In AP Chemistry, a closed system is one that can exchange energy (like heat) with its surroundings but not matter, which is why gases can't escape and a reversible reaction can settle into equilibrium with constant concentrations or partial pressures (Topics 6.1 and 7.1).
A closed system lets energy cross the boundary between the system and the surroundings, but matter stays put. Think of a sealed flask sitting in a water bath. Heat can flow in or out through the glass, but no molecules leave or enter. Contrast that with an open beaker, where water vapor or gas products can just drift away.
This matters in two places in the CED. In thermochemistry (Topic 6.1), you track heat moving between system and surroundings, so you need a boundary that energy can cross. In equilibrium (Topic 7.1), the system has to be closed for a reversible process to actually reach equilibrium. If PCl3 and Cl2 could escape the container, the reverse reaction would never have anything to work with, and the forward reaction would just run until the PCl5 was gone. Trapping the matter is what lets forward and reverse rates eventually match.
Closed systems show up in Unit 6 (Thermochemistry) and Unit 7 (Equilibrium). LO 6.1.A asks you to connect experimental observations to energy changes, and the whole exothermic/endothermic framework (EK 6.1.A.3) depends on energy flowing between the system and surroundings while you keep track of the matter inside. LO 7.1.A asks you to explain how reversible processes establish equilibrium, and EK 7.1.A.2 describes the result, which is constant concentrations or partial pressures with reactants and products both present. That constancy only happens when nothing can leak out. Almost every equilibrium MCQ stem on the exam starts with the phrase 'in a closed system,' and that phrase is doing real work. It's the exam's way of telling you equilibrium is even possible.
Keep studying AP Chemistry Unit 6
Chemical Equilibrium (Unit 7)
Equilibrium requires a closed system. A reversible reaction like PCl5 ⇌ PCl3 + Cl2 can only reach the point where forward and reverse rates are equal if the products are trapped in the container and available to react backward.
First Law of Thermodynamics (Unit 6)
Energy is conserved, so in a closed system any heat the reaction releases must show up in the surroundings, and vice versa. That's the bookkeeping logic behind calorimetry problems, where the water bath's temperature change tells you the system's energy change.
Partial Pressure (Units 3 & 7)
Constant partial pressures are the experimental fingerprint of equilibrium in a closed gas-phase system. If pressures stop changing over time, the reaction hasn't stopped. The forward and reverse rates have just become equal.
Conservation of Mass (Unit 4)
A closed system is conservation of mass made visible. Since no atoms enter or leave, the total mass inside the container never changes, which is why ICE tables and stoichiometry work cleanly in equilibrium problems.
You'll almost never get a question that asks 'define closed system.' Instead, the phrase appears in MCQ stems to set up equilibrium reasoning. A typical question describes a reaction in a closed system where partial pressures remain constant, then asks what must be true. The answer hinges on EK 7.1.A.2 and 7.1.A.3, meaning concentrations are constant because forward and reverse rates are equal, not because the reaction stopped. Other stems ask which microscopic process is happening at equilibrium (both reactions, simultaneously, at equal rates). On FRQs, the closed-system condition is usually given in the prompt, and your job is to use it. If asked why removing the lid or letting a gas escape changes things, explain that the system is no longer closed, so equilibrium can't be maintained.
The three system types differ in what crosses the boundary. An open system exchanges both matter and energy (an uncovered beaker), a closed system exchanges only energy (a sealed flask that can still heat up or cool down), and an isolated system exchanges neither (an idealized perfect thermos). AP Chem equilibrium problems assume closed, not isolated. Heat can still flow, which is exactly why you can run an equilibrium at a fixed temperature.
A closed system exchanges energy with its surroundings but not matter, like a sealed flask that can still be heated or cooled.
Equilibrium can only be established in a closed system, because escaping reactants or products would prevent the reverse reaction from balancing the forward one.
At equilibrium in a closed system, concentrations and partial pressures are constant, but the system is dynamic, with forward and reverse reactions still running at equal rates.
In thermochemistry, the closed boundary lets you track heat flow cleanly, so energy lost by an exothermic reaction equals energy gained by the surroundings.
Closed is not the same as isolated. Closed blocks matter only, while isolated blocks both matter and energy.
A closed system can exchange energy (such as heat) with its surroundings but cannot exchange matter. It's the standard setup for equilibrium problems in Unit 7 and for tracking heat flow in Unit 6.
No. Equilibrium is dynamic (EK 7.1.A.3). The forward and reverse reactions both keep running, but at equal rates, so concentrations and partial pressures stay constant even though molecules are constantly reacting.
A closed system blocks matter but allows energy transfer, while an isolated system blocks both. AP equilibrium problems use closed systems, since heat can still flow in or out to keep the temperature constant.
If products like Cl2 in PCl5 ⇌ PCl3 + Cl2 could escape, the reverse reaction would have nothing to react, and the forward reaction would run to completion instead. Trapping all species is what lets forward and reverse rates eventually equalize.
Yes. That's the defining feature. Only matter is blocked, so an exothermic reaction in a closed flask still transfers heat to the surroundings, which is how calorimetry measurements work in Topic 6.1.