🧂Physical Chemistry II Unit 8 – Advanced Thermodynamics

Key Concepts and Fundamentals

• Thermodynamics studies the interrelationships between heat, work, and energy in systems
• Macroscopic properties (temperature, pressure, volume) describe the state of a system
• Microscopic properties (molecular interactions, kinetic energy) influence macroscopic behavior
• Internal energy ($U$) represents the total energy of a system, including kinetic and potential energies
• Enthalpy ($H$) measures the heat content of a system at constant pressure ($H = U + PV$)
• Entropy ($S$) quantifies the disorder or randomness of a system and relates to the second law of thermodynamics
• Gibbs free energy ($G$) predicts the spontaneity of processes at constant temperature and pressure ($G = H - TS$)
  • Processes with $\Delta G < 0$ are spontaneous, while those with $\Delta G > 0$ are non-spontaneous

Laws of Thermodynamics Revisited

• The zeroth law establishes thermal equilibrium and the concept of temperature
  • If two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other
• The first law states that energy is conserved in a closed system ($\Delta U = Q + W$)
  • Heat ($Q$) and work ($W$) are forms of energy transfer between a system and its surroundings
• The second law introduces entropy and states that it always increases in a closed system
  • Heat flows spontaneously from hot to cold objects, never the reverse
• The third law states that the entropy of a perfect crystal approaches zero as temperature approaches absolute zero
• The laws of thermodynamics have wide-ranging applications in chemistry, physics, and engineering
  • Examples include heat engines, refrigeration cycles, and chemical reactions

Statistical Mechanics and Ensemble Theory

• Statistical mechanics bridges the gap between microscopic properties and macroscopic thermodynamic behavior
• Ensembles are large collections of microstates (specific configurations of a system) that share common macroscopic properties
• The microcanonical ensemble (NVE) describes isolated systems with constant number of particles ($N$), volume ($V$), and energy ($E$)
• The canonical ensemble (NVT) represents systems in thermal equilibrium with a heat bath at constant temperature ($T$)
• The grand canonical ensemble ($\mu$VT) allows for the exchange of particles and energy with a reservoir at constant chemical potential ($\mu$)
• The partition function ($Z$) is a key concept in statistical mechanics that relates microscopic properties to macroscopic thermodynamic quantities
  • For the canonical ensemble, $Z = \sum_i e^{-E_i/kT}$, where $E_i$ is the energy of microstate $i$, $k$ is the Boltzmann constant, and $T$ is the temperature
• Thermodynamic properties can be derived from the partition function, such as internal energy ($U = -\partial \ln Z/\partial \beta$) and entropy ($S = k \ln Z + kT \partial \ln Z/\partial T$), where $\beta = 1/kT$

Thermodynamic Potentials and Relations

• Thermodynamic potentials are state functions that characterize a system's energy and provide criteria for spontaneity and equilibrium
• The four main thermodynamic potentials are internal energy ($U$), enthalpy ($H$), Helmholtz free energy ($A$), and Gibbs free energy ($G$)
• Maxwell relations are derived from the equality of mixed partial derivatives of thermodynamic potentials
  • For example, $(\partial S/\partial V)_T = (\partial P/\partial T)_V$ is derived from the Helmholtz free energy
• Legendre transformations relate thermodynamic potentials through changes in natural variables
  • Enthalpy is a Legendre transform of internal energy: $H = U + PV$
  • Gibbs free energy is a Legendre transform of enthalpy: $G = H - TS$
• The Gibbs-Duhem equation relates changes in chemical potential to changes in temperature and pressure for a multi-component system
  • $SdT - VdP + \sum_i N_i d\mu_i = 0$, where $N_i$ is the number of particles of species $i$ and $\mu_i$ is its chemical potential

Phase Equilibria and Transitions

• Phase equilibria occur when two or more phases coexist at the same temperature, pressure, and chemical potential
• The Gibbs phase rule ($F = C - P + 2$) relates the number of degrees of freedom ($F$) to the number of components ($C$) and phases ($P$) in a system
• The Clapeyron equation describes the slope of a phase boundary in a pressure-temperature diagram
  • $dP/dT = \Delta H/T\Delta V$, where $\Delta H$ and $\Delta V$ are the enthalpy and volume changes associated with the phase transition
• The Clausius-Clapeyron equation is a special case for vapor-liquid equilibrium
  • $\ln(P_2/P_1) = -\Delta H_{vap}/R(1/T_2 - 1/T_1)$, where $P_1$ and $P_2$ are the vapor pressures at temperatures $T_1$ and $T_2$, and $\Delta H_{vap}$ is the enthalpy of vaporization
• Critical points occur at the end of a phase equilibrium curve, where the properties of the two phases become identical
  • The critical temperature ($T_c$), pressure ($P_c$), and volume ($V_c$) are important parameters for describing the behavior of fluids

Chemical Equilibrium in Complex Systems

• Chemical equilibrium is achieved when the forward and reverse reaction rates are equal, and the concentrations of reactants and products remain constant
• The equilibrium constant ($K$) is related to the Gibbs free energy change of a reaction ($\Delta G^\circ = -RT \ln K$)
  • For a reaction $aA + bB \rightleftharpoons cC + dD$, $K = [C]^c[D]^d/[A]^a[B]^b$, where the brackets denote equilibrium concentrations
• Le Chatelier's principle states that a system in equilibrium will shift to counteract any external stress applied to it
  • Examples include changes in concentration, pressure, temperature, or volume
• The van 't Hoff equation describes the temperature dependence of the equilibrium constant
  • $d \ln K/dT = \Delta H^\circ/RT^2$, where $\Delta H^\circ$ is the standard enthalpy change of the reaction
• Coupled reactions occur when the product of one reaction serves as a reactant for another, influencing the overall equilibrium
  • ATP hydrolysis is often coupled to energetically unfavorable reactions in biological systems to drive them forward

Non-Ideal Solutions and Mixtures

• Non-ideal solutions deviate from the behavior predicted by Raoult's law due to intermolecular interactions between components
• Activity coefficients ($\gamma_i$) account for non-ideality by relating the activity ($a_i$) of a component to its mole fraction ($x_i$)
  • $a_i = \gamma_i x_i$, where $\gamma_i = 1$ for an ideal solution
• The Gibbs-Duhem equation for non-ideal solutions relates changes in activity coefficients to changes in composition
  • $x_1 d \ln \gamma_1 + x_2 d \ln \gamma_2 = 0$ for a binary mixture
• Excess properties (e.g., excess Gibbs free energy, $G^E$) quantify the deviation of a mixture from ideal behavior
  • $G^E = RT(x_1 \ln \gamma_1 + x_2 \ln \gamma_2)$ for a binary mixture
• Models such as the Margules, van Laar, and Wilson equations describe the composition dependence of activity coefficients
• Azeotropes are mixtures that boil at a constant temperature and composition, forming a local maximum or minimum in the vapor-liquid equilibrium diagram
  • Examples include ethanol-water (95.6% ethanol) and hydrochloric acid-water (20.2% HCl)

Applications in Materials Science and Engineering

• Thermodynamics plays a crucial role in understanding the stability, properties, and processing of materials
• Phase diagrams depict the equilibrium phases present in a system as a function of composition, temperature, and pressure
  • Binary phase diagrams (e.g., Fe-C) are essential for designing alloys with desired properties
• Ellingham diagrams show the temperature dependence of the Gibbs free energy of formation for various oxides
  • Used to predict the stability of oxides and the feasibility of reduction reactions
• Defect chemistry examines the thermodynamics of point defects (vacancies, interstitials, substitutional atoms) in crystalline solids
  • Defect concentrations influence properties such as electrical conductivity and diffusion rates
• Interfacial thermodynamics governs the behavior of surfaces, grain boundaries, and heterogeneous interfaces
  • Surface energy and interfacial tension affect phenomena such as wetting, adhesion, and sintering
• Thermodynamic modeling and computational methods (e.g., CALPHAD) enable the prediction of phase equilibria and properties in complex multicomponent systems
  • Facilitates the design of advanced materials with tailored compositions and microstructures