h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

what effect does the number of carbon atoms have on the amount of energy given out during a reaction?also if you could give me a brief expanation of why this is i would be greatful!

wow

Hi wow

It depends on each reaction - do you mean, for example, the differences in the amount of energy released when the same number of moles of different alkanols are completely combusted?

RhoMuNuQ

say for example i wanted to heat 100ml of water, what would do this quickest, butanol or ethanol (butanol having more carbon atoms)and then why does this happen?sorry to bother you!dont bother answering if you dont want to!!

Of course I'll reply.

If you burned an equal number of moles (where 1 mol is approximately 6.02 x 10^23 molecules) of ethanol and butanol both in plentiful oxygen, more heat would be produced by the butanol burning than by the ethanol burning. Therefore, the water would be heated more rapidly by the butanol burning than by the ethanol burning.

During complete combustion, 1367kJ of energy is released when 1 mol ethanol (~46g) is burned but 2676kJ of energy is released when 1 mol butan-1-ol (~74g) is burned. As an equal number of alkanol molecules are present in 1 mol ethanol as in 1 mol butanol, the amount of energy released per molecule of butanol burned must be greater than the amount of energy released per molecule of ethanol burned. The same difference in energy release, incidentally, still applies during incomplete combustion - where there is insufficient oxygen for complete combustion so the alkanol combusts to produce carbon monoxide and soot, as well as the carbon dioxide and water produced during complete combustion. However, less than 1676kJ energy would be released when 1 mol ethanol was burned and less than 2676kJ would be released when 1 mol butanol was burned.

The reason for the difference in the amount of energy released are the two extra CH2 groups present in the butanol that aren't present in ethanol. Before combustion begins, more energy must be supplied to break the extra C-C and C-H bonds in the butanol than are present in the ethanol and to break the extra O=O bonds in the extra molecules of oxygen required for butanol to burn than for ethanol to burn. Assuming that carbon dioxide and water are the only products, however, forming the extra C=0 and O-H bonds releases more energy than breaking the extra bonds for butanol to burn than for ethanol to burn initially. Therefore, adding CH2 groups results in more energy being released per mole alkanol molecules burned, due to more energy being released in the extra bond formation than the extra energy input required to initially break all the bonds.

I hope that helps.
RhoMuNuQ

Thanks!!!Just out of interest though where did you learn your science from, becuse I am very interested in science, mostly physics though, and I often find it hard to find information relating to subjects that I am interested in (popular science books never cover enough detail!).
Anyway thanks again,

WOW

That was taught to me during the A-Level course. Popular science books are good reads, but, as you say, almost never go into enough detail! Have you tried reading A-level textbooks? They would go into far more detail than the vast majority of popular science books.

RhoMuNuQ

Yeah, I would but it's hard when Im in secondary school.plus i never know which ones are worth their money, anyway thanks you've been a great help, hope you enjoy h2g2!cheers

wow

If you continue with chemistry, you'll be bound to do organic chemistry at some point.

If you're interested in everything, try clicking this link a few times: <./>RandomEditedEntry</.>. Every time you click it, you'll get a different Edited Entry.

RhoMuNuQ