[ds]

Hi all,

I would like to know why the heat of combustion (or enthalpy change) differs for alkanes, alcohols, alkenes and cycloalkanes. I have collected data from the NIST chemistry web book and after looking at it, it seems that alkanes have the highest heat of combustion, followed by alkenes and cycloalkanes and alcohols at the bottom.

I would like to know why this occurs. I realise that there are different functional groups and structures, but I would love an explanation of why the -OH in alcohols, why the double bond in alkenes and why the cyclic structure affects the heat of combustion/causes the heat content of the fuel to be lower.

Any help is greatly appreciated. Thank you

[cliverlong]

One approach would be to use mean bond enthalpies

Identify the nett bonds broken (endothermic) and the nett bonds formed (exothermic) and see if calculated  :delta: Hr agrees with your existing values from data booklet. Then look at the bond enthalpies and see if the discrepancies can be explained in terms of a lot of energy required to initally break bonds or fewer, low energy bonds are made in products

For example

CH₄ + 2O₂  CO₂ + 2H₂O

Bonds broken 4 x C- H,  2 x O=O
Bonds formed 2 x C = O, 4 x O - H