why do heats of combustion increase with chain length increase of alkanols? (1 Viewer)

[.ben]

Can anyone tell me?
thanks

 

[priesty]

from memory (i did hsc chem last year) i belive it is to do with the greater number of bonds and intermolecular forces that are present. for example the -OH on the alkanol has the ability to form H-bonds with another molecule's -OH. So therefore as well as larger molecules obviously having larger number of dispersion forces present in the sample, there is the ability to form H-bonds which leads to more energy being required to break these bonds for the compounds to be broken down to water and carbon dioxide..... hmm, someone correct me if im wrong, but look into that and im sure u'll find more detail.

hope this helps

 

[insert-username]

When chemical bonds are formed they release energy and when they are broken they require energy. Comparatively, it takes less energy to break the bonds in an alkanol (C-H, C=O, C-O) and in oxygen (O=O) during combustion than is released by bonds forming in CO₂ (C=O) and H₂O (O-H) (products of combustion), so the enthalpy change is negative. In longer chain alkanols, you have more CO₂ and H₂O forming, which releases more energy than it takes to break the extra bonds in the alkanol and oxygen.

I'm pretty sure that's right, but it's outside the syllabus - you only need to identify why longer chained alkanols have higher melting and boiling points, off the top of my head.


I_F

 

[sikeveo]

You should know all the names of the bonds, i lost marks for that last year.

 

[.ben]

oic now thanks gugys

 

[tristambrown]

isnt the answer to this simpy - becayse there is more fuel per mole. keep in mind this is comparing MOLE to MOLE not mass to Mass .... (bonding issues relate more to BP, MP and determining exact heat of combustion taking into account energy required to break bonds vs energy released by reaction- all of which is not relevant/too complicated for HSC syllabus)

as a fuel issue:

eg one methane molecule CH3 has only one carbon atom to oxidise (to burn) and three hydrogens. In a mole of this substance it is important to note that there is one mole of Carbon and Three moles of Hydrogen available for combustion

Ethane on the other hand CH3-CH3 has 2 carbons and 6 hydrogens to burn - HEAPS more energy to release.. In one mole of this substance note that there is TWO moles of Carbon and Six moles of Hydrogen available for combustion. therefore one mole of ethane has alot more potential heat energy in it that the molecule one carbon shorter than itself (methane)

and so on for more carbon atoms added to the chain ....