A liquid–vapor mixture of a volatile solvent is undergoing an exothermic chemical reaction in an uncovered beaker placed on an insulated bench top. Vapor produced during boiling escapes freely into the room. After the reaction has gone to completion, the liquid in the beaker and the surrounding laboratory air return to the initial room temperature.
Consider three different choices of system boundary and analyze each in turn.
(a) System A: the reacting liquid alone (contents of the beaker). Classify the system type and predict the sign of its total entropy change from start of reaction until the beaker contents have returned to room temperature. Justify with reference to energy and matter exchanges.
(b) System B: the reacting liquid plus the beaker walls, but not the escaping vapor. Repeat the classification and entropy-sign prediction for this definition.
(c) System C: everything described—the reacting liquid, beaker walls, escaping vapor, and surrounding laboratory air. Identify the system type and argue the sign of its overall entropy change.
(d) Compare and reconcile the three results, commenting on how inclusion or exclusion of mass flow alters compliance with the second law while leaving the physical reality unchanged.