2.7 Thermodynamic potentials

Thermodynamic potentials are key functions in statistical mechanics that link microscopic particle behavior to macroscopic system properties. They allow us to calculate various thermodynamic quantities and predict how systems behave under different constraints.

These potentials include internal energy, enthalpy, Helmholtz free energy, and Gibbs free energy. Each has unique natural variables and applications, from determining equilibrium conditions to predicting spontaneous processes and phase transitions.

Fundamental thermodynamic potentials

• Thermodynamic potentials serve as cornerstone functions in statistical mechanics, providing a bridge between microscopic particle behavior and macroscopic system properties
• These potentials enable the calculation of various thermodynamic quantities and prediction of system behavior under different constraints
• Understanding thermodynamic potentials is crucial for analyzing complex systems in equilibrium and non-equilibrium states

Internal energy

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• Represents the total energy contained within a thermodynamic system
• Denoted by U, includes kinetic and potential energies of all particles
• Fundamental to the First Law of Thermodynamics: $dU = δQ - δW$
• Depends on extensive variables (entropy, volume, particle number)
• Natural variables: entropy (S) and volume (V)

Enthalpy

• Defined as H = U + PV, where P is pressure and V is volume
• Measures the total heat content of a system
• Particularly useful for processes occurring at constant pressure (isobaric)
• Changes in enthalpy (ΔH) indicate heat absorbed or released during chemical reactions
• Natural variables: entropy (S) and pressure (P)

Helmholtz free energy

• Expressed as A = U - TS, where T is temperature and S is entropy
• Represents the useful work obtainable from a closed system at constant temperature
• Minimized at equilibrium for systems at constant temperature and volume
• Used to determine the direction of spontaneous processes in isothermal, isochoric conditions
• Natural variables: temperature (T) and volume (V)

Gibbs free energy

• Defined as G = H - TS or G = U + PV - TS
• Measures the maximum reversible work that can be extracted from a system
• Minimized at equilibrium for systems at constant temperature and pressure
• Crucial for determining chemical equilibrium and spontaneity of reactions
• Natural variables: temperature (T) and pressure (P)

Mathematical formulations

Legendre transformations

• Mathematical technique used to switch between thermodynamic potentials
• Allows conversion between conjugate variables (extensive and intensive)
• Preserves information content while changing the independent variables
• General form: $F(x,y) = f(x,y) - y \frac{\partial f}{\partial y}$
• Enables derivation of new potentials from existing ones (U to H, A to G)

Partial derivatives relationships

• Connect different thermodynamic quantities through mathematical relations
• Fundamental equation: $dU = TdS - PdV + \mu dN$
• Derived relationships:
  • $(\frac{\partial U}{\partial S})_{V,N} = T$
  • $(\frac{\partial U}{\partial V})_{S,N} = -P$
  • $(\frac{\partial U}{\partial N})_{S,V} = \mu$
• Enable calculation of one quantity from measurements of others

Maxwell relations

• Set of equalities between mixed partial derivatives of thermodynamic potentials
• Derived from the symmetry of second derivatives of thermodynamic potentials
• Four fundamental Maxwell relations:
  • $(\frac{\partial T}{\partial V})_S = -(\frac{\partial P}{\partial S})_V$
  • $(\frac{\partial T}{\partial P})_S = (\frac{\partial V}{\partial S})_P$
  • $(\frac{\partial S}{\partial V})_T = (\frac{\partial P}{\partial T})_V$
  • $(\frac{\partial S}{\partial P})_T = -(\frac{\partial V}{\partial T})_P$
• Simplify complex thermodynamic calculations and relate different measurable quantities

Properties and applications

Equilibrium conditions

• Thermodynamic equilibrium occurs when a system's properties remain constant over time
• Characterized by the minimization or maximization of appropriate thermodynamic potentials
• Conditions for different types of equilibrium:
  • Thermal equilibrium: uniform temperature throughout the system
  • Mechanical equilibrium: uniform pressure throughout the system
  • Chemical equilibrium: no net chemical reactions or mass transfer
• Equilibrium states correspond to extrema of thermodynamic potentials under specific constraints

Spontaneity criteria

• Determine the direction of natural processes in thermodynamic systems
• Based on the Second Law of Thermodynamics and the concept of entropy increase
• Criteria for spontaneous processes:
  • Constant T and V: dA < 0 (decrease in Helmholtz free energy)
  • Constant T and P: dG < 0 (decrease in Gibbs free energy)
  • Isolated system: dS > 0 (increase in entropy)
• Reversible processes occur when the equality holds in these criteria

Phase transitions

• Transformations between different states of matter or phases within a substance
• Classified as first-order (discontinuous changes in first derivatives of thermodynamic potentials) or second-order (continuous first derivatives, discontinuous second derivatives)
• Characterized by changes in order parameters and symmetry
• Gibbs phase rule determines the number of degrees of freedom in a system: $F = C - P + 2$
• Critical points mark the end of phase coexistence lines and exhibit unique behavior

Thermodynamic potential diagrams

Energy surfaces

• Graphical representations of thermodynamic potentials as functions of their natural variables
• Provide visual insights into system behavior and stability
• Common representations:
  • U(S,V) surface for internal energy
  • G(T,P) surface for Gibbs free energy
• Allow identification of equilibrium states, phase transitions, and critical points
• Curvature of surfaces indicates system stability and susceptibility to fluctuations

Stability conditions

• Determine whether a system will maintain its current state or undergo spontaneous changes
• Derived from the second derivatives of thermodynamic potentials
• General stability criteria:
  • $(\frac{\partial^2 U}{\partial S^2})_V > 0$ (thermal stability)
  • $(\frac{\partial^2 U}{\partial V^2})_S > 0$ (mechanical stability)
• Stable states correspond to local minima in the appropriate thermodynamic potential
• Instabilities lead to phase transitions or spontaneous processes

Critical points

• Special states where distinctions between phases disappear
• Characterized by the vanishing of first and higher-order derivatives of thermodynamic potentials
• Exhibit unique physical properties:
  • Divergence of susceptibilities and compressibility
  • Long-range correlations and fluctuations
  • Universal behavior described by critical exponents
• Examples include the liquid-gas critical point and the Curie point in ferromagnets

Connections to statistical mechanics

Partition function relationships

• Partition functions (Z) provide a statistical description of thermodynamic systems
• Connect microscopic states to macroscopic thermodynamic potentials
• Key relationships:
  • Helmholtz free energy: $A = -kT \ln Z$
  • Internal energy: $U = kT^2 (\frac{\partial \ln Z}{\partial T})_V$
  • Entropy: $S = k \ln Z + kT (\frac{\partial \ln Z}{\partial T})_V$
• Enable calculation of thermodynamic quantities from statistical mechanical principles

Ensemble averages

• Statistical mechanics uses ensembles to represent all possible microstates of a system
• Ensemble averages correspond to macroscopic thermodynamic quantities
• Different ensembles relate to different thermodynamic potentials:
  • Microcanonical ensemble (NVE) → Internal energy
  • Canonical ensemble (NVT) → Helmholtz free energy
  • Grand canonical ensemble (μVT) → Grand potential
• Ergodic hypothesis connects time averages to ensemble averages in equilibrium systems

Fluctuations and response functions

• Thermodynamic fluctuations arise from microscopic variations in system properties
• Magnitude of fluctuations related to system size and thermodynamic derivatives
• Fluctuation-dissipation theorem connects equilibrium fluctuations to non-equilibrium response
• Important response functions:
  • Heat capacity: $C_V = T(\frac{\partial S}{\partial T})_V$
  • Isothermal compressibility: $\kappa_T = -\frac{1}{V}(\frac{\partial V}{\partial P})_T$
• Fluctuations become particularly important near critical points and in small systems

Experimental relevance

Measurable quantities

• Thermodynamic potentials are not directly measurable but can be inferred from observable quantities
• Commonly measured properties:
  • Temperature (T)
  • Pressure (P)
  • Volume (V)
  • Heat capacity (C)
  • Compressibility (κ)
• Combinations of these measurements allow calculation of thermodynamic potentials and their derivatives
• Experimental techniques often focus on measuring changes in potentials rather than absolute values

Calorimetry techniques

• Methods for measuring heat transfer and energy changes in chemical and physical processes
• Types of calorimeters:
  • Bomb calorimeter: measures heat of combustion at constant volume
  • Flow calorimeter: measures enthalpy changes in flowing systems
  • Differential scanning calorimeter (DSC): measures heat capacity and phase transitions
• Calorimetry data provide direct information about enthalpy changes and heat capacities
• Enable determination of reaction enthalpies, phase transition energetics, and thermodynamic parameters

Equation of state derivations

• Equations of state relate thermodynamic variables (P, V, T) for a given substance
• Derived from experimental data or theoretical models
• Common equations of state:
  • Ideal gas law: $PV = nRT$
  • Van der Waals equation: $(P + \frac{an^2}{V^2})(V - nb) = nRT$
• Provide a bridge between microscopic interactions and macroscopic behavior
• Allow calculation of thermodynamic potentials and their derivatives for real systems

Advanced concepts

Massieu functions

• Alternative thermodynamic potentials introduced by Henri Louis Le Châtelier
• Defined as the negative of free energies divided by temperature
• Examples:
  • $Y = -A/T = S - U/T$ (Massieu function)
  • $X = -G/T = S - (U + PV)/T$ (Planck potential)
• Simplify certain thermodynamic calculations and relationships
• Particularly useful in statistical mechanics for connecting partition functions to thermodynamics

Grand potential

• Thermodynamic potential for systems with variable particle number
• Defined as Ω = U - TS - μN, where μ is the chemical potential
• Natural variables: temperature (T), volume (V), and chemical potential (μ)
• Minimized at equilibrium for systems at constant T, V, and μ
• Particularly important in the grand canonical ensemble of statistical mechanics
• Relates to the pressure in homogeneous systems: $\Omega = -PV$

Landau theory of phase transitions

• Phenomenological approach to describing phase transitions and critical phenomena
• Based on the expansion of free energy in terms of an order parameter
• General form of Landau free energy: $F = F_0 + a\phi^2 + b\phi^4 + ...$
• Predicts critical exponents and universality classes for continuous phase transitions
• Applicable to a wide range of systems (ferromagnets, superconductors, liquid crystals)
• Provides a bridge between microscopic theories and macroscopic behavior near critical points