In thermodynamics, the study of the properties of a system allows us to understand how energy, work, and heat interact within that system.
A thermodynamic property is any characteristic of a system that can be measured or calculated and that describes its state. Properties provide insight into how a system may experience changes in energy, work, and heat during a process. These properties are divided into two main types: intensive properties , which do not depend on the amount of matter, and extensive properties , which do depend on the amount of matter in the system.
Intensive thermodynamic properties
Intensive properties are those that do not depend on the amount of matter present in the system. This means that if a system is divided into smaller parts, the value of an intensive property remains the same in each part. Intensive properties describe the intrinsic nature of a substance or system, regardless of the size or amount of material.
A classic example to illustrate this is temperature . If you have a litre of water at 25 °C and you divide it into two half-litre containers, the temperature will still be 25 °C in both containers. Similarly, other intensive properties such as pressure, density and specific volume are not affected by the size of the system.
Examples of intensive properties
- Density : Density is the relationship between the mass of a substance and the volume it occupies. Its formula is:
Density = Mass / Volume
It does not matter if it is a small sample or a large volume of substance, the density remains constant, as long as the temperature and pressure do not change. - Specific volume : Specific volume is the volume occupied by a unit of mass of a substance. It is the inverse of density:
Specific volume = 1 / Density
It is another intensive property, since it does not depend on the total amount of substance present, but on how the volume is distributed in relation to the mass. - Pressure : Pressure is the force exerted by a system per unit area on its boundaries. For example, in a confined gas, the pressure does not change when dividing the volume, provided that the conditions of temperature and quantity of gas are not altered.
- Temperature : Temperature is a measure of the thermal state of a substance and is related to the average kinetic energy of the particles in the system. It is an intensive property because its value is independent of the size or amount of the substance.
- Composition : The chemical composition of a substance, such as its concentration or the proportion of components in a mixture, is another intensive property. For example, the salinity of a salt solution remains the same no matter how much of the solution there is.
Importance of intensive properties
Intensive properties play a crucial role in the identification and characterization of materials and systems, since they are independent of the size or amount of matter.
This is especially useful in industrial and scientific processes, where a small sample of material can be analyzed to obtain information applicable to a larger quantity.
Extensive thermodynamic properties
Extensive properties , on the other hand, depend directly on the amount of matter present in the system.
If a system is divided into two parts, the value of an extensive property is also divided between those two parts. For example, the volume of a system is an extensive property: if you have a volume of 1 m³ and you divide it into two equal parts, each part will have a volume of 0.5 m³.
Extensive properties add up in a system made up of subsystems. For example, if you have two subsystems with different masses, the total mass of the system will be the sum of the masses of the subsystems.
Examples of extensive properties
- Mass : Mass is a measure of the amount of matter in a system. Clearly, it depends on the size of the system. If you divide a system into parts, the mass of each part will be proportional to the size of the part.
- Volume : Volume is the space occupied by a substance. Like mass, it is an extensive property because the volume of a system is the sum of the volume of all its parts.
- Internal Energy : Internal energy is the sum of all microscopic energies in a system, including the kinetic and potential energies of the molecules that compose it. It is an extensive property, since it depends on the total amount of matter in the system.
- Enthalpy : Enthalpy is a measure of the total energy of a system, including internal energy and the energy required to displace its surroundings at constant pressure. It is extensive because it depends on the amount of matter in the system.
- Entropy : Entropy is a measure of disorder or the amount of energy that cannot be converted to work in a system. It is an extensive property because the greater the amount of matter, the greater the disorder and total entropy of the system.
Relationship between extensive and intensive properties
An extensive property can be converted into an intensive property when it is expressed in terms of a unit of mass, volume, or moles. For example:
- Density : It is obtained by dividing the mass (extensive) by the volume (extensive), which results in an intensive property.
- Specific volume : It is obtained by dividing the volume (extensive) by the mass (extensive), which generates an intensive property.
This type of transformation is useful for normalizing properties and making comparisons between systems of different sizes.
Relationship between properties and state equations
The relationships between the thermodynamic properties of a system are determined by the equations of state .
An equation of state is a mathematical relationship that connects several intensive and extensive properties of a system, allowing the behavior of the system to be predicted under different conditions.
The most commonly known equation of state is the ideal gas equation , which relates the pressure (P), volume (V), and temperature (T) of an ideal gas to the amount of substance (n) by means of the ideal gas constant (R):
P·V=n·R·T
In more complex systems, such as liquids and solids, more elaborate equations of state are needed to relate thermodynamic properties.
Thermodynamic variables
Thermodynamic variables are physical quantities that describe the state of a system in equilibrium.
Like properties, thermodynamic variables can be intensive or extensive. They are also called state functions , since they depend only on the current state of the system and not on how that state was reached.
State functions
State functions are those quantities whose value depends exclusively on the current state of the system, regardless of the path followed to reach that state. This means that it does not matter how a certain temperature or pressure was reached, what is important is the final value of the variable, not the intermediate steps.
Examples of state functions include internal energy, enthalpy, and entropy. These functions are essential for performing energy and efficiency analysis on thermodynamic systems.