Chemical FE Exam: Common Mistakes

Before writing any balance equation, draw a clear boundary around your system and label every stream crossing it. Identify all species, their flow rates, and compositions. An ambiguous system boundary leads to missing terms in your balance equations and incorrect answers.

Use Cp for open systems and constant-pressure processes. Use Cv for closed systems at constant volume. For ideal gases, Cp = Cv + R. Using the wrong heat capacity changes your energy balance results significantly, especially for gas-phase calculations where the difference between Cp and Cv matters most.

Ideal gas behavior breaks down at high pressures, low temperatures, and near the critical point. Check whether the problem conditions justify ideal gas assumptions by calculating the reduced temperature and pressure. Use the compressibility factor (Z) or an equation of state when conditions are far from ideal.

The design equations differ for batch, CSTR, and PFR reactors. A CSTR operates at exit conditions (use exit concentrations in the rate expression), while a PFR requires integration along the reactor length. Using CSTR math for a PFR problem (or vice versa) produces fundamentally wrong sizing calculations.

Raoult's law without activity coefficients only works for ideal mixtures. For non-ideal systems (which most real systems are), you must include activity coefficients in VLE calculations. Ignoring non-ideality leads to significant errors in bubble point, dew point, and flash calculations.