Entropy & the Second/Third Laws Flashcards
(16 cards)
Give the thermodynamic definition of entropy.
Entropy is a state function quantifying the degree of dispersal of energy and matter, commonly interpreted as “randomness” or “disorder.”
State the units of entropy in the SI system.
J K⁻¹ mol⁻¹.
How does composite mass affect entropy magnitude?
For chemically similar substances under comparable conditions, the greater the molar mass, the larger the molar entropy because more microstates are accessible.
Can entropy change (ΔS) be negative? Under what circumstance?
Yes; ΔS < 0 when a process results in decreased molecular dispersal (e.g., gas compression or crystallization).
Define a microstate in statistical thermodynamics.
A microstate is a unique distribution of particles and energies consistent with a system’s macroscopic condition.
How does Boltzmann relate entropy to the number of microstates (W)?
S = kB ln W, where kB is the Boltzmann constant.
Explain why gas expansion into a vacuum is entropically favored.
The expanded state offers vastly more positional microstates, maximizing W and thus S.
What qualitative ordering of phase entropies is always valid?
S solid < S liquid ≪ S gas.
State the Second Law succinctly.
In every spontaneous process, the entropy of the universe increases.
Write the mathematical criterion for spontaneity in entropy terms.
ΔS univ = ΔS sys + ΔS surr > 0.
Why can energy be conserved while entropy increases?
Energy conservation pertains to quantity, whereas entropy measures quality (dispersion); conversions disperse energy even as total energy remains constant.
Give an everyday example illustrating the Second Law.
Hot coffee cooling in a room: heat flows to the surroundings, increasing environmental entropy more than the cup’s entropy decreases.
State the Third Law.
The entropy of a perfect crystalline substance approaches zero as temperature approaches absolute zero (0 K).
Explain why residual entropy can exist even at 0 K.
If a crystal possesses positional or orientational disorder (e.g., CO ice), multiple microstates remain accessible, giving S > 0.
How does the Third Law permit absolute entropies to be tabulated?
Because it supplies a universal reference point (S 0 K = 0), enabling direct calorimetric integration of Cp/T from 0 K to the temperature of interest.
Why does entropy rise continuously with temperature even in a single phase?
Thermal agitation populates additional vibrational, rotational, and translational states, increasing the number of accessible microstates.
