A Detailed Study on Criteria for Equilibrium and Spontaneity

According to the second law of thermodynamics, heat energy cannot be transferred from a lower temperature to a higher temperature without the addition of energy.

If you’ve ever broken a glass, you’ll know there’s no way of getting the unbroken glass back. This is known as irreversibility.

The Second Law of Thermodynamics in Practice:

1. Heat always flows from a body at a higher temperature to a body at a lower temperature, according to the law. This law applies to all sorts of heat engine cycles, including Otto, Diesel, and others, as well as all types of working fluids. This law has helped in the advancement of modern cars. 
2. Refrigerators and heat pumps based on the Reversed Carnot Cycle are another application of this law. Heat produces work in the original Carnot Cycle, whereas work is provided to transport heat from a lower temperature reservoir to a higher temperature reservoir in the Reversed Carnot Cycle.

3)Removing heat from food products in the refrigerator and discarding it in a higher temperature environment is not an automatic process. 

To make this happen in the refrigerator, we’ll need to supply external energy through the compressor.

Criteria for spontaneity include the following:

Any process’s spontaneity is determined by three variables.

When a process’s enthalpy change is negative, the process is exothermic and may be spontaneous. (H stands for negative)

If a process’s entropy change is positive, the process may arise spontaneously. (S is a positive letter)

For a reaction to occur spontaneously, the gibbs free energy, which is the combination of the aforementioned two (H -TS), must be negative, i.e. the necessary condition for a reaction to occur spontaneously is H -TS = 0

Thermodynamic Equilibrium: 

When no spontaneous change in any macroscopic attribute is seen and the system is isolated from its surroundings, the system is considered to be in thermodynamic equilibrium.

There are three thermodynamic equilibriums in total:

Chemical Equilibrium is a term used to describe a state of equilibrium in chemistry.

Mechanical Equilibrium and Thermal Equilibrium are two different types of equilibrium.

Chemical Equilibrium: 

When no chemical reactions occur within the system or between the system and its surroundings, the system is considered to be in chemical equilibrium. The chemical composition will be consistent throughout the system, and the system’s chemical balance will not be disturbed. This is a reaction that can be reversed.

When Calcium Carbonate (CaCO3) is heated to 1073K, it produces CaO and CO2.

Mechanical Equilibrium:

 When there is no imbalanced force within the system or between the system and its surroundings, the system is considered to be in mechanical equilibrium.

It is about the force. The pressure in the system is constant throughout and does not change over time.

For example, a treadmill is a gym machine on which we run but do not move forward since the force you are trying to push forward is the same force that is pushing you backward. As a result, one of the best instances of mechanical equilibrium may be seen here.

Thermal Equilibrium: 

When there is no temperature difference and the temperature remains constant at all points, the system is considered to be in thermal equilibrium.

E.g:

The temperature of a hot cup of tea is higher than the ambient temperature, hence it is not in thermal equilibrium.

However, if you leave it out in the open for a while, the temperature begins to radiate into the environment, and the ambient temperature and the temperature of a cup of tea remain the same at that point, we may say it is in thermal equilibrium.

Criteria for Equilibrium:

We say an isolated system is at equilibrium when it is incapable of spontaneous change. Finally, this phrase clarifies what we mean when we say “equilibrium.” We have requirements for equilibrium in every macroscopic system based on our statement of the second law of thermodynamics:

An isolated system can go through any change that causes the system’s entropy to rise. The reverse is also true: an isolated system with increasing entropy can change. Any such shift is referred to as “spontaneous.” If an isolated system cannot alter in a way that causes its entropy to grow, the system is said to be in equilibrium.

Any change in a system that is not isolated can result in an increase in the entropy of the cosmos, and vice versa. Such shifts are also referred to as “spontaneous.”

The change is not spontaneous if it occurs in a non-isolated system while the entropy of the universe remains constant. The amount and sign of the entropy changes in the system and its surroundings are equal. It is said that the transformation is reversible.

Conclusion: 

The second rule of thermodynamics is crucial because it discusses entropy, which, as we all know, “determines whether or not a process or reaction will be spontaneous.”

For example, Every day, we have coffee. In 10 minutes, what happens to our cup of hot coffee? The coffee begins to cool, or, in thermodynamic terms, the hot coffee emits heat into the environment, causing the coffee to chill. This Phenomenon is known as Entropy.