A thermodynamic system is a part of the physical universe with a specific limit for observation. This limit can be defined by real or imaginary walls.
A system contains what is called a study object. A study object is a substance with a large number of molecules or atoms. This object is made up of a geometric volume of macroscopic dimensions subjected to controlled experimental conditions.
A thermodynamic system can undergo internal transformations and exchange energy and/or matter with the external environment.
Definition of thermodynamic system
A thermodynamic system is defined as a quantity of matter or a region in space on which attention is focused on the analysis of a problem.
Everything that is part of the exterior of the system is called an environment or environment. The system is separated from the environment by the system boundary.
The limit can be fixed or mobile. A system and its surroundings together.
Types of thermodynamic systems
Within thermodynamics there are the following types of systems:
A system is open if it interacts with its surroundings. It can exchange mass, energy, or both.
An example of an open system is a pool filled with water. The water can enter or leave the pool. It can be heated by a wind heating and cooling system.
A system is closed if it allows an energy flow with the outside environment, through its boundary, but not mass.
An example is a cylinder kept closed by a valve. The cylinder can be heated or cooled however, it does not lose mass. While the cylinder itself behaves like an open system if we open the valve.
An adiabatic system does not transfer any heat into or out of the system. The PV=constant equation is only valid for an adiabatic system in a simple system of an ideal gas for a certain number of moles.
A system is said to be isolated if
It does not allow the exchange of matter with the external environment.
It does not allow the transfer of energy with the external environment.
An example is the universe. Most astronomers also consider the universe as an isolated system. It does not allow the entry or exit of matter or energy.
Thermodynamic processes do not affect the total amount of energy of the system according to the zeroth law of thermodynamics.
Each of these systems can still be schematized due to their internal complexity, there is the possibility of subdividing them into smaller subsystems. In this way we will obtain that an open, adiabatic open, closed, adiabatic and isolated system can be:
Simple thermodynamic system. A system is simple if it is limited by a limit, within which there are no other walls.
Composite thermodynamic system. A system is composed if it is delimited by a limit, within which other walls exist.
Systems can also be classified according to their homogeneity. In this way we speak of systems:
Homogeneous systems, in these systems the macroscopic properties and equations of state are the same anywhere. The thermodynamic state of the system is the same everywhere.
Heterogeneous systems, when the above does not occur. The properties of the system could differ in different points. For example, a liquid in the presence of its vapor.
What does it mean that a system is in thermodynamic equilibrium?
A thermodynamic equilibrium is a state in which a thermodynamic system has a thermal and mechanical equilibrium and an equilibrium reaction.
The state of thermodynamic equilibrium is determined by intensive variables. The intensive parameters are thermodynamic variables that do not depend on the size of the system. For example, pressure and temperature.
An extensive variable is a variable that depends on the size of the system. Two examples of extensive variables are pressure and volume.
Although the name "equilibrium state" suggests otherwise, this state is not reached spontaneously in all isolated systems.
Many 'chemical systems' do not reach their ideal equilibrium state. It is like that because the chemical reaction that should lead to that equilibrium lacks the necessary activation energy.