Thermodynamics and Phase Equilibria
Thermodynamics underpins separation, reaction and energy analysis. Key concepts: the first and second laws; enthalpy, entropy and Gibbs free energy; ideal gas and real gas behavior (compressibility factor, virial and cubic equations of state); departure functions; fugacity and activity coefficients; Raoult's law and modified Raoult's law; VLE (bubble point, dew point, flash calculations); Henry's law for dilute solutes; and phase diagrams (T-x-y, P-x-y, azeotropes).
Traps: assuming ideal behavior when activity coefficients are far from one; confusing bubble point (first vapor) with dew point (first liquid); forgetting that an azeotrope cannot be separated by simple distillation; and misapplying the Clausius-Clapeyron equation without consistent units. Watch for questions where the answer depends on whether the system is at equilibrium - only at equilibrium are fugacities of each component equal in all phases.
Worked example (rule of thumb): for a binary ideal mixture at bubble point, y_i = x_i·P_sat_i/P, and Σy_i = 1. If Σx_i·P_sat_i/P > 1, the mixture is above its bubble point (superheated vapor region). For non-ideal systems, replace x_i with activity a_i = γ_i·x_i. Always verify Σy = 1 as a consistency check.