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What irreversible processes occur in a gasoline engine? Why are they irreversible?
Question Date: 2013-04-26
Answer 1:

Combustion is an example of an irreversible process that occurs inside a gasoline engine. In a combustion reaction, an organic compound that has some combination of carbon and hydrogen atoms is reacted with oxygen to produce carbon dioxide gas and water. Why are they irreversible?

Physically, you can see this in the following way: imagine trying to compress carbon dioxide and water to form the original gasoline you put in the car. We could wait a very, very long time, but this is probably not going to happen. So it has something to do with the stability of the products formed, which can be expressed in terms of the ΔG (change in Gibbs Free Energy) of the reaction.

Let's consider an example. The gasoline we put in our cars is actually a complex mixture of over 500 different kinds of hydrocarbons (chemicals with the general formula CxHy, where x is a number between 5-12 and y is a number that depends on what x is). What I am about to say is fairly general and can be applied to any of the different kinds of hydrocarbons in gasoline, but let's consider an example for octane one of the particular hydrocarbons we might find.

The combustion reaction for octane is: C8H18(g) + 25/2 O2 (g) → 8CO2 (g)+ 9 H20(l), ∆G° = -5641.4 kJ/mol.

Notice that ∆G° is a very large, negative number. You may have learned in your chemistry class that a large negative number for ∆G° for a reaction indicates that the reaction is very spontaneous -- the reaction goes "downhill" in free energy, which means the products are more stable. But you may also recall that to determine the ∆G° for a reaction in the opposite direction (i.e. going from water and carbon dioxide back to octane and oxygen) we just need to flip the sign on ∆G°. So for the reverse reaction we have:

8CO2 (g)+ 9H20(l) → C8H18(g) + 25/2 O2 (g) , ∆G° = +5641.4 kJ/mol.

Now, ∆G° is a large positive number, which means it is very unspontaneous! So the reverse reaction is very unlikely to happen, and thus the combustion of gasoline is essentially irreversible.

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