AS Chemistry Help: Enthalpy Change in Chain Alkanes

[psychedelia]

Hi, this question is really bugging me.
Thanks in advance.

Homework Statement

The difference between the enthalpy change of combustion if sucessive chain alkanes is around -650 kJ/mol

Homework Equations

Why does the value of enthalpy change of the sections of the chains have a different value...

That wasn't very clear was it?

The Attempt at a Solution

http://img516.imageshack.us/img516/5919/whyqf0.jpg

 

[GCT]

Hi, this question is really bugging me.
Thanks in advance.

Homework Statement

The difference between the enthalpy change of combustion if sucessive chain alkanes is around -650 kJ/mol

Homework Equations

Why does the value of enthalpy change of the sections of the chains have a different value...

That wasn't very clear was it?

The Attempt at a Solution

http://img516.imageshack.us/img516/5919/whyqf0.jpg

For one calculating the the enthalpy of a reaction using tabulated bond enthalpies is an estimation - the context around which the bond is situated may alter its common bond enthalpy. E.G. You may have utilized the bond enthalpy of the carbon hydrogen bond in methane however note that your current molecule is relatively unstable ... its been a while since I've faced this topic so the actual answer may be related to a distinctly unrelated matter.