thermochemistry question(rating twice to satisfactory ans)?

Question

The dissociation energy of methane and ethane is 360 an 620 cal/mol. The bond energy of C-C is 80kcal. please explain how

Answer 1

There is no C-C bond in methane, and therefore the dissociation energy of methane is just breaking the four C-H bonds. Breaking energy for C-H bond is 90 Kcal/mole and therefore, for Methane Breaking energy will be 90 kcal/mole per C-H bond X 4 = 360kcal/mol. For ethane, we have 6 C-H bonds at 90kcal/mol = 540kcal/mol + one C-C bond 80kcal/mol = 620kcal/mol.

Answer 2

In chemistry, bond energy (E) is a measure of bond strength in a chemical bond. For example the carbon-hydrogen bond energy in methane E(C–H) is the enthalpy change involved with breaking up one molecule of methane into a carbon atom and 4 hydrogen radicals divided by 4. Bond energy (E) should not be confused with bond dissociation energy.

Another example: an O–H bond of a water molecule (H–O–H) has 493.4 kJ mol-1 of bond dissociation energy, and 424.4 kJ mol-1 is needed to cleave the remaining O–H bond. The bond energy of the O–H bonds in water is 458.9 kJ mol-1, which is the average of the values.
Bond strength (energy) can be directly related to the bond distance. Therefore we can use the metallic, ionic, or covalent radii of each atom in the molecule to determine the bond strength. For example, the covalent radius of boron is estimated at 83.0 pm, but the bond length of B–B in B2Cl4 is 175 pm, a significantly larger value. This would indicate that the bond between the two boron atoms is a rather weak single bond. In another example, the metallic radius of rhenium is 137.5 pm, with a Re–Re bond length of 224 pm in the compound Re2Cl8. From this data, we can conclude that the bond is a very strong bond or a quadruple bond. This method of determination is most useful for covalently bonded compounds

Answer 3

The disociation energy of methane is just breaking the 4 C-H bonds; so [360kcal/mol]/4 = 90 kcal/mole per C-H bond.

Now for ethane, we have 6 C-H bonds at 90kcal/mol = 540kcal/mol + 80kcal/mol for the C-C bond = 620kcal/mol.

Answer 4

bond energies can only ever be a rough guide, since electrons are not restricted to atoms, they're delocalised over the whole molecule. a particular type of bond can be easier or harder to break depending on what other bonds surround it.