Entropy and Spontaneity in Law of Thermodynamics

A spontaneous process in chemistry is one that occurs without the use of external energy . Because spontaneity has nothing to do with kinetics or response rate, it can happen quickly or slowly . Entropy is a measure of a system’s unpredictability or chaos . The entropy of gases is comparatively higher than liquids, while in case of entropy of liquids is always higher than that of solids . In physical systems, the concept of order and disorder is critical (also known as randomness) .

Spontaneous process:

A spontaneous process in thermodynamics is one that occurs without the introduction of matter or electrical energy into the system .A spontaneous process in a fully isolated system is characterised by an increase in entropy . In a closed system, a spontaneous process at constant temperature is accompanied by a drop in the system’s free energy and a temporal development towards a more thermodynamically stable state (closer to thermodynamic equilibrium) . Any spontaneous activity that happens at constant pressure and temperature decreases the Gibbs free energy, but any spontaneous process that occurs at constant volume and temperature decreases the Helmholtz free energy . The second law of thermodynamics is directly responsible for these conclusions . Some spontaneous processes, such as ice melting at 5 degrees Celsius, require thermal energy input, whereas others, such as steam condensation below the equilibrium boiling point, require mechanical energy input (compression work) . Processes that require the input of electrical energy, such as electrolysis, are not spontaneous . In the precise thermodynamic sense, a spontaneous process is not reversible . A spontaneous reaction is a chemical reaction that occurs spontaneously at the conditions of interest .

Randomness In entropy:

In English, the word entropy has numerous meanings, but it always alludes to some kind of unpredictability, chaos, or uncertainty . In physics, for example, the phrase refers to a measurement of a system’s thermal energy, which is related to the level of random mobility of molecules and hence a measure of disorder . The unpredictability or spread-outness of a set of molecules is referred to as entropy . Increasing the level of randomness is beneficial . There is an increase in entropy (randomness) involved with the development of a solution, which thermodynamically favours the solution over the two original states . When the temperature rises, the randomness reduces . The third law asserts that at absolute zero temperature, a substance’s entropy is zero, indicating the maximum degree of order . The degree of randomness in a system is measured by entropy . Entropy rises when the degree of unpredictability rises . H cannot be the only factor influencing spontaneity . The unpredictability factor cannot be used as the sole criterion for a process’ spontaneity . The process of liquefaction would be impossible if the unpredictability element was the only condition  .

Entropy and spontaneity:

It is critical to carefully analyse the definition of the system and surroundings when considering the entropy change of a process to determine spontaneity . The second rule of thermodynamics asserts that if the entropy of an isolated system grows over time, the process will be spontaneous . The sentence must be changed for open or closed systems to say that the total entropy of the combined system and surroundings must increase, or

∆Stotal =  ∆Ssystem + ∆Ssurrounding ≥ 0

Entropy changes during phase transition:

• As the temperature rises, the random movement of the molecules increases, resulting in an increase in the rate of entropy .
• When a phase melts, the entropy changes during phase transition rise because the internal energy change is greater . However, if the phase is moving toward lower internal energy, the system’s entropy decreases, resulting in the phase transitioning from gaseous or liquid to solid .
• When the phase transition occurs quasi–statistically at thermodynamic equilibrium, the overall entropy change of the system and surroundings is zero .

Conclusion:

The change in Entropy is the best measure of spontaneity in a reaction (S or DS) . According to the Second Law of Thermodynamics, for a reaction to be spontaneous, it must rise in entropy . Although entropy is sometimes characterised as a measure of a system’s disorder, this is not a very precise definition . Different events can be observed following identical initial circumstances, according to the definition of random models . Under the principles of physics, such randomness cannot occur in a universe dominated by determinism  .