#1
Yeah i got a chemistry assignment due tommorow and we did these tests on petrol and ethanol blends. we found that as the percentage of petrol within the blend increased, so too did the volatility of the blend. obviously this means that petrol is more volatile than ethanol and is what combusts first and then sets off a chain reaction, igniting the ethanol.

but what i want to knwo is, why is petrol (C8H18) more volatile than ethanol (C2H5OH). thanks guys..

the person who answers will be credited in my bibliography ^.^
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#2
because there is more "stuff" in it to burn with the oxygen and make CO2 and H2O

I made that up
but it could be right
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#3
I believe it's because their bonds have more energy stored in them, so when they break during the combustion, more energy is released. I could be wrong, though.

And as to why their bonds have more energy, I have no idea.
#4
no no no no!

lol guys the question is: why is petrol more volatile than ethanol?

i mean petrol is a heavier molceule right? . the answer has nothing to do with energy released in combustion or anything
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There are most certainly not the same thing. They share the same notes, but then again a hamburger and vomit could share the same molecular composition.


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Lmao. You, sir, just made my year.
#5
Petrol has more Carbon and Hydrogen molecules bonded together, so when the bonds are broken (as in a combustion reaction), more energy will be released than in ethanol, which has fewer hydrogen-carbon bonds. Therefore, petrol is a stronger hydrocarbon?
That makes sense to me, but honestly, I'm not 100% sure. I take AP Chem, by the way

Edit: The energy released would be how volatile it is, no? If you are looking at weight, the atomic weight of each compound can be easily calculated using the periodic table.
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#6
My guess is that since ethanol has hydrogen bonding (Cd+ -- Od- -- Hd+) it has stronger intermolecular forces than octane, which only has dispersion forces. Hence it takes less energy to evaporate the substance (since there is less energy holding the molecules together). I'm not entirely sure though.

N.B. Energy is ABSORBED when bonds are broken. Energy is EMITTED when bonds are made.
The reason, then, octane has a higher molar heat of combustion than ethanol is not because more energy is used to break the bonds, it is because there are more weaker C-H and C-C bonds in the reactants to make the stronger H-O and C-O bonds in the products.

The reason the combustion of any hydrocarbon is exothermic is because of the relative strength of the bonds in the reactants and products.

Also, volatility = the tendency of a liquid to pass into the vapor state at a given temperature.
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Last edited by Yamiyo at Nov 29, 2007,
#7
yamiyo is correct...

Ethanol can hydrogen bond, which are strong forces

petrol doesn't even have dipole-dipole interactions, only vanderwaals forces
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