[Marky_sparky]

Hey all i hope you can help me!!
1. I was wondering if anyone knows anything about the expansion ratio of gasoline?
2. Is there any source to find the energy densities of certain materials? (I found a wikipedia but i don't understand where they got that data from [there are no annotations] i am looking for materials like wood, h2, propane, coal.  I don't know how accurate wiki is and was hoping there was a site or book.)

thank,
MT

[Arkcon]

Hey all i hope you can help me!!

I'll try, making some assumptions from the context.  Be forewarned you're heavily using jargon that's not absolutely connected with chemistry, but more with engineering topics.

1. I was wondering if anyone knows anything about the expansion ratio of gasoline?

You mean how much the fluid expands or contracts in bulk depending on temperature, right.  That can be looked up, but I'd like to take a moment to let you know, the number is likely very small.

2. Is there any source to find the energy densities of certain materials? (I found a wikipedia but i don't understand where they got that data from [there are no annotations] i am looking for materials like wood, h2, propane, coal.  I don't know how accurate wiki is and was hoping there was a site or book.)

You can start by looking for values from table of enthalpy.  That's a sort of measure of the heat energy used to form the molecule.  But the "energy density" is a sort of combination of enthalpy of reactants - enthalpy of products, trying to take into account inefficiencies based on generalized reaction conditions, etc.  Also, wood, for example is not a rigidly defined substance -- yeah, I know a tree when I see it too, but the relative amounts of combustible compounds is highly variable from sample to sample.

[Marky_sparky]

thanks for the reply... the gasoline question has to do when it is reacted (combusted) i am sorry that i wasn't clear on that!  So my new question would be... how much does gasoline expand when it is combusted?

As for energy density... I have asked many engineering profs and they don't seem to have an idea for a resource on that topic so i figured maybe anyone in chemistry would know.

[Arkcon]

thanks for the reply... the gasoline question has to do when it is reacted (combusted) i am sorry that i wasn't clear on that!  So my new question would be... how much does gasoline expand when it is combusted?

OK, lets start with the typical student's give-and-take, that's the way we like it on this forum:  Can you try to write a balanced equation for the complete combustion of a compound in gasoline?

As for energy density... I have asked many engineering profs and they don't seem to have an idea for a resource on that topic so i figured maybe anyone in chemistry would know.

Yeah, that's what I said.   The specifics are rather poorly defined, so a simple straight answer is likewise hard to reach.  Probably the most correct answer would be a range of values, and maybe probabilities for the tightness of the most likely values in the range.  Perhaps if someone would specify a set of conditions, controlled experiments could be performed, and we could extrapolate.  Such this are probably done all the time in Engineering Technical journals.

[Marky_sparky]

C8H18(l) +12.5O2 --> 9H2O(g) + 8CO2(g)

[Arkcon]

Good, now you'll understand this web page better:http://www.webmo.net/curriculum/heat_of_combustion/heat_of_combustion_key.html  It uses the published heats of formation d_fH of a variety of fuels (searchable in the NIST web book) and the enthalpy of O2, CO2 and H2O, to give heat generated.  You see, you get heat energy when you break apart fuel and oxygen, but you use some of it it make CO2 and water.  That page also uses published density of fuels, to calculate energy, like you wanted.

Bear in mind that gasoline isn't 100% octane.  I actually thought the petroleum fraction called gasoline was a mixture of pentanes to octanes, I was wrong, it goes all the way up to 12 carbon chains.  Not that the gasoline fraction of crude oil is useful for modern engines, real fuel contains a number of additives, from aromatics to other solvents, like alcohols and ethers, to make a useful fuel.

[Marky_sparky]

so volume per volume gasoline combusted will expand about 17 times?  that doesn't seem correct

[Arkcon]

Hmmm...assume the gasoline is entirely vaporized, it isn't, but we're dealing with ideals here.  Assume all gases behave idealy, none of them do, but we're stabbing in the dark otherwise.  Using n-octane:

C₈H18 + 12.5 O2 --> 8 CO₂ + 9 H₂O

You start with 13 and a half volumes of "stuff" as a gas, you end up with 17 volumes of "stuff" as a gas -- you end up, at the end with more individual particles, which the ideal gas law says bounce around independently.  This can be a little confusing, basic chemistry says that we can't create matter.  But we treat each individual gas molecule as a point source of pressure, so they do take up more space.

Secondly, after combustion, you've made a bunch of hot gas products.  Again, using the ideal gas law, PV=nRT, depending on the pressure, (which we don't know) the volume is the product of how much gas you have, and it's temperature, and a constant.  Put more pressure on it and you can squeeze it into any space you want.  But ...

OK ... summary time.  Internal combustion engines work by a serious of controlled explosions.  So gasoline and air mixtures can be considered explosives.  Pretty much, a simple definition of explosive is: take a small amount of solid or liquid, make it into lots of gas, quickly, and maybe make it hot, so it takes up lots of space.  I haven't worked out the math, as you can see, but yeah, 17x expansion of volume doesn't shock me.

[Marky_sparky]

How come this type of procedure to see how a liquid would expand to a gas doesn't work with for instance H2O2?  I read that the expansion of 2H2O2 --> 2H2O + O2  (70% concentration) expands 2700 times (vol/v) http://www.h2o2.com/intro/properties/thermo08b.gif

[Arkcon]

Hotlinked chart lacks a proper title, where on that site, did it come from?  Are you confusing the thermal expansion of a liquid with the expansion of a gas, in spite of or because of the liquid reacting to form a gas?

[Marky_sparky]

I got the information from here: if you read  the footnote it says that the adiabatic curve is for rapid expansion (as if i were to use a catalyst of Ir/Al2O3 or Ag/Al2O3)
http://www.h2o2.com/intro/properties/thermodynamic.html

[Arkcon]

Yes, like the title on that page says, the chart is for the volume expansion of the decomposition of hydrogen peroxide.  Engineering sources may have a similar chart for various grades (read that as mixtures) of gasoline combusting with air, but I don't know where to find such a reference.

[Marky_sparky]

one more thing... when i read that graph is it saying... if i react 1mL of H2O2 i will get 2700mL of products at 70% concentration?

[Arkcon]

one more thing... when i read that graph is it saying... if i react 1mL of H2O2 i will get 2700mL of products at 70% concentration?

Yes, 1 ml of 70% H₂O₂, decomposes into 2.7 liters of water, steam and oxygen gas, if you let it expand until the pressure reaches, what is essentially, atmospheric, since it likely started at atmospheric.  That is a lot of gas, from a tiny starting point, which makes 70% H₂O₂, yeah, you guessed it -- a dangerous explosive.

[Marky_sparky]

do you have any suggestions of where i could find similar charts to that of the H2O2 expansion for different materials?