Systems
High-Yield Summary
- The system is the part of the universe being analyzed; the surroundings are everything else; the boundary separates them and governs what can be exchanged.
- Isolated systems exchange neither energy nor matter; closed systems exchange energy only; open systems exchange both.
- State functions (temperature, pressure, volume, energy, enthalpy) depend only on a system's current state, not the path taken to get there.
- Path functions (heat, work) depend on the process taken between states — they aren't properties of the system itself.
Types of Thermodynamic Systems
| System Type | Energy / Matter Exchange |
|---|---|
| Isolated (e.g., bomb calorimeter) | No energy, no matter |
| Closed (e.g., steam radiator) | Energy yes, matter no |
| Open (e.g., pot of boiling water) | Energy yes, matter yes |
State Functions vs. Path Functions
| State Functions | Path Functions |
|---|---|
| Temperature, pressure, volume, energy, enthalpy | Heat, work |
| Depend only on current state | Depend on the process/path taken |
| Path-independent | Path-dependent |
Common MCAT Trap
- Heat and work are not state functions, even though they change a system's state — the same net change in state can be reached via different combinations of heat and work.
- Don't confuse 'closed' with 'isolated' — a closed system can still exchange energy (e.g., heat) with its surroundings; only an isolated system exchanges nothing.
Quick Recall
What can cross the boundary of an open system?
Why aren't heat and work considered state functions?