Most of human society is powered by the oxidation of hydrocarbons (ex. Octane {C8H18}-- a principal component of gasoline) in combustion reactions.

Question

Most of human society is powered by the oxidation of hydrocarbons (ex. Octane {C8H18}-- a principal component of gasoline) in combustion reactions.

a) In terms of matter spread and energy spread, what happens when a hydrocarbon like gasoline is

oxidized? Justify your answer using the chemical equation and observable characteristics of the

reaction.

b) Is the oxidation of a hydrocarbon like octane always spontaneous, never spontaneous or only

spontaneous at specific temperatures? Justify your answer.

Answer 1

Hello, Jenn,

I'm not sure if the problem wants a simple, unbalanced, version of the chemical equation:

C₈H₁₈ + O₂ = CO₂ + H₂O

with a simple explanation: "The oxidation of octane produces carbon dioxide and water in an exothermic process that gives off large amounts of heat."

If it is asking for a balanced chemical equation, which it should, it adds a little more work. I'd be happy to explain techniques for balancing chemical equations, but please ask because the process I use takes more time than I have at the moment to explain clearly.

The big hint is tostart with the most complex molecule and assign it a coefficient of 1, in pencil, to get started. Since we need whole molecules, see is you can distribute all the atoms from 1 molecule of octane to the two products. Since octane has 8 carbons, we see that we must have at least 8 CO₂ molecules, so put a 8 for it's coefficient. The 18 H atoms all go into H₂O molecules, so place a 9 for it's coefficient. Now we need to check on O₂. From the one molecule of octane, wehave, so far, 8 CO₂ and 9 H₂O molecules. That adds to (16 + 9 =) 25 O atoms required. The problem is oxygen is diatomic, so it only comes in pairs, O₂. So we need an even number of O atoms in the products. It nis OK to use fractions to start the process, which I'll do here. To get 25 O atoms from O₂, I'll use a coefficient of 12.5, so the equation is numerically balanced, if not technically possible. But all we need to do is multiply all the coefficients by a number that would make them whole, which would be 2, in this case.

So we go from:

C₈H₁₈ + 12.5 ₂ = 8CO₂ + 9H₂O

to

2C₈H₁₈ + 25 ₂ = 16CO₂ + 18H₂O

Although unwieldly, the equation is now balanced.

I'm unable to answer part b with a high level of justification, but I can say that the reaction is spontaneous above a certain temperature. One the spark is released, or the match thrown in, the heat released by the reaction supplies mure than enough energy to keep the process going, as long as oxygen is available. If it were always spontaneous there could be no invention such as a car, at least one that would survive the first filling. The reaction has a sufficient activation energy requirement to keep it safe enough to remain the major source of energy for transportation.

I hope this helps,

Bob