Octane is an organic molecule, with the chemical formula C
8H18. There are two forms of octane, each with a different
structural formula.
Note that it is possible to rotate these 3D models by clicking and dragging
on them, so that it is possible to see their structure more completely.
The carbons are always at the intersection of four lines (except for carbon
dioxide, in which the carbon is at the intersection of two lines), and hydrogens
are at the ends of all white lines. Oxygens are represented by red lines. If you
can't see the models, click on their labels.
You can see that there are 8 Carbon atoms, and 2 Hydrogen atoms per
Carbon atom, plus 2 Hydrogens on the end of the chain, hence C
8 H18. Another formula for this molecule could
be CH3(CH2)6CH3.
The structural formula for this molecule is more complex. On one side,
there are three CH3- groups, joined by a Carbon atom,
creating an ion of C(CH3)-. On the other side, there
are only two CH3- groups, with one H-, all
connected by a Carbon atom. The ion for this side is (CH3)
2CH-. These two sides are joined by a CH2
2+ group (Carbon can be a 4+ or a 4-). The logical combination of
this information would be (CH3)3C(CH2)C(CH
3)2H. Yes, this does not follow any ordinary atomic ordering
conventions, nor is it easy to read and work with; this is why C8
H18 is more commonly used.
Both of the forms of octane burn in oxygen and create CO2
and H2O. Incomplete burning is possible, causing toxins like
CO to form; this will be discussed in detail in a future chapter in our textbook,
and is not necesary to understand the importance of octane in gasoline.
Description Equations (factors in moles of atoms):
2C8H18 + 25O2 --> 16CO2
+ 18H2 O D-7,372kJ
(according to chapter 8 in the textbook, specifically p.'s 224-225)
The 'kJ' at the end of the equation is a concept that will be introduced
in Ch. 8; it is a measure of the net enthalpy in the reaction. The equation
is balanced, and energy is given off; this will all be explained in the future,
and it is not all necesary for understanding this process. The kJ, or kilojoule,
is the kilo- variant of the standard unit of energy (the Joule); 1kJ is,
by definition, the energy required to accelerate a 2000kg mass by 1meter/second.
The equation for energy is therefore Energy = 1/2 x Mass x Velocity2. Click here to see a calculator that uses this formula to calculate how much gasoline cars use in certain conditions.
Of course, gasoline is not pure octane, even though octane is the primary
component of it; it is a homogeneous mixture of many different chemicals,
the exact chemicals and their ratios regulated by the gasoline producing
companies. The effect of octane on fuel will be discussed in more detail
in the next section.
