The dissociation energy of CH4 and C2H6 are respectively 360 and 620 kcal/mol. The bond energy of C−C bond is:
The given that
Bond dissociation enthalpy of CH4=360 is the energy required to break 4 C−H bonds in it.
Thus dissociation enthalpy of 1 C−H
Bond disssociation enthalpy of CH44=3604=90 kcal/ mol.
Bond dissociation enthalpy of C2H6=620 is the energy required to break 6 C−H bonds in it.
1 C−H bonds.
Thus, Bond dissociation enthalpy for C−C = Bond dissociation enthalpy for C2H6− 6 (Bond dissociation enthalpy for 1C−H)
⇒620−6(90)=620−540
⇒80 kcal/ mol.
This is the required solution.