🔥Thermodynamics I Unit 12 – Thermodynamic Property Relations

Key Concepts and Definitions

• Thermodynamic properties describe the state of a system and include pressure ($P$), volume ($V$), temperature ($T$), internal energy ($U$), enthalpy ($H$), and entropy ($S$)
• Extensive properties depend on the size of the system (mass, volume, energy) while intensive properties are independent of size (temperature, pressure, density)
• State functions are properties that depend only on the current state of the system, not the path taken to reach that state
  • Examples of state functions include pressure, temperature, volume, internal energy, enthalpy, and entropy
• Process functions depend on the path taken between two states (work and heat)
• Equilibrium is a state where the system's properties remain constant over time and there are no net flows of matter or energy
  • Types of equilibrium include thermal (no heat flow), mechanical (no change in pressure), and chemical (no change in composition)
• Quasi-static processes occur slowly enough that the system remains infinitesimally close to equilibrium at all times
• Reversible processes can be reversed without any net change to the system or surroundings, while irreversible processes cannot be reversed without external intervention

Fundamental Thermodynamic Relations

• The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another
  • Mathematically, $\Delta U = Q + W$, where $\Delta U$ is the change in internal energy, $Q$ is heat added to the system, and $W$ is work done by the system
• The second law of thermodynamics states that the entropy of an isolated system always increases over time
  • This law introduces the concept of irreversibility and explains why certain processes, such as heat transfer from cold to hot objects, do not occur spontaneously
• The fundamental thermodynamic relation combines the first and second laws: $dU = TdS - PdV$
  • This relation connects changes in internal energy ($dU$) with changes in entropy ($dS$) and volume ($dV$)
• The Gibbs free energy ($G$) is another important thermodynamic potential, defined as $G = H - TS$
  • Changes in Gibbs free energy determine the spontaneity of processes at constant temperature and pressure
• Enthalpy ($H$) is defined as $H = U + PV$ and represents the total heat content of a system
• Helmholtz free energy ($A$) is defined as $A = U - TS$ and is useful for analyzing processes at constant temperature and volume

Equations of State

• An equation of state is a mathematical relationship between state variables (pressure, volume, and temperature) that describes the behavior of a substance
• The ideal gas law, $PV = nRT$, is a simple equation of state that assumes molecules have negligible size and do not interact with each other
  • $P$ is pressure, $V$ is volume, $n$ is the number of moles, $R$ is the universal gas constant, and $T$ is temperature
• Real gases deviate from ideal behavior, especially at high pressures and low temperatures
  • The van der Waals equation, $\left(P + \frac{a}{V^2}\right)\left(V - b\right) = RT$, accounts for molecular size ($b$) and intermolecular attractions ($a$)
• Other equations of state, such as the Redlich-Kwong and Peng-Robinson equations, provide more accurate descriptions of real gas behavior
• Equations of state can also be developed for liquids and solids, although they are generally more complex due to the stronger intermolecular interactions in these phases
• The virial equation of state, $\frac{PV}{RT} = 1 + \frac{B}{V} + \frac{C}{V^2} + ...$, expresses the compressibility factor as a power series in inverse volume
  • The coefficients $B$, $C$, etc., are called virial coefficients and depend on temperature and the specific substance

Maxwell Relations

• Maxwell relations are a set of equations that relate the partial derivatives of thermodynamic potentials (internal energy, enthalpy, Helmholtz free energy, and Gibbs free energy) with respect to different variables
• These relations are derived from the fundamental thermodynamic relation and the fact that the mixed second partial derivatives of a smooth function are equal
• The four Maxwell relations are:
  • $\left(\frac{\partial S}{\partial V}\right)_T = \left(\frac{\partial P}{\partial T}\right)_V$
  • $\left(\frac{\partial S}{\partial P}\right)_T = -\left(\frac{\partial V}{\partial T}\right)_P$
  • $\left(\frac{\partial T}{\partial V}\right)_S = -\left(\frac{\partial P}{\partial S}\right)_V$
  • $\left(\frac{\partial T}{\partial P}\right)_S = \left(\frac{\partial V}{\partial S}\right)_P$
• Maxwell relations are useful for deriving other thermodynamic relationships and calculating properties that may be difficult to measure directly
• For example, the relation $\left(\frac{\partial S}{\partial P}\right)_T = -\left(\frac{\partial V}{\partial T}\right)_P$ can be used to calculate the change in entropy with pressure from measurements of volume as a function of temperature at constant pressure

Thermodynamic Potentials

• Thermodynamic potentials are state functions that provide a complete description of a system's thermodynamic state
• The four primary thermodynamic potentials are internal energy ($U$), enthalpy ($H$), Helmholtz free energy ($A$), and Gibbs free energy ($G$)
• Internal energy is the total kinetic and potential energy of a system's particles
  • $dU = TdS - PdV$ (fundamental thermodynamic relation)
• Enthalpy is the sum of internal energy and the product of pressure and volume
  • $H = U + PV$, $dH = TdS + VdP$
  • Useful for processes at constant pressure
• Helmholtz free energy is the maximum amount of work a system can perform at constant temperature and volume
  • $A = U - TS$, $dA = -SdT - PdV$
  • Useful for processes at constant temperature and volume
• Gibbs free energy is the maximum amount of non-expansion work a system can perform at constant temperature and pressure
  • $G = H - TS$, $dG = -SdT + VdP$
  • Useful for processes at constant temperature and pressure
• The choice of thermodynamic potential depends on the constraints of the problem (constant temperature, pressure, volume, etc.)

Partial Derivatives and Their Applications

• Partial derivatives are used to describe how thermodynamic properties change with respect to specific variables while holding other variables constant
• Examples of partial derivatives in thermodynamics include:
  • $\left(\frac{\partial U}{\partial S}\right)_V = T$ (temperature)
  • $\left(\frac{\partial U}{\partial V}\right)_S = -P$ (pressure)
  • $\left(\frac{\partial H}{\partial S}\right)_P = T$ (temperature)
  • $\left(\frac{\partial H}{\partial P}\right)_S = V$ (volume)
• Partial derivatives can be used to derive relationships between thermodynamic properties, such as heat capacities and compressibilities
• Heat capacity at constant volume: $C_V = \left(\frac{\partial U}{\partial T}\right)_V = T\left(\frac{\partial S}{\partial T}\right)_V$
• Heat capacity at constant pressure: $C_P = \left(\frac{\partial H}{\partial T}\right)_P = T\left(\frac{\partial S}{\partial T}\right)_P$
• Isothermal compressibility: $\kappa_T = -\frac{1}{V}\left(\frac{\partial V}{\partial P}\right)_T$
• Adiabatic compressibility: $\kappa_S = -\frac{1}{V}\left(\frac{\partial V}{\partial P}\right)_S$
• The Jacobian method can be used to transform partial derivatives between different sets of variables, which is useful when applying Maxwell relations

Practical Applications and Examples

• Thermodynamic property relations are essential for designing and optimizing various engineering systems, such as power plants, refrigeration cycles, and chemical processes
• In a Rankine cycle (used in steam power plants), the efficiency depends on the thermodynamic properties of water and steam at different stages of the cycle
  • Equations of state and partial derivatives are used to calculate the work output and heat transfer in each component (boiler, turbine, condenser, and pump)
• Refrigeration cycles, such as the vapor-compression cycle, rely on the thermodynamic properties of refrigerants to transfer heat from a low-temperature reservoir to a high-temperature reservoir
  • Maxwell relations and partial derivatives are used to analyze the performance of the compressor, condenser, expansion valve, and evaporator
• Chemical processes, such as the production of ammonia (Haber-Bosch process), involve reactions at high temperatures and pressures
  • Thermodynamic potentials, particularly Gibbs free energy, are used to determine the equilibrium composition and optimize reaction conditions
• In meteorology, thermodynamic property relations are used to understand atmospheric processes, such as the formation of clouds, precipitation, and weather systems
  • The Clausius-Clapeyron equation, derived from Maxwell relations, relates the vapor pressure of a substance to its temperature and latent heat of vaporization
• Thermodynamic property relations are also crucial in the development of new materials, such as high-temperature superconductors and advanced alloys for aerospace applications
  • The stability and performance of these materials depend on their thermodynamic properties, which can be analyzed using equations of state and partial derivatives

Common Pitfalls and Tips

• Remember that partial derivatives are taken while holding other variables constant, which is indicated by subscripts (e.g., $\left(\frac{\partial U}{\partial S}\right)_V$)
• Be careful when using equations of state, as they have limitations and may not accurately describe the behavior of a substance under all conditions
  • Always consider the assumptions behind an equation of state and whether they are valid for the specific problem
• When applying Maxwell relations, make sure to use the correct sign and order of the partial derivatives
  • A common mistake is to forget the negative sign in relations like $\left(\frac{\partial S}{\partial P}\right)_T = -\left(\frac{\partial V}{\partial T}\right)_P$
• Pay attention to the units of thermodynamic properties and constants, as inconsistent units can lead to errors in calculations
  • Use a consistent set of units (e.g., SI units) and convert values as needed
• When solving problems involving thermodynamic cycles (e.g., Rankine or refrigeration cycles), break the process down into steps and analyze each component separately
  • Use the appropriate thermodynamic potential and equation of state for each step, depending on the constraints (constant temperature, pressure, volume, etc.)
• Remember that real processes are irreversible and always involve some degree of entropy generation
  • While ideal, reversible processes are useful for setting theoretical limits, actual systems will have lower efficiencies due to irreversibilities
• Consult property tables, charts, and software tools to find accurate values for thermodynamic properties, especially for complex substances like refrigerants and mixtures
  • Interpolation and curve-fitting techniques may be necessary when working with tabulated data