Question

The bond dissociation energy of $CH$ in \[C{H_4}\] ​from the equation
$C(g) + 4H(g) \to C{H_4}(g)$ , $\Delta H = - 397.8kcal$
A) +99.45kcal
B) -99.45kcal
C) +397.8kcal
D) +198.9kcal

Answer 1

Hint:Carbon and Hydrogen are in their elementary states so their enthalpies will be zero (0). Bond dissociation energy measures the strength of bond. Or we can say that bond dissociation energy is the energy required to dissociate the bond.

Complete answer:
Given balanced equation is $C(g) + 4H(g) \to C{H_4}(g)$ , $\Delta H = - 397.8kcal$
Since Carbon and Hydrogen in this reaction are in their elementary state so their enthalpies will be “0”
And enthalpy for the reaction is $\Delta H = - 397.8kcal$
So as enthalpies of carbon and hydrogen are zero therefore enthalpy of \[C{H_4}\]= $397.8kcal$
And since there are $4CH$ bonds in \[C{H_4}\]
Therefore it can be written as, $\Delta H$ = heat released in formation of $4CH$ bonds in \[C{H_4}\]
$\Delta H$=$4 \times $ bond dissociation energy of$CH$ bond in \[C{H_4}\]
$397.8kcal$ = $4 \times $bond dissociation energy of $CH$ bond in ​\[C{H_4}\]
Bond Dissociation energy of $CH$ bond in ​\[C{H_4}\]=$397.8 \div 4$.
Therefore, Bond Dissociation energy of $CH$ bond in ​\[C{H_4}\]= $+99.45kcal$

Hence, the correct answer is option ‘A’.

Note:Bond dissociation energy is one which measures strength of bond. Elements or compounds which are in their elementary state have enthalpies zero as in this question enthalpies of carbon and hydrogen is zero because they are present in their elemental states.