System, Surroundings and Types of System

In thermodynamics, the first and most important step in analysing any process is to draw a clear boundary around the small portion of the universe we actually want to study. The part inside this boundary is called the system; everything else — literally the rest of the universe — is called the surroundings. The real or imaginary dividing surface between them is the boundary, and it can be rigid or movable, and permeable or impermeable to matter and heat.

Systems are classified into three types based on what they are allowed to exchange with their surroundings across the boundary:

• An open system can exchange both matter and energy (heat and work) with its surroundings. A beaker of hot water sitting open on a bench is an open system — it loses both heat and water vapour to the air around it.
• A closed system can exchange energy but not matter with its surroundings. A gas sealed inside a cylinder fitted with a movable, airtight piston is a closed system — the gas amount stays fixed, but heat can flow through the cylinder walls and work can be done as the piston moves.
• An isolated system can exchange neither matter nor energy with its surroundings. A perfectly insulated, sealed thermos flask is the standard approximation of an isolated system — in reality no system is truly perfectly isolated, but a well-insulated, closed vacuum flask comes close enough over short time-scales to be treated as one for most purposes.

Choosing the system correctly, and identifying which of these three types it is, is the essential first step before any of the energy bookkeeping developed in the rest of this chapter — the First Law, enthalpy, Hess's law, entropy, and Gibbs free energy — can be applied to a real chemical process. A chemical reaction taking place in an open beaker is normally treated as an open (or, if the gas produced is negligible, approximately closed) system at constant atmospheric pressure; a reaction run in a sealed bomb calorimeter is a closed, constant-volume system. Getting this classification right is what lets us decide, later in the chapter, whether the heat exchanged should be identified with $\Delta U$ (constant volume) or $\Delta H$ (constant pressure).