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# Bond enthalpy help watch

1. Struggling with this summary question in my textbook about bond enthalpy:
The difference between the enthalpy change of combustion of successive straight chain alkanes is around -650kJmol-1. This is because each alkane differs from the previous one by a methyl fragment, -CH2.
a. Write an equation for the combustion of one of the carbon and two hydrogen atoms in this fragment.
I've done this and my answer is correct, its CH2 + 1/2O2 → CO2 + H2O.
b. Use bond enthalpies to calculate a value for the enthalpy change of the reaction in part a (including the breaking of the C-C bond).
The answer in the back of the book says -618kJmol-1, but I've managed to work it out to -1116kJmol-1 and have no idea how you get to -618.

Help will be much appreciated!!!!!
2. (Original post by uselessdoctor)
Struggling with this summary question in my textbook about bond enthalpy:
The difference between the enthalpy change of combustion of successive straight chain alkanes is around -650kJmol-1. This is because each alkane differs from the previous one by a methyl fragment, -CH2.
a. Write an equation for the combustion of one of the carbon and two hydrogen atoms in this fragment.
I've done this and my answer is correct, its CH2 + 1/2O2 → CO2 + H2O.
b. Use bond enthalpies to calculate a value for the enthalpy change of the reaction in part a (including the breaking of the C-C bond).
The answer in the back of the book says -618kJmol-1, but I've managed to work it out to -1116kJmol-1 and have no idea how you get to -618.

Help will be much appreciated!!!!!

Your answer is not correct - the equation you have given is not balanced!.
3. (Original post by charco)
Your answer is not correct - the equation you have given is not balanced!.
What a stupid mistake haha, I completely missed the 1 1/2 in the answers and just read it as 1/2. Balancing equations - not my strong point. Thanks!

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