[Amar]

Hello everyone!

I have a paper I need to deliver about the energy a gasoline driven car releases, particularly the Gibbs Free Energy.
The molecule I've chosen is Octane. My balanced chemical formula is 2C8H18(l) + 25O2(g) → 16CO2(g) + 18H2O(g) | (Assuming complete combustion)

I've calculated entropy and enthalpy based on 1 mol of octane, which is dH= -5074kJ and dS= 483.4 J K-1.

Now I need to calculate the Gibbs Free Energy (dG). I tried doing that by using the standard temperature of 298.15 K and the formula dG = dH - TdS, but I got that wrong. (Delivered the paper once, got it back with a note saying I had to use the reaction temperature and not 25C). So my question is this - how do I calculate the reaction temperature? Or is there perhaps a list of "standard combustion temperatures" ?

I need to deliver the paper by tomorrow, which means I need help as soon as possible Thank you!

[Corribus]

Maybe this is helpful.

http://en.wikipedia.org/wiki/Adiabatic_flame_temperature

[Amar]

Thanks for the quick response.

So should I just take the temperature from the table? Octane isn't on the table...

Can you be a little bit more specific?

Thanks in advance.

Edit: I now realised that "Medium fuel oil" (Petroleum) is on the list, and that Octane is widely used in Petrol...Should I just use this as my reference temperature?

[Corribus]

Here's the relevant bit:

In daily life, the vast majority of flames one encounters are those of organic compounds including wood, wax, fat, common plastics, propane, and gasoline. The constant-pressure adiabatic flame temperature of such substances in air is in a relatively narrow range around 1950 °C.

Granted there are probably some variations in the actual temps due to engine designs, but this seems like a good approximation.

[Amar]

Thanks for the help, Corribus!

Just to be 100% sure, could you just take a look at my final calculation of the Gibbs Free Energy and tell me if this is realistic or not?

ΔG = -5074kJ – 2100K(0.4834 kJ K-1) = -6089.14 kJ. (Reaction is spontaneous)

[Corribus]

Well... sure? I mean, I don't know what the actual value is but it seems reasonable. Keep in mind that this is a molar quantity, so you should include that in your units.