Second Law of Thermodynamics
High-Yield Summary
- Entropy (S) measures a system's disorder/randomness and how evenly energy is distributed within it: ΔS = q/T.
- The Second Law: total entropy of a system and its surroundings always increases in any natural process.
- Entropy is sometimes called "time's arrow" — its one-way increase is what distinguishes past from future.
- Entropy of the universe is always increasing: ΔS_universe = ΔS_system + ΔS_surroundings > 0.
- Spontaneity depends on Gibbs free energy, ΔG = ΔH − TΔS; a process is spontaneous when ΔG < 0. Positive ΔS favors spontaneity but doesn't guarantee it.
Entropy and Gibbs Free Energy
ΔS = q/T | ΔS_universe = ΔS_system + ΔS_surroundings > 0 | ΔG = ΔH − TΔS
- q = Heat transferred
- T = Absolute temperature
- ΔG = Gibbs free energy change — negative means spontaneous
- ΔH = Enthalpy change
Must Know
- Systems evolve toward maximum disorder — energy spontaneously spreads out unless something hinders it.
- A positive ΔS favors spontaneity but doesn't guarantee ΔG < 0; enthalpy and temperature also matter.
- Entropy as "time's arrow": you can tell forward from backward in a video of a natural process because entropy only increases forward in time.
Common MCAT Trap
- A positive ΔS doesn't automatically mean a process is spontaneous — spontaneity depends on the full ΔG = ΔH − TΔS equation, not entropy alone.
- The Second Law applies to the system + surroundings together, not the system in isolation — a system's own entropy can decrease (e.g., a freezing liquid) as long as the surroundings' entropy increases by more.
Quick Recall
Does the Second Law forbid a system's entropy from ever decreasing?
What determines whether a process is spontaneous?