Thermodynamics: Systems, Transformations and the First Law
Thermodynamic Systems and Transformations
Item 4: A thermodynamic system is any portion of the universe that is subject to study. The environment is the rest of the universe that is outside the system, although for merely practical purposes, the environment is the area near the system capable of interacting with it.
System Boundaries and Limits
The limits of a system can be:
- Real
- Imaginary
Classification of Thermodynamic Systems
Exchanges between the system and the environment can be:
- Open: Can exchange both matter and energy with the environment.
- Closed: Can exchange energy but not matter with the environment.
- Isolated: Cannot exchange matter or energy with the environment.
Phases and System Composition
A phase is a region where the composition and properties are homogeneous. Systems can be classified as:
- Homogeneous: If the system appears as a single phase.
- Heterogeneous: If the system consists of more than one phase.
State of a System and State Functions
To know the state of a system is to know its composition, location, and energy at a determined moment. Thermodynamic variables are called state functions when their value depends only on the current state of the system and not on how it reached that state.
The Three Conditions for Equilibrium
A system is in equilibrium when three conditions are met simultaneously:
- Mechanical Equilibrium: No macroscopic movement is observable within the system.
- Thermal Equilibrium: The temperature is equal at all points of the system.
- Chemical Equilibrium: Its composition does not vary with time.
Thermodynamic Transformations
A transformation or change is any variation of the thermodynamic state of a system. These involve an initial state and a subsequent final state. Transformations can be reversible or irreversible.
Types of Thermodynamic Processes
- Isochoric: No variation in volume.
- Isobaric: No variation in pressure.
- Adiabatic: No heat is transferred between the system and the environment.
- Isothermal: The temperature of the system does not vary.
The First Law of Thermodynamics
The First Law of Thermodynamics ensures that in an isolated state, total energy is conserved. There are two types of energy transfer:
- Heat (Q)
- Work (W): Refers to energy transfers other than heat.
The total internal energy variation (ΔU) of a system is equal to the sum of the heat and work exchanges between the system and the environment.
Internal Energy and Enthalpy
Internal Energy (U) is the total energy of the components of a system, such as rotating molecules and atomic vibrations. It is represented by the function U and is incalculable for a specific determined state; we only measure its changes.
Enthalpy (H) is a state function like internal energy, and we can only know its variations. It is defined by the formula:
H = U + pV
Therefore, the variation is: ΔH = ΔU + PΔV + VΔP