[RK1992]

Hi, I have an AS organic chemistry exam on Monday (OCR Chem - Chains, Energy and Resources in case anyone else has this paper) and was wondering how to best explain the trend of enthalpies of alkanes with respect to the chain lengths. I believe they become less exothermic as the chain increases; that is, the enthalpy increases (by approx 650kJmol^-1 per CH2
group). This isn't mentioned in my class notes (it just says I "should know why") so I thought I should guess and ask here.


My best guess is the idea of bond enthalpies - bond breaking being exothermic, making of bonds being exothermic.

For methane you have 6 bonds being broken and 6 being made:
CH₄ + 2O₂    CO₂ + 2H₂O
so you have a ratio of 6:6 or 1:1

For ethane you have:
C₂H₆ + 3.5 O₂    2CO₂ + 3H₂O
so there's an effective ratio of 10.5:10 or 1.05:1

and the patern increases such that the ratio of bonds broken to bonds made is 4.5n+1.5 : 4n+2 (n being the number of carbon atoms in the chain)... does this adequately explain the trend? Simply that the longer the chain, the greater the proportion of bonds broken to made..? Are there other factors I should know about?

Thanks in advance