Energy changes and bonding
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I am currently analysing my results for a practical assignment to find out how the molecular structure of an alcohol affects the enthalpy change of combustion. However, I have encountered a problem; I cannot explain the very slight difference in theoretical and practical enthalpy changes between alcohols with the same chain length but different position of OH or hydroxyl groups. e.g. propan-1-ol and propan-2-ol. Is it something to do with electronegativity? Please could you help me out?
Igloo replies
No, it’s nothing to do with electronegativity.
All chemical reactions, including combustion, involve bonds being broken and subsequently formed in a different configuration.
Now, although propan-1-ol and propan-2-ol share the same molecular formula and even the same functional group, the hydroxyl (-OH) group, slightly different amounts of energy are involved in breaking all the bonds in one mole of these different isomers.
The reason for this is that bond energies vary slightly with their environment (i.e. which other bonds are close neighbours), and the consequence of this is that although the same amount of energy is released when CO2 and H2O are formed from the atomised elements of carbon and hydrogen, the energies involved in breaking the bonds in the two alcohols are not identical.
That is why most of the values for bond energies in tables of data are described as 'average' or 'mean' bond energies.
This is why your data book will give you slightly different values for the enthalpies changes of combustion for these two alcohols, and indeed, for any set of structural isomers.
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updated: 09 February 2005
