heat of combustion of ethanol (1 Viewer)

[lucyinthehole]

is it greater or less than that of a normal fuel, say octane?

 

[stoydgen]

It's less than, thus more ethanol needs to be combusted in order for it to produce the same heat as octane

EIDT: too late lol

 

[Sirius Black]

yes *less than*

http://members.nuvox.net/~on.jwclymer/rq/hoctable.html

 

[velox]

Thats why octane is used in fuels, and ethanol isn't (well to a less extent i suppose) Except in other countries, but thats besides the point.

 

[icxdragon]

per gram, per litre, unfort. less-->hence you have fill up more, and a disadvantage to add to that lil dot point

 

[nit]

Hmm..looks like this thread has been rejuvenated. Well anyway, for equal masses of ethanol and octane, approximately 60% less energy is extracted from ethanol as from octane (delta Hcomb ethanol = -1367kJ/mol vs -5470kJ/mol for ethanol if i remember correctly). When considering the two in liquid form as is most useful if inverstigating ethanol as a fuel, the ratio converts to 70% due to ethanol's density of 0.785g/mL vs 0.678g/mL for octane. Furthermore, when blended at the 10% level, approximately 3.5% more petrol is required compared to what would be needed if there was no oxygenating ethanol present in the fuel.

 

[Budz]

Less

is it greater or less than that of a normal fuel, say octane?

The basic idea is that as th number of carbons increase, the heat released increases. This is because combustion of alkanes is exothermic and production of heat is a result of the reforming of bonds.
Conservation of matter states matter is neither created nor destroyed
Octane with 8 carbons has more bonds then ethanol with two carbons
So therefore there is more products CO2's and H2O's needed to balance the equation.
The more CO2 + H2O produced the more bonds a formed, and the formation of bonds releases energy in this case Heat...

So in other words ethanol produces less heat(energy) than say Octane

 

[Sirius Black]

ethanol has a 113 octane rating (where "octance rating" is a relative measure of fuel's energy output per gram comparing to octane)
in this case, shouldn't ethanol release more energy than octane per gram?

 

[Frigid]

ethanol has a 113 octane rating (where "octance rating" is a relative measure of fuel's energy output per gram comparing to octane)
in this case, shouldn't ethanol release more energy than octane per gram?

mole for mole, octane > ethanol. however, because octane number is a measure of energy/mass, and the mass of ethanol is less than octane (therefore, more ethanol molecules is present in one gram than octane molecules), therefore ethanol has a higher octane number.

this of course, is coming from a person who hasn't studied chem for over a year, and my words must be taken with a pinch of NaCl.

 

[Xayma]

Per gram, ethanol has a heat of combustion about 3/4 of that of octane.