Year 12s: what will be the first way you research your future options? >>

Enthalpy: Confusion

Hey! Having done some work on enthalpy change of combustion of ALCOHOLS: methanol, ethanol, propanol etc, i thought i would see that the more carbon atoms present, the higher the enthalpy change, but having plotted the values, I see that it begins to curve off for higher alcohols.

Why is this????? Is it becausethe O-H bonds dominate bottom end and C-C bonds dominate top end.

I'm really confused!!!!!!!!!!!!!!!!!

HELP!!!!!!!!!!!!!

Reply 1

Eau

Yes, I think because even though the chain increases, the number of -OH bonds are still the same. And the enthalpy of dissociation for -OH bonds is greater than C-C bonds. Well actually this is what your graph suggests.

Reply 2

charco

Study Forum Helper

OK here's an idea - I'm not saying it defo, but just chucking in what seems to me to be plausible. Feel free to shoot down in flames, but listen without prejudice:

The enthalpy change will always be the difference between bonds formed and bonds broken. This is always the same for successive alcohols. You break 1.5 oxygen molecules to react with the carbon and hydrogen from one extra -CH2- group, and the products are always one more water molecule and one more carbon dioxide molecule.

However, as you get bigger and bigger alcohols the bonding within the alcohol itself (i.e. the intermolecular forces) increases. These bonds must also be broken to vaporise the alcohol in order to then break the bonds (intramolecular) and reform new ones (products).

Clearly as the alcohols get larger their van der Waals forces increase and the b.p. increases supporting this hypothesis.

The theoretical values for enthalpy change do not take this into account. Hence the energy released is experimentally less than the theoretical value.