Thermodynamics is the branch of physics that studies the exchanges of energy and matter in systems. One of its fundamental concepts is the thermodynamic state , which fully describes a system in terms of a set of variables that allow its behavior to be predicted.
Thermodynamic systems are characterized by a series of physical, chemical and mechanical properties that determine their state, and this can be manipulated or altered through thermodynamic processes.
What is a thermodynamic state?
A thermodynamic state is the set of all state variables of a thermodynamic system at a given time. These variables may include, among others, temperature, pressure, volume, internal energy, and enthalpy.
State variables are those quantities that allow the condition of a system to be fully described. In other words, it does not matter how the system got to that state; the only thing that matters is the value of the variables at the current time.
A simple example is a gas contained in a closed container such as the piston of a heat engine. To describe the state of the gas, it is necessary to know its pressure, volume and temperature. Once these variables are determined, the state of the system is completely defined.
State variables
State variables or state parameters are physical quantities that describe the state of a thermodynamic system in equilibrium without the need to know its previous history.
These can be classified into two types:
- Extensive variables : These are those that depend on the quantity of matter in the system, such as volume, internal energy, enthalpy and mass.
- Intensive variables : They do not depend on the amount of matter, such as temperature, pressure and density.
For any system, the combination of a suitable number of these state variables is sufficient to completely describe its thermodynamic state.
The relationships between these variables are determined by the equations of state.
State functions and their importance
A state function is any property of a system that depends only on the current state of the system, regardless of the path or process by which it arrived at that state.
State functions are fundamental in thermodynamics because they allow us to predict how the system will change without having to trace its entire history. This greatly simplifies thermodynamic analysis.
Examples of state functions
Some examples of state functions include:
- Internal energy (U) : It is the sum of the kinetic and potential energy of all the particles in a system.
- Enthalpy (H) : Represents the total energy of a system, including both the internal energy and the product of its pressure and volume.
- Entropy (S) : It is a measure of the disorder or randomness in a system.
- Pressure (P) : Force exerted by the system per unit area.
- Temperature (T) : A measure of the average kinetic energy of the particles in the system.
- Volume (V) : Space occupied by the system.
State equations: relationship between state variables
State equations are mathematical relationships that link the state variables of a system.
In general, equations of state provide a way to predict how the properties of a system will change in response to changes in external conditions, such as temperature or pressure.
For an ideal gas, for example, the equation of state is the familiar ideal gas equation:
P·V=n·R·T
Where:
- P is the pressure,
- V is the volume,
- n is the amount of substance (in moles),
- R is the ideal gas constant, and
- T is the absolute temperature.
For more complex systems, such as real gases, liquids, or solids, the equations of state can be much more complicated.
Thermodynamic equilibrium
A fundamental concept in thermodynamics is that of thermodynamic equilibrium. A system is in a state of equilibrium when its macroscopic properties do not change over time.
This implies that the system has reached a state where the forces and flows of energy or matter are balanced.
There are several types of equilibrium that a system can achieve:
- Thermal equilibrium : It is reached when the temperature is uniform throughout the system and there is no heat flow between the different parts of the system or with its surroundings.
- Mechanical equilibrium : Occurs when the internal and external forces acting on the system are balanced, so that there is no net movement of matter or change in pressure.
- Phase equilibrium : Occurs when the mass of each phase of a system remains constant over time. A classic example is a system in which a liquid and its vapor coexist in equilibrium.
- Chemical equilibrium : It occurs when the chemical reactions within a system have reached a point where the rates of forward and reverse reactions are equal, and the concentrations of the reactants and products do not change.
In practical terms, a system that has reached thermodynamic equilibrium no longer experiences spontaneous changes in its macroscopic properties.
Thermodynamic state diagrams
A useful way to represent the states and transitions of a system is by using thermodynamic diagrams . These diagrams allow us to visualize how the state variables of a system change during a process.
- PV (Pressure-Volume) Diagram : This is one of the most common diagrams and is used to represent processes in gas systems. In this diagram, the area under the curve in an isobaric process (constant pressure) represents the work done by the system.
- TS (Temperature-Entropy) Diagram : This diagram is particularly useful in the analysis of thermodynamic cycles, such as the Carnot cycle. The area under the curve in a closed cycle in a TS diagram represents the heat exchanged.
- HS Diagram (Enthalpy-Entropy) or Mollier Diagram : Widely used in engineering to study turbines, compressors and other power equipment.
Thermodynamic processes
Thermodynamic systems can undergo thermodynamic processes , which are transitions from one equilibrium state to another. During these processes, the state variables change, and exchanges of energy and matter with the environment can occur.
Some common types of processes are:
- Isobaric process : Occurs at constant pressure.
- Isochoric process : It is carried out at constant volume.
- Isothermal process : It takes place at constant temperature.
- Adiabatic process : There is no heat exchange with the surroundings.
The laws of thermodynamics and thermodynamic states
The laws of thermodynamics are fundamental principles that govern thermodynamic states and processes.
- Zeroth law of thermodynamics: It states that if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This means that there will be no net heat flow between these systems when they are in contact with each other, which implies that they all have the same temperature.
- First law of thermodynamics (conservation of energy): States that energy can neither be created nor destroyed, it can only be transformed from one form to another. In terms of a thermodynamic system, the change in internal energy is equal to the heat added minus the work done by the system.
- Second law of thermodynamics : Introduces the concept of entropy and states that in any spontaneous process, the total entropy of the system and its surroundings always increases. This means that irreversible processes tend to increase disorder.
- Third law of thermodynamics : It postulates that, as it approaches absolute zero, the entropy of a system approaches a minimum value, and in some cases, it can reach a zero value in perfectly ordered systems.
Examples of thermodynamic states:
Here are several examples of thermodynamic states for various systems:
Heat transfer fluid system in a solar collector
A heat transfer fluid, such as glycol, flows through a solar thermal collector, reaching a temperature of 120°C (393 K) under a pressure of 2 atm.
The fluid is used to transfer the thermal energy absorbed by the collectors to a heat exchanger to heat water or generate steam.
This state describes the thermal fluid heated by solar energy in a solar thermal energy system.
Solar heat storage tank
A thermal energy storage tank containing water heated to 90°C by solar thermal energy under a pressure of 1.5 atm.
This is a common state in solar heat storage systems, where the hot water is subsequently used for heating or to generate steam in a concentrating solar power (CSP) plant.
The relevant variables are the temperature, pressure and volume of water.
Solar powered steam system
Water vapor at 200°C (473 K) generated by a field of solar thermal collectors that concentrate solar radiation.
The steam is at 15 atm and is used to drive a turbine in a concentrated solar power (CSP) electricity generation system.
Here, solar radiation is the source of energy to increase the internal energy of the water, converting it into steam that performs work on the turbine.
Solar powered heat exchange system
A system where a fluid, such as thermal oil, flows through tubes that absorb solar radiation, reaching a temperature of 300°C and a pressure of 3 atm.
The heat from the fluid is then transferred to a heat exchange system to heat water and produce steam. This is a typical component of a solar thermal plant, where thermal oil carries the absorbed solar energy.
Refrigerant system in a refrigeration cycle
A refrigerant, such as R-134a , circulates in a vapor compression cycle, used in a refrigeration system. In one stage, the refrigerant is in a vapor state at a temperature of -10°C and under a pressure of 2 atm after having passed through the evaporator. This state describes the refrigerant in the process of absorbing heat from the environment, a common application in air conditioning systems and refrigerators.
Internal combustion engine
In the Otto cycle of an internal combustion engine, air and fuel are in a cylinder just after combustion, at a temperature of 1500 K, a pressure of 30 atm and a small volume (close to the minimum cylinder volume).
This is a key state in the engine operating cycle, where the air-fuel mixture reaches its peak temperature and pressure, which drives the piston.
Compressed air system in a factory
Compressed air stored in a tank at 298 K (25°C), under a pressure of 8 atm and with a volume of 50 liters.
Compressed air is used to power pneumatic tools or control systems in a factory. The state variables that describe compressed air are temperature, pressure and volume.
Solid in state of sublimation
A block of dry ice (solid carbon dioxide) exposed to ambient air at room temperature of 20°C (293 K) and atmospheric pressure of 1 atm.
In this state, dry ice sublimates, going directly from solid to gas, and its mass decreases with time. The state variables in this system include temperature, pressure, and mass of the solid/gas in this phase transition process.