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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