C\left(s\right)+3H_2\left(g\right)⟶C_2{H}_6\left(g\right);{\Delta }_{f}H=-19.46Kcal$ Let the heat of atomisation is $xkcal$.
Bond energies of reactant = $2x+3\times (H--H)$ $=2x+3\times 104.18$
$=2x+312.54$
Bond energy of products = $(C--C)+6(C--H)$
$=80+6\times 99$ $=674$
The enthalpy change for the formation of ethane is the difference between the bond energy of reactants and the bond energy of products.
$\Delta H=2x+312.54-674$
$=-19.46$

$\therefore x=171kcal$
Hence, the heat of atomisation of graphite (and the value of x) is $171kcal$