Microscopic Entropy, Entropy Changes, and the Third Law

All flashcards

Microstate

A specific microscopic arrangement of a system’s particles that is consistent with its observable state.

Entropy as a state function

Entropy change depends only on the initial and final states, not on the process path.

Entropy change at constant specific heat

For mass $m$ and constant specific heat $c$, heating from $T_A$ to $T_B$ gives $\Delta S=mc\ln(T_B/T_A)$, with temperatures in kelvins.

Entropy change in ideal-gas free expansion

For an ideal gas expanding from $V_1$ to $V_2$, $\Delta S=nR\ln(V_2/V_1)$.

Third law of thermodynamics

Absolute zero cannot be reached through any finite number of cooling steps.

Quiz questions

1. For an irreversible thermodynamic process, what happens to the entropy of the universe?Answer: It increases overall, even if one part becomes more ordered.
2. Why can a reversible path be used to calculate entropy change for an irreversible process?Answer: Entropy change depends on the initial and final states, not the path.
3. During melting at a fixed temperature, how is a substance’s entropy change calculated?Answer: Divide the heat transferred reversibly by the absolute temperature.
4. What does the third law state about reaching absolute zero?Answer: It cannot be reached through any finite number of cooling steps.
5. An ideal gas expands freely from volume $V_1$ to $V_2$. What is its entropy change?Answer: $nR\ln(V_2/V_1)$