The thermodynamic quantity called entropy, symbolized by the letter s, is related to degree of organization of a system. The greater the disorder in the system, the greater the entropy.
For example, imagine that we put sodium chloride (NaCl) in water. What happens is their ionic dissociation, releasing ions into the water, as shown below:
1 NaCl(s) → 1 In+(here) + 1 Cl-(here)
Note that 1 mole of salt molecules gives rise to 2 mole of dissociated ions. The ions in solution are more disorganized than in the solid, which means that the entropy of this system has increased.
THE entropy variation, ∆S, is measured by:
The entropy and disorder of a system has to do with the spontaneity of physical processes. If entropy and disorder increase, it means that the process is spontaneous. For example, consider the fall of a glass, this is a spontaneous process, in which the disorder of the system increases. The opposite process, that is, the broken glass shards going up and recovering the glass, does not occur, is not spontaneous and is irreversible.
Another case is the fall of water from dams, which is a spontaneous process; in this case we can conclude that entropy increases. However, the water returning by itself to the top of the dam is not spontaneous, an external action would be needed, such as a water pump to accomplish this. And if it were possible, the entropy would decrease.
Therefore, in any natural process the entropy of the Universe or the system always increases.
The variation of entropy can also be measured, in isothermal systems (of the same temperature) by the following equation:
On what:
whatrev = energy reversibly as heat;
T = temperature.
Since the variation in entropy is directly proportional to the temperature, we have that at lower temperatures, the disorganization will be less, and vice versa
Another way to calculate the variation in entropy is relating it to heat:
The variation in entropy is directly proportional to the variation in energy, and this proportionality is given by the temperature T.
According to Lord Kelvin (William Thomson, 1824-1907), it is impossible to build a thermal machine in which all the heat from the source is used fully in work, that is, its yield will never be 100%. The energy that is dissipated in the form of heat is transformed into entropy, increasing the disorder in the system.
We have then that the increase in entropy is very important, because without it nothing would happen, it is responsible for the occurrence of the phenomena. This is related to the meaning of the word “entropy”, which comes from the Greek en, which means "in" and stumbles, which is “change”.
By Jennifer Fogaça
Graduated in Chemistry