állapotfüggvényeknek

A state function, known in Hungarian as állapotfüggvény, refers to a thermodynamic property whose value depends solely on the current state of a system, regardless of the path taken to reach that state. Common examples include internal energy, enthalpy, entropy, Gibbs free energy, and Helmholtz free energy. Because their values are determined only by the system’s state variables such as temperature, pressure, volume, and composition, state functions provide a convenient way to describe equilibrium conditions and energy exchanges.

State functions are fundamental to the formulation of the first and second laws of thermodynamics. The first law, the conservation of energy, can be expressed in terms of the differential change in internal energy (ΔU = Q − W), where Q and W are path-dependent heat and work quantities, respectively, but the internal energy U itself is a state function. Similarly, the second law introduces entropy (S) as a state function, leading to inequalities that distinguish reversible from irreversible processes (ΔS_total ≥ 0 for real processes). The Gibbs free energy (G = H − TS) and Helmholtz free energy (F = U − TS) are also state functions that provide criteria for spontaneity and equilibrium at constant temperature and pressure or temperature and volume, respectively.

Mathematically, state functions are described by potential functions that satisfy the property of exact differentials. For example, the differential of internal energy is expressed as dU = TdS − PdV, where the right‑hand side is an exact differential because the partial derivatives satisfy the integrability conditions. This feature allows integration along any path between two states to determine the change in the state function without needing to specify the route.

In practical applications, state functions enable the calculation of thermodynamic cycle efficiencies, determine phase equilibria, and support engineering design and analysis. Their path‑independent nature simplifies complex thermodynamic calculations, making them indispensable tools in both theoretical studies and industrial processes.