External vs Internal equilibrium: How important for me

The difference between external equilibrium and internal equilibrium means a lot to an engineer. As an engineer, understanding and managing internal and external equilibrium in a chemical reaction in a reactor is of great importance.

There are two types of equilibrium, external which is when a system is in equilibrium with the surroundings, and internal when the system is in equilibrium with itself internally. For easy understanding, right in the beginning, I will give you an example that is familiar " Isentropic systems" Isentropic systems are an excellent example of internal equilibrium. In an isentropic process, there is no heat transfer between the system and its surroundings, and there are no irreversibilities such as friction or heat losses. This means that the system remains in a state of internal equilibrium throughout the process.

Fundamentals

Internal equilibrium refers to the state where a system does not have any net internal forces or unbalanced energy distribution. It is crucial to ensure that components within a system are in equilibrium to guarantee stability and prevent unwanted behavior or failure.

External equilibrium, on the other hand, refers to maintaining a balance between a system and its surroundings. This includes managing heat, mass, and energy transfer between the system and its environment.

Let us take an example of a glass of still water on your table and analyze the role of external and internal equilibrium.

External equilibrium

In simple thermodynamics, the still water is externally in equilibrium with the surroundings. It is free energy is near zero having no ability to perform work with enthalpy and entropy balancing each other at the given temperature and pressure. This is called external equilibrium. This is what we generally perceive about equilibrium

Internal equilibrium

It is complex

The still water is not internally in equilibrium with itself. The molecules are colliding, intermolecular bonds are making and breaking so enthalpy (potential energy) is changing. As bonds break and make the new microstates are changing entropy.

Therefore, the free energy dG = dH -TdS, of water molecules are constantly changing internally not letting it reach minimum free energy in the still water

Why you do not see an apparent change in internal free energy in still water?

Because under isothermal/ isobaric conditions the energy comes from the surroundings and returns to the surroundings

A real-life example:

?Internal equilibrium in gases

Gases maintain internal equilibrium through a process known as diffusion. Diffusion is the movement of gas molecules from an area of higher concentration to an area of lower concentration until the concentration is equalized or in equilibrium. This occurs due to random motion and collisions between gas molecules.

When a gas is in a closed system, the gas particles continuously collide with each other as well as the walls of the container. During these collisions, the gas particles exchange kinetic energy and momentum, leading to the redistribution of gas molecules throughout the system. This process eventually results in a uniform distribution of gas molecules, meaning that the concentration of the gas is the same throughout the system. At this point, the gas is in internal equilibrium.

How important is internal and external equilibrium in chemical processes

One specific case in the chemical industry where both internal and external equilibrium matters is in a chemical reactor. Internal equilibrium in a chemical reactor refers to the state where the chemical reactions taking place within the reactor have reached a balance, meaning that the forward and reverse reaction rates are equal. Achieving internal equilibrium ensures that the desired chemical reactions proceed efficiently, without the formation of unwanted byproducts or undesired side reactions.

External equilibrium in a chemical reactor involves maintaining a balance between the reactor and its surroundings. This includes managing heat transfer to ensure the reactor is at the desired temperature, maintaining pressure levels within safe and optimal ranges, and controlling the flow of reactants and products. Maintaining both internal and external equilibrium in a chemical reactor is crucial for the overall performance and efficiency of the process.

Internal equilibrium is not achievable

A system can go near internal equilibrium but it cannot reach internal equilibrium

?Internal equilibrium is not achievable even at absolute zero because enthalpy, a measure of the total energy of a system, is not zero at this temperature. Entropy, which is a measure of molecular disorder, decreases as the temperature approaches absolute zero. However, even at absolute zero, there can still be residual energy in the system. At absolute zero temperature, gases would still possess residual energy, known as zero-point energy, which arises from the inherent vibrations and motions of atoms and molecules.

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