Chemistry > Physical Chemistry > Thermodynamics

Thermodynamics is a specialized subfield within physical chemistry that focuses on the relationships between heat, work, temperature, and energy. It provides a macroscopic description of systems and is integral for understanding the physical and chemical transformations that substances undergo. The term “thermodynamics” itself is derived from the Greek words “thermo” (heat) and “dynamics” (force), indicating its central concern with energy transformations.

Fundamental Concepts:

1. System and Surroundings:
   • A system refers to the part of the universe that is being studied, while the surroundings encompass everything outside of the system. Systems can be open, closed, or isolated based on their ability to exchange energy or matter with the surroundings.
2. State Functions:
   • State functions are properties that depend only on the current state of the system, not on the path taken to reach that state. Key state functions include internal energy (\(U\)), enthalpy (\(H\)), entropy (\(S\)), and Gibbs free energy (\(G\)).

Laws of Thermodynamics:

1. Zeroth Law:
   • Establishes the concept of temperature. It states that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
2. First Law (Law of Energy Conservation): \[ \Delta U = Q - W \]
   • This law posits that the change in internal energy (\( \Delta U \)) of a closed system is equal to the heat added to the system (\( Q \)) minus the work done by the system (\( W \)). It emphasizes that energy can be transformed but not created or destroyed.
3. Second Law: \[ \Delta S_{\text{universe}} \geq 0 \]
   • States that the entropy of an isolated system will tend to increase over time, approaching a maximum value at equilibrium. This law introduces the concept of entropy (\( S \)), a measure of randomness or disorder. Processes that increase the total entropy are spontaneous.
4. Third Law:
   • As the temperature of a system approaches absolute zero (0 Kelvin), the entropy of a perfect crystal approaches zero. This provides a reference point for the measurement of entropy.

Key Equations in Thermodynamics:

1. Enthalpy (H): \[ H = U + PV \]
   • Enthalpy is used to measure the heat content of a system at constant pressure.
2. Gibbs Free Energy (G): \[ G = H - TS \]
   • Gibbs free energy is essential for predicting the spontaneity of processes at constant temperature and pressure. A negative \( \Delta G \) indicates a spontaneous process.
3. Helmholtz Free Energy (A): \[ A = U - TS \]
   • Helmholtz free energy is useful for systems at constant volume, relating to the work obtainable from a thermodynamic system.

Applications:

• Chemical Reactions: Thermodynamics helps predict reaction feasibility, equilibrium positions, and energy changes.
• Biological Systems: Understanding the energetics of biochemical reactions, cellular processes, and metabolic pathways.
• Engine and Refrigeration Cycles: Designing efficient heat engines, refrigerators, and understanding phase transitions.
• Material Science: Studying phase diagrams, alloy formation, and the thermodynamic stability of materials.

Thermodynamics forms a foundational basis for many modern scientific and engineering disciplines, allowing us to quantify and predict the behavior of matter and energy in diverse systems.