Chemical Engineering Thermodynamics
Core concepts
Thermodynamics underpins every separation, reaction and energy balance in the exam. You must be fluent with the first and second laws for open systems, the definitions of enthalpy, entropy and Gibbs and Helmholtz free energies, and the criteria for equilibrium (constant T,P gives minimum G). Know the ideal-gas relations, departure functions for real gases, and equations of state (van der Waals, Redlich–Kwong, Peng–Robinson) including how to evaluate fugacity and fugacity coefficient. For mixtures, master partial molar properties, chemical potential, activity coefficients, and the excess Gibbs energy models (Margules, van Laar, Wilson, NRTL, UNIQUAC). Vapour–liquid equilibrium via modified Raoult's law and the azeotrope condition (equal composition in both phases) is a perennial topic. Residual and excess properties, the Gibbs–Duhem equation, and consistency tests appear in numericals.
Traps
Watch units (bar vs Pa, kJ vs J), and never apply ideal-gas formulas to high-pressure or strongly non-ideal systems. Distinguish standard-state definitions (Lewis–Randall vs Henry's law). In VLE problems, check whether an azeotrope exists before assuming simple distillation. Remember Cp and Cv differ, and that ΔH and ΔU differ by Δ(PV).
Worked rule of thumb
For an ideal binary system at 1 atm, the bubble point satisfies 1 = x1·Psat1/P + x2·Psat2/P. If both pure vapour pressures bracket the total pressure, a bubble point exists between the two saturation temperatures; solve by trial on T. A shortcut: use Antoine constants and iterate once or twice with linear interpolation between the two Tsat values.
Exam strategy
Set up the material and energy balances symbolically before plugging numbers; most marks come from correct formulation.