# Comparing Decane and Octane

1. Jun 5, 2014

### 462chevelle

So I was trying to figure some random stuff out, trying to figure out something other than gasoline to supplement inject into an engine in a stand alone with nitrous other than gasoline. I know there are a bunch of things I don't know about chemistry, but I have a couple questions before I can even proceed. I took Octane and balanced the equation and found its change in enthalpy during combustion, then I found the change in enthalpy of Decane the same way. I found Decane to have a higher change in enthalpy during combustion, please correct me if im wrong. But, i think that means that Decane has more energy per mol. Then I used a reference of -1X10^11 kJ to see how they compared for a certain amount of energy. I used the molar mass of Octane and Decane to convert these to kg used for that amount of energy. Since the molar mass of Decane is higher than Octane I found that you need less octane in kg to make this amount of energy, by quite a bit I might add. So my conclusion is that it would be a waste of time to try to use Decane. As if I even know where to find it or enough about chemistry to actually get everything working correctly. Please correct any mistakes I've made in my logic here. I'm completely self taught in Chemistry. Thanks.

2. Jun 5, 2014

### Staff: Mentor

This is kinda unreadable wall'o'text, this post of yours.

Decane having higher energy per mole looks OK, octane having higher energy per kg by quite a bit - not so. Energy per kg should be quite similar (not identical, but I would be surprised by difference of more than just a few percent).

3. Jun 5, 2014

### SteamKing

Staff Emeritus
The OP is what happens if you take too much caffeine while pondering the difference between octane and decane.

4. Jun 6, 2014

### 462chevelle

Sorry, I get a little worked up and lose all organization skills, limited on a normal day. Without my numbers in front of me octane had something like 2.8X10^6 and decane had 2.3X10^6. I attributed the different to the molar masses, octanes somewhere around 114.2 and decane somewhere around 144.4. Again, going off of memory so I could be off a few figures. And yes, I drink waaaay to much caffeine and think about stuff that is beyond my knowledge at the time of starting the thought process.

5. Jun 6, 2014

### Staff: Mentor

This is close to 10:8 - which is ratio of number of atoms of carbon in both compounds.

For linear molecules combustion enthalpy can be approximated by 2*(terminal hydrogen)+n*(-CH2- group). Non-linear molecules are quite close. This is why combustion enthalpy depends on the molar mass, but not so on the mass.

You can also think in terms of number of C-C bonds and C-H bonds. In typical molecules (ie not some exotic, strained things like cyclopropane) their energy is reasonably constant. This will be equivalent to the rule listed above.

6. Jun 7, 2014

### 462chevelle

So my next question, where does a person find Decane? Is there any way for me to find out what kind of O2-Decane ratio is needed for optimal combustion? and also what kind of reactions is will have with N2O. My chem book is watered down (just a gen chem 1-2 book), I need to find a better one. If you have a recommendation that would be great.

7. Jun 7, 2014