Material and Energy Balances
Material and energy balances are the backbone of process analysis. You must be comfortable with: steady-state and unsteady-state (accumulation) balances; system boundary selection and control volume definition; recycle, bypass and purge streams; tie components and degrees-of-freedom analysis; extent of reaction and atomic balances for reactive systems; and the general energy balance (enthalpy in/out, heat, work, accumulation). Reference-state selection for enthalpy is a frequent topic.
Traps to expect: forgetting to include accumulation in transient problems; choosing a basis that makes the problem harder (pick 100 mol or 1 kg of feed when compositions are given); mishandling purge streams in recycle loops - purge is essential when inert or impurity buildup would otherwise be unbounded; and sign conventions for heat and work (define your convention explicitly). Degrees-of-freedom questions often hinge on whether a reaction provides an extra independent relation.
Worked example (rule of thumb): for a reactive system with C components and R independent reactions, the number of independent atomic/species balances is C − R plus the atomic balance constraints; if unknowns exceed equations, look for a specification or recycle relation you overlooked. For energy balances, always set a reference temperature (commonly 25 °C) for all species and compute sensible heat as m·Cp·ΔT for constant Cp, or via enthalpy tables otherwise.