What is Thermodynamics?
Thermodynamics is a branch of physics that studies heat, energy, and work. It is capable of analyzing a wide range of thermal systems without taking into account their detailed microstructures, which means that it falls under the macroscopic domain of physics.
The three main laws of thermodynamics are described on separate slides. Each law leads to the definition of thermodynamic properties, which helps us in understanding and predicting the behavior of a physical system.
What does Equilibrium Mean?
The term "equilibrium" refers to the state of balance within the system as well as between the system and its surroundings. Equilibrium denotes a state of disappearing driving forces or gradients in which everything remains unchanged. When a system is in equilibrium, it maintains its current state since there are no driving forces causing anything to change. There are no imbalanced potentials in a system that is in equilibrium.
System and Surrounding
The part of the universe under consideration is referred to as a system. The remainder of the universe, i.e. everything other than the system, is referred to as the surrounding. The system is distinguished from its surroundings by the presence of a boundary around it. It can be real or imaginary, fixed or mobile.
equilibrium-in-thermodynamics
A system can either be homogeneous or heterogeneous. A homogenous system, as the name suggests (homo means same), has all of its constituents in the same physical state. For instance, a mixture of gases, a completely soluble mixture of liquids, and so on. A heterogeneous system, on the other hand, has constituent particles in various physical states. An oil-and-water mixture can be considered an example of a heterogeneous system.
Types of system
Broadly, the system can be classified into 3 categories: open, closed, and isolated. Open system: In an open system, both mass and energy can be transferred between the system and the surrounding.
Closed system: A closed system allows only the transfer of energy within its surroundings.
Isolated system: In an isolated system, neither energy nor mass can be exchanged with the surrounding.
Thermodynamic Equilibrium
The concept of thermodynamic equilibrium is foundational in thermodynamics. When the values of characteristics are the same at all points in a system, it is said to be in thermodynamic equilibrium. In this state, there is no spontaneous change in any macroscopic attributes within the system. This is known as a thermodynamic equilibrium because the system is isolated from its environment. There are no net macroscopic flows of matter or energy inside or between systems in thermodynamic equilibrium.
Thermodynamic equilibrium is a property of a system that results from its constantly accumulating small charges. These small charges add up, forming larger ones until the system reaches its final condition. This is how thermodynamic equilibrium works. This concept is not only applicable in physics but also in everyday life.
The thermodynamic equilibrium is characterized when some of the thermodynamic potentials reach their minimum level. Gibbs free energy and Helmholtz free energy can be regarded as examples.
Helmholtz free energy (H): Helmholtz free energy is a thermodynamic potential that assesses how much 'useful' work a closed thermodynamic system can produce when the temperature and volume are kept constant. For such a system, the negative of the difference in Helmholtz energy equals the greatest amount of work that can be taken from a thermodynamic process with constant temperature and volume. It is minimized when the system reaches equilibrium.
Gibbs free energy (G): It is a thermodynamic potential that assesses how much 'useful' or process-starting work an isothermal, isobaric thermodynamic system can produce. The Gibbs free energy is the maximum amount of non-expansion work that can be taken from a closed system or the maximum amount of non-expansion work that can be obtained solely through a perfectly reversible process.
On the contrary, for a system to remain in thermodynamic equilibrium, there should be no change in the macroscopic particles with time. Hence, the system’s Entropy(S) should be maximum. Entropy is indicative of disorderliness in a system.
It can be said that,
Entropy (S) is greatest in an isolated system.
For a thermodynamic equilibrium of a closed system at constant volume and temperature, Helmholtz free energy is minimum.
For a thermodynamic equilibrium of a closed system at constant pressure and temperature, Gibbs free energy is minimum.
The concept of thermodynamic equilibrium can be understood by Zeroth law of Thermodynamics. As per the Zeroth Law of Thermodynamics, If two bodies are individually in thermal equilibrium with a third body, the first two bodies are in thermal equilibrium with each other. This indicates that if system A is in thermal equilibrium with system C and system B is in thermal equilibrium with system C, then both systems are in thermal equilibrium.
Equilibrium
The values of a system's intense parameters, such as pressure and temperature, determine its local state at thermodynamic equilibrium.
Types of Thermodynamic Equilibrium
The following three types of equilibrium requirements should be fulfilled to consider any system within the thermodynamic equilibrium: mechanical equilibrium, thermal equilibrium, and chemical equilibrium.
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. For a system to remain in mechanical equilibrium, the pressure in the system must be constant throughout and not change over time. Because there are no pressure gradients, there is no bulk flow of fluids.
An example of mechanical equilibrium is a Treadmill. It is a gym machine on which we run but do not move ahead since the force pushing you forward is the same force pushing you backward. As a result, it can serve as one of the best examples of mechanical equilibrium.
Thermal Equilibrium: When there is no temperature difference and the temperature remains constant throughout a system, it is considered to be in thermal equilibrium. It is possible to do this when all of the objects in the system reach the same temperature. Since there is no thermal gradient, no heat exchange will take place in thermal equilibrium.
Consider a hot cup of tea as an example. The temperature of a hot cup of tea is higher than the ambient temperature, hence it is not in thermal equilibrium. However, when it is left 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 become the same. At this point, it can be considered in thermal 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. Consider this equation as an example:
CaCO3 → CaO + CO2
When Calcium Carbonate (CaCO3) is heated to 1073K, CaO and CO2 are produced.
All intensive (temperature, pressure) and extensive thermodynamic properties (U, G, A, H, S, and so on) should remain constant for a thermodynamic system to be in equilibrium. As a result, at equilibrium, the total change in any of those attributes must be zero.
Conditions for thermodynamic equilibrium
For any system to be in thermodynamic equilibrium, the following conditions must be satisfied:
Every part of the system must have the same temperature.
On any component or all of the system, there should be no net unbalanced forces.
Chemical reactions should not cause any changes.
Local and Global Thermodynamic Equilibrium
The exchanges within a system, as well as those between the system and the outside world, are governed by intensive factors or properties in thermodynamics.
In Global Thermodynamic Equilibrium [GTE], the system's intensive parameters remain uniform everywhere, i.e., are homogeneous throughout the entire system. Conversely, the Local Thermodynamic System [LTE] denotes that certain intensive parameters vary in space and time, but at such a slow rate that thermodynamic equilibrium can be assumed in some neighborhoods around any location. It is worth noting that this local equilibrium applies only to large particles.
Summary
Thermodynamic equilibrium is a condition or state of a thermodynamic system in which the attributes of the system do not change over time and can only be changed at the expense of other systems. A thermodynamic equilibrium system with given energy has higher entropy than any other state with the same energy. A thermodynamic equilibrium state's Gibbs free energy at a given pressure and temperature is less than that of any other state at the same pressure and temperature.