System

The part of the universe chosen for thermodynamic analysis. Open systems exchange both matter and energy with surroundings; closed systems exchange only energy; isolated systems exchange neither. The choice of system boundary affects how thermodynamic quantities are defined.

Defining the system is the first step in any thermodynamic analysis. Chemical reactions are analysed as systems to calculate heat, work, enthalpy, and entropy changes. Industrial reactor design treats the reaction vessel as the system for heat and mass balance calculations.

Isolated systems in thermodynamics are theoretical constructs. In practice, chemical processes approach isolated systems when thermal insulation is used, which can lead to heat accumulation and thermal runaway if exothermic reactions are not controlled.