Entropy
Entropy (S) measures how many ways energy and particles can be arranged, often called disorder. Gases have more entropy than liquids, and liquids have more than solids. Entropy rises when a substance is heated, dissolved or broken into more particles.
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Analogy
A tidy deck of cards has few arrangements. A shuffled deck has billions. Entropy counts the arrangements.
Example
H₂O(g) > H₂O(l) > H₂O(s)2 mol gas > 1 mol gasN₂ + 3H₂ → 2NH₃ΔS is negative (4 gas moles → 2)
In practice
Ice melting in a warm room increases entropy because water molecules can move more freely than in the rigid crystal.
Practice
Flashcards
- Which has higher entropy: 1 mol of gas or 1 mol of liquid?The gas.
Interview questions
- Explain how ΔH and ΔS combine to decide whether a reaction is spontaneous at different temperatures.
- ΔG = ΔH − TΔS
- Spontaneous if ΔG is negative
- Four sign combinations
- Temperature matters only when ΔH and ΔS have the same sign
Questions people ask
Does entropy always mean disorder?
Disorder is a handy picture, but a more exact idea is the number of ways energy can be spread over particles. A system with more possible arrangements has higher entropy. Calling it disorder works for gases and mixing, but it can mislead for subtler cases.
What is the third law of thermodynamics?
The third law says a perfect crystal at absolute zero (0 K) has zero entropy, because there is only one possible arrangement. This gives us a true zero for entropy, so absolute entropy values can be listed in tables, unlike enthalpy values, which need a reference point.