Θερμοδυναμική: Βασικές Έννοιες και Νόμοι

Fundamental Concepts

• Thermodynamics studies energy transformations, focusing on macroscopic system properties, not microscopic details.
• Key concepts include heat, work, temperature, internal energy, enthalpy, entropy, and free energy.
• Thermodynamics laws govern energy behaviour in processes.

The Laws of Thermodynamics

• First Law: Energy is conserved, only transformed. Change in internal energy (ΔU) equals heat added (Q) minus work done (W): ΔU = Q – W.
• Second Law: Isolated system entropy always increases over time. Spontaneous processes increase entropy in isolated systems.
• Third Law: Perfect crystal entropy is zero at absolute zero temperature.

Systems and Surroundings

• System: The portion of the universe under study.
• Surroundings: The rest of the universe interacting with the system.
• Open system: Exchanges both matter and energy with surroundings.
• Closed system: Exchanges energy but not matter with surroundings.
• Isolated system: Exchanges neither matter nor energy with surroundings.

Processes

• Isothermal: Constant temperature.
• Adiabatic: No heat exchange with surroundings.
• Isobaric: Constant pressure.
• Isochoric: Constant volume.

Thermodynamic Properties

• Internal energy (U): Total energy within a system.
• Enthalpy (H): Thermodynamic potential useful at constant pressure: H = U + PV.
• Entropy (S): Measure of disorder or randomness in a system; higher entropy means more disorder.
• Gibbs free energy (G): Maximum reversible work a system can perform at constant temperature and pressure: G = H – TS.

Cycles

• Carnot cycle: Theoretical thermodynamic cycle, describing maximum heat engine efficiency between two heat reservoirs; consists of two isothermal and two adiabatic processes.

Applications

• Heat engines: Convert heat energy to mechanical work (e.g., internal combustion engines, steam turbines).
• Heat pumps: Reverse heat engine direction, moving heat from cold to hot (e.g., refrigerators, air conditioners).
• Chemical reactions: Thermodynamics predicts spontaneous reaction under given conditions.
• Phase transitions: Thermodynamics explains phase changes (solid, liquid, gas) of substances.
• Engineering designs: Thermodynamic principles are vital in efficient, reliable power plant and engine designs.

Key Equations

• First Law: ΔU = Q – W
• Enthalpy: H = U + PV
• Gibbs free energy: G = H – TS

State Functions

• Internal energy (U): Depends only on current system state, not path.
• Enthalpy (H): Also a state function.
• Entropy (S): A state function.
• Gibbs free energy (G): A state function.

Equilibrium

• Systems tend towards equilibrium with no net macroscopic changes.
• Chemical equilibrium: Forward and reverse reaction rates are equal.

Spontaneous Processes

• Spontaneous processes occur without external intervention.
• Predicting spontaneity involves Gibbs free energy and the Second Law of thermodynamics.