[megh44]

about bond breaking, compare energy required to break the first c-h bond in methane to the average c-h bond energy of 414 kj/mol. Why are these values different?

i think the answer is because extra energy is required to begin the process of breaking bonds...or that as there are less bonds then there is less energy required to break a bond... i am not sure...can someone please help me understand?
thanks!

[enahs]

The energy required to break a bond is influenced by the environment it is in.

When you remove the first hydrogen, the environment (other then the hydrogen being removed) consists of 1 carbon atoms and 3 other hydrogen atoms, with their appropriate electrons.

After that first hydrogen is removed, the environment has changed. If you know try to remove a second hydrogen, the environment affecting that is now only a carbon and 2 hydrogens. And so on.

The average bond energy is the energy required to remove all 4 hydrogens from the carbon, divided by the number of carbons. It takes 1,656 kj/mol of methane to break it all apart. 1,656/4 = 414 kj/mol

http://www.webchem.net/notes/how_far/enthalpy/enthalpy_diagrams.htm