2.5.1 · D5Thermodynamics (Chemical)
Question bank — System vs surroundings; open, closed, isolated
Before we start, some anchors so every line reads cleanly. The single figure below turns the "three switches" idea into a picture you can point at while answering.
Read the figure left to right: each column is one independent switch, and the labels "open / closed / isolated" only appear once all three switches are set. Keep it in view — every trap below is really a question about which switch a phrase is secretly setting.
True or false — justify
TF1. A closed system cannot exchange energy with its surroundings.
False. Closed means impermeable to matter only; from both energy channels ( and ) stay open — a sealed can in a fire gains heat, so .
TF2. Every isolated system is also a closed system.
True. Isolated blocks matter and energy; closed only requires blocking matter. Blocking more is still blocking matter, so isolated satisfies the closed condition.
TF3. Every closed system is also isolated.
False. The converse fails — a closed system still lets energy through ( or is allowed), so it need not be isolated.
TF4. A rigid boundary automatically makes the system closed.
False. "Rigid" is a mechanical switch (fixed volume → work ). Matter could still cross a rigid but porous wall, so rigidity alone says nothing about openness.
TF5. For any isolated system, .
True. No heat and no work (of any kind) cross the boundary, so in both terms are zero: ; internal energy is fixed.
TF6. For any closed system, .
False. Closed only fixes mass. In the and terms can still be nonzero — a sealed flask being warmed has , so .
TF7. The universe is a closed system.
Technically true but too weak. Since the universe is isolated (nothing crosses, so , , ) and every isolated system is also closed (TF2), calling it "closed" is not wrong — it is just the loosest correct label. The precise description is isolated; saying only "closed" wrongly hints that energy could still be exchanged.
TF8. An open system in steady state has .
True, but it's a special case. Steady state means input rate equals output rate, so the internal energy of the system stops changing even though matter and energy keep flowing through.
TF9. A pot of boiling water with the lid off is a closed system because the water stays "in the pot."
False. Steam (matter) escapes, so it is open. What matters is whether matter crosses the boundary, not whether it stays in the container.
TF10. Choosing what counts as the "system" is a physical fact fixed by nature.
False. The boundary is our choice, drawn around whatever we want to analyse; nature does not label the system for us.
TF11. An adiabatic boundary means no energy at all can cross.
False. Adiabatic sets only. If the wall is movable, energy can still cross as work — e.g. an adiabatic gas compressed by a piston (, so ; see Adiabatic Processes).
Spot the error
SE1. "The steel bomb calorimeter is rigid, so no matter escapes — that's why it's a closed system."
The reasoning confuses two switches. It is closed because the wall is impermeable; it has (for work) because it is rigid. Rigid ≠ impermeable.
SE2. "This container is adiabatic, therefore ."
Adiabatic gives , so , not . Internal energy still changes if work is done (unless the wall is also rigid and no non- work runs, forcing ).
SE3. "The room is closed off from the lab, so it's isolated — nothing gets in or out."
Everyday "closed off" ≠ thermodynamic isolated. Heat and light still leak through walls (), so at best it is a (leaky) closed system, not isolated.
SE4. "A living cell exchanges nutrients and waste, so it must be closed."
Exchanging matter is precisely what makes it open, not closed. Closed forbids matter transfer.
SE5. "The gas expands and pushes the piston, but the container is sealed, so no energy left the system."
Pushing the piston is work done by the system, so — energy left as work even though no matter escaped. Sealed controls matter, not energy.
SE6. "We heated the isolated thermos and the water warmed up, so for the system."
If it truly is isolated then by definition. Either the "heating" came from inside the system, or the thermos isn't actually isolated.
SE7. " always, for any system."
The Second-Law statement applies to an isolated system (or the universe). A non-isolated system's entropy can decrease () if it dumps entropy to its surroundings — see Entropy and Second Law.
SE8. "Rigid walls mean ."
Rigid means work (no volume change), not . A rigid diathermal wall lets heat pour in freely, e.g. the constant-volume heating case.
SE9. "The container is rigid, so no work of any kind can be done — guaranteed."
Rigid only kills work. A sealed rigid box containing a battery still does electrical work across its terminals, so is possible even with .
Why questions
WHY1. Why do chemists design most lab experiments as closed systems?
Fixing the moles removes mass-flow bookkeeping, so the First Law reduces to the clean and heat can be measured directly by Calorimetry.
WHY2. Why must the Second Law be stated for an isolated system specifically?
Only when nothing crosses the boundary is entropy generation forced to stay inside; then measures irreversibility without a surroundings term hiding leaks.
WHY3. Why is "rigid" listed as a separate boundary property from "impermeable"?
They answer different questions — impermeable asks "can matter cross?", rigid asks "can volume change?". A wall can be rigid yet porous, or flexible yet sealed.
WHY4. Why does a closed rigid container (with no electrical or other non- work) give ?
Rigid forces , so the work , where is the external pressure; with no other work mode running, , so all the heat becomes internal energy.
WHY5. Why can an open system accumulate energy even though it "exchanges freely"?
"Exchange" describes flow across the boundary, but measures what stays inside. If input exceeds output, energy piles up and .
WHY6. Why do we treat the boundary as possibly imaginary?
Thermodynamics only needs to know what crosses a surface; that surface can be a mathematical shape in space (e.g. a region of the atmosphere) with no physical wall at all.
WHY7. Why does an adiabatic (but movable) piston still let internal energy change?
No heat crosses (), but pushing or being pushed transfers work, so — the basis of Adiabatic Processes.
WHY8. Why is the constant-pressure heat convenient for open beakers on a bench?
Such systems sit at constant atmospheric pressure. Enthalpy is defined so that, at constant pressure, the heat absorbed equals — folding the expansion work in automatically (see Enthalpy and Constant Pressure). Here is the constant-pressure heat capacity.
WHY9. Why can't we assume "" always means ""?
Because is all non-heat energy transfer. Electrical, surface-tension, and shaft work cross the boundary with no volume change, so writing is only valid when the sole work mode is expansion.
Edge cases
EC1. A perfectly insulated flask with a movable piston — matter sealed, adiabatic, but not rigid. Isolated?
No. It blocks matter and heat, but the movable piston lets work cross (). Isolated requires blocking work too, i.e. rigid and adiabatic and impermeable (and no non- work path).
EC2. A sealed, rigid, diathermal steel can. Which type, and what is ?
Closed (matter blocked, heat allowed); the work because rigid forbids volume change. If no wires run in, .
EC3. Two water samples mixing inside one perfect thermos — is each sample isolated?
No. The combined contents are isolated ( to outside), but each sample exchanges heat with the other. Isolation is a property of your chosen boundary, and here it encloses both.
EC4. The limiting "perfect thermos" or "insulated bomb calorimeter" — are these truly isolated?
Only as idealizations. Real ones leak a little heat over time, so they are isolated on short timescales and drift toward closed on long ones.
EC5. Degenerate case: what if we draw the boundary around the entire universe?
There is nothing outside to exchange with, so both and are identically zero — the universe is the definitional example of an isolated system (and, being isolated, trivially closed too).
EC6. A test tube open to air but at steady state (constant temperature, no reaction). Open or closed, and what is ?
Still open (air and vapour can cross the mouth), but because nothing inside is actually changing. Openness is about possible crossing, not net change.
EC7. Zero--work but nonzero-heat limit — a rigid wall that is also diathermal. Which two switches are set how?
The movable/rigid switch is set to rigid (so work ), and the diathermal/adiabatic switch is set to diathermal (so heat is allowed, ). With no other work mode, this isolates , the $C_V$ route.
EC8. A sealed rigid electrolysis cell with current flowing in through wires. Closed? Is ?
Closed (matter blocked, rigid so no work). But : electrical work enters through the wires, so — a clean case where "rigid" does not force .
Recall Two-line summary to lock it in
Closed vs isolated ::: Closed blocks matter only — its heat- and work-switches are left free, so a closed system may be diathermal or adiabatic, movable or rigid. Isolated blocks matter and energy (, , so ). The universe is ::: best called isolated (no outside means no exchange); it is also trivially closed, but "isolated" is the precise label and is why .