What is Entropy and the Second Law of Thermodynamics?
Entropy (S) measures disorder in a system — the second law states that entropy always increases in isolated systems. This law explains why reactions proceed in certain directions and why some processes are irreversible.
Entropy is disorder; the second law says ΔS_universe > 0 for spontaneous processes — disorder always increases overall.
- 1↓Initial stateOrdered, low entropy
- 2↓Process occursMolecules move randomly
- 3Final stateDisordered, high entropy
Step-by-step worked examples
Ice melts at room temperature. Is this spontaneous? Why?
ΔS > 0: solid ordered → liquid disordered ΔS_universe > 0 → spontaneous (favourable entropy)
Gas expands into vacuum. ΔS positive or negative?
Gas molecules spread over larger volume ΔS = +ve (higher disorder)
At 298 K, ΔH = 40 kJ/mol, ΔS = 150 J/(mol·K). Is melting spontaneous?
ΔG = ΔH − TΔS = 40 − 298×0.150 = 40 − 44.7 = −4.7 kJ ΔG < 0 → spontaneous
Flashcards
Quick quiz
Q1.Second law: entropy of isolated system…
Q2.Melting ice has what entropy change?
Q3.A perfectly ordered crystal at 0 K has…
Q4.Reversible process: entropy change?
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Common mistakes
Entropy = heat. — Correct: Entropy is disorder; heat is energy transfer.
Entropy always increases in any system. — Correct: Only in isolated systems; local decrease is possible if universe increases.
Second law forbids all disorder. — Correct: It states disorder increases — reactions favour disorder, not order.
Reversible processes have zero entropy. — Correct: Reversible: ΔS_universe = 0, but ΔS_system can be non-zero.
FAQ
What is entropy in simple terms?
Entropy is a measure of how disordered or random a system is. Higher entropy = more chaos.
Why does the second law matter?
It explains why some reactions happen spontaneously and others don't — disorder increases in isolated systems.
Can entropy ever decrease?
In one part of a system, yes. But the universe's total entropy always increases for spontaneous processes.
What is the third law of thermodynamics?
Perfect crystals at absolute zero (0 K) have zero entropy — the baseline for measuring S.