A preference for an increase in the random motion of an object is a
simple statement of the second law of thermodynamics. Formally, in addi-
tion to energy, objects have another property called entropy, which represents
the molecular disorder of a system. According to the second law, processes
will occur spontaneously if the entropy increases. It is the direction of
entropy that leads to the direction of processes that occur in the universe.
Whether considering the progression of stars through different states as their
energy output decreases or the motion of bacteria in search of nutrients,
processes are headed away from organized states to random states.
Entropy
The increase in disorder is evident in our everyday lives. Each day, food
is consumed and largely converted into heat, which is a more disordered
state than the original food items. In general, it could be argued that
all life forms have devised methods of taking ordered resources, such
as molecules or food, and converting them into disordered states in order
to generate energy. If all processes are moving to a disordered state, how
can highly organized states such as crystals or biological cells be created?
The answer lies in considering the overall state and not just one part. For
example, your bedroom may tend to become very disordered with daily
use until you decide to clean it up. While the clothes and other objects in
the room may be more ordered, the process of cleaning the room involves
the expenditure of energy in the form of heat. Thermodynamics would
say that the disorder associated with the heat is greater than the order
associated with the folding and stacking of the clothes.
In a sense, the change in order of an object provides a direction for
chemical reactions. When a process results in a change from one state to
another that occurs in an irreversible way, the process is called spontan-
eous. Only irreversible changes are spontaneous; truly reversible processes
are not. Truly reversible processes do not occur in nature, as it would
require all forces to be perfectly balanced with no driving force for the
system to move. However, by moving objects very slowly while keeping
forces in nearly perfect balance, processes that are very close to reversible
can be created. As an example, gas inside a piston expanding under con-
stant pressure inside and outside does not have any net force to drive the
expansion, and the piston does not move. By making the imbalance very
small, the piston moves, although at a very slow rate. Such a motion would
take a considerable amount of time to be completed, but the rate is not
under consideration here, only the direction.
To quantify the concept that matter and energy tend to be dispersed,
a new state function is introduced, entropy, S, which is a measure of the
disorder of a system. As energy and matter disperse, entropy is defined
to increase. The concept of entropy is explicitly defined in terms of the