Thermodynamics flash cards

1. 1.What is thermodynamics?

   The branch of science that deals with the study of energy changes (heat, work) accompanying physical and chemical transformations. It deals only with initial and final states, not the rate or mechanism.

   Hint: Energy transformations in matter.

2. 2.Define a system and surroundings in thermodynamics.

   System = the part of the universe chosen for study. Surroundings = everything else in the universe outside the system. System + Surroundings = Universe.

   Hint: Under study vs. rest.

3. 3.What separates the system from the surroundings?

   The boundary — a real or imaginary wall that separates the system from the surroundings. Energy and/or matter may cross it.

   Hint: Real or imaginary wall.

4. 4.Distinguish open, closed and isolated systems.

   Open: exchanges both matter and energy. Closed: exchanges only energy, not matter. Isolated: exchanges neither matter nor energy.

   Hint: Think: cup of tea (open), sealed flask (closed), thermos (isolated).

5. 5.Give an example of an isolated system.

   A thermos flask (or coffee in a stoppered vacuum flask) — ideally exchanges neither matter nor energy with surroundings.

   Hint: Perfectly insulated, sealed.

6. 6.What is a state function?

   A property whose value depends only on the current state of the system (defined by state variables like $T$, $P$, $V$), not on the path taken to reach that state.

   Hint: Path-independent.

7. 7.Name common thermodynamic state functions.

   Internal energy $U$, enthalpy $H$, entropy $S$, Gibbs energy $G$, pressure $P$, volume $V$, temperature $T$.

   Hint: $U, H, S, G, P, V, T$.

8. 8.What is a path function? Give two examples.

   A quantity whose value depends on the path/process followed, not just initial and final states. Examples: heat ($q$) and work ($w$).

   Hint: Depends on how you get there.

9. 9.Define the state of a system.

   The condition of a system described by its measurable macroscopic properties (state variables) such as $P$, $V$, $T$ and composition.

   Hint: Set of $P,V,T$ values.

10. 10.What is an extensive property? Give examples.

    A property that depends on the amount (quantity) of matter present. Examples: mass, volume, internal energy, enthalpy, entropy, heat capacity, Gibbs energy.

    Hint: Scales with amount.

11. 11.What is an intensive property? Give examples.

    A property independent of the amount of matter. Examples: temperature, pressure, density, molar volume, refractive index, viscosity, molar heat capacity.

    Hint: Independent of amount.

12. 12.Is molar heat capacity intensive or extensive? Why?

    Intensive — it is heat capacity per mole, so the amount is normalized out. (Total heat capacity is extensive.)

    Hint: Per mole → intensive.

13. 13.What is internal energy ($U$)?

    The total energy stored within a system — the sum of all kinetic and potential energies of its molecules (translational, rotational, vibrational, electronic, nuclear, and interaction energies). It is a state function.

    Hint: Total microscopic energy stored inside.

14. 14.Can the absolute value of internal energy be measured?

    No. Only the change $\Delta U$ can be measured, not the absolute value of $U$.

    Hint: Only $\Delta U$ is measurable.

15. 15.State the First Law of Thermodynamics (law of conservation of energy).

    Energy can neither be created nor destroyed, only transformed. Mathematically $\Delta U = q + w$, where $q$ = heat absorbed by system and $w$ = work done on system.

    Hint: Energy is conserved.

16. 16.Write the mathematical form of the first law and the sign convention.

    $\Delta U = q + w$. Sign convention: $q > 0$ if heat is absorbed by system; $w > 0$ if work is done on the system. $q < 0$ if heat released; $w < 0$ if work done by system.

    Hint: Heat in +, work done on system +.

17. 17.For an isolated system, what does the first law give?

    $q = 0$ and $w = 0$, so $\Delta U = 0$. The internal energy of an isolated system is constant.

    Hint: No exchange → $\Delta U = 0$.

18. 18.What is work in thermodynamics (pressure–volume work)?

    Mechanical work associated with change in volume against an external pressure. For expansion/compression, $w = -P_{ext}\,\Delta V$ (work done on the system).

    Hint: $w = -P_{ext}\Delta V$.

19. 19.Write the expression for work done in irreversible isothermal expansion of an ideal gas.

    $w = -P_{ext}(V_2 - V_1)$, where $P_{ext}$ is the constant external pressure against which expansion occurs.

    Hint: Constant $P_{ext}$ against volume change.

20. 20.Write the expression for work done in reversible isothermal expansion of an ideal gas.

    $w = -2.303\,nRT\log\dfrac{V_2}{V_1} = -2.303\,nRT\log\dfrac{P_1}{P_2}$.

    Hint: Log of volume/pressure ratio.

21. 21.Which gives maximum work — reversible or irreversible expansion?

    Reversible (isothermal) expansion gives the maximum work, because the gas works against the largest possible opposing pressure at every step.

    Hint: Reversible = maximum work.

22. 22.What is a reversible process?

    A process carried out infinitesimally slowly so that the system remains in equilibrium with surroundings at every stage; the driving and opposing forces differ only infinitesimally. It can be reversed by an infinitesimal change.

    Hint: Infinitely slow, always at equilibrium.

23. 23.What is free expansion and its work?

    Expansion of a gas into vacuum ($P_{ext} = 0$). Work done $w = -P_{ext}\Delta V = 0$. No work is done.

    Hint: Into vacuum → $w=0$.

24. 24.For an ideal gas expanding into vacuum isothermally, what are $q$, $w$ and $\Delta U$?

    $w = 0$ (free expansion), $\Delta U = 0$ (isothermal, ideal gas depends only on $T$), so $q = 0$.

    Hint: All three are zero.

25. 25.Define an isothermal process.

    A process carried out at constant temperature ($\Delta T = 0$). Heat may flow in/out to maintain temperature.

    Hint: Constant $T$.

26. 26.Define an adiabatic process.

    A process in which no heat is exchanged between system and surroundings ($q = 0$). Hence $\Delta U = w$.

    Hint: $q = 0$; insulated.

27. 27.Define isobaric and isochoric processes.

    Isobaric: constant pressure ($\Delta P = 0$). Isochoric: constant volume ($\Delta V = 0$, so $w = 0$).

    Hint: Constant $P$ vs. constant $V$.

28. 28.For a process at constant volume, relate $q_v$ to internal energy.

    At constant $V$, $w = -P\Delta V = 0$, so $\Delta U = q_v$. Heat absorbed at constant volume equals the change in internal energy.

    Hint: $q_v = \Delta U$.

29. 29.Define enthalpy ($H$).

    Enthalpy is the total heat content of a system at constant pressure: $H = U + PV$. It is a state function and extensive property.

    Hint: $H = U + PV$.

30. 30.For a process at constant pressure, relate $q_p$ to enthalpy.

    At constant $P$, $\Delta H = \Delta U + P\Delta V = q_p$. Heat absorbed at constant pressure equals the enthalpy change.

    Hint: $q_p = \Delta H$.

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