The second law of Thermodynamics
The first law of Thermodynamics explains what happens when a reaction takes place. How
is the energy transferred? How does the internal energy of the system change….?
The second law explain why a reaction takes place.
To explain this, we first introduce the idea of spontaneous change. It explains to us that
some reactions can happen without the need of external influences. These reactions don’t
need to happen fast, they will just happen slow or fast. ( or they just have a tendency to
happen). Like a hot object will cool down when you place it in a cold environment. So we
know that there is a natural direction for a reaction to happen in. The reaction can also
happen in its unnatural direction. This requires energy. We force the reaction to happen in
that way.
But the spontaneous change does explain why reactions happen. It just explains that some
reactions are more to happen than others.
This leads to the idea of entropy.
When a hot object is placed in a cold environment the fast moving/vibrating atoms of the hot
object will collide with the slow moving atoms of the environment. This will make transfer
some of their energy into the low energy atoms which increases their energy.
We begin to see a pattern. The spontaneous change will always happen in the direction in
which the energy is more spread out.
This is when we define entropy. Entropy is a measure of disorder or a measure of how the
energy is spread out. In a system, the energy can be spread out in different ways. Each of
these ways represent a microstate. The entropy of the system is directly related to the
amount of these microstates. A higher amount of microstates will give a higher amount of
disorder or
entropy.
The idea is: an isolated system that undergoes a spontaneous change will always have a
higher level of entropy after the reaction.
If again a hot object is placed in a cold environment, the natural change is for the hot object
to become less hot and pass on its energy to the colder environment. This will increase the
energy of the environment, which increase the entropy of the environment and the hot
object.
You think of it as the prison with the prisoners as the hot object and the society with the civils
as the environment. The prison has a high level of entropy or disorder. The society has a
lower level of disorder since the people try to follow the rules. Assume all the prisons break
down and the prisoners are released. The prisoners now invade the environment and live
without following the rules. Obviously this will lead to a higher level of disorder than when the
disorder was concentrated into one place: the prison.
The entropy is a state function. It does not depend on the path taken to achieve its current
state.