Absolute Entropy
Definition and meaning of Absolute Entropy in chemistry.
The absolute entropy of a substance is the total entropy it possesses at a given temperature, measured relative to a perfect crystal at absolute zero, which the third law of thermodynamics assigns an entropy of zero.
In more detail
Because there is a natural zero point, a perfectly ordered crystal at 0 K, entropy can be given an absolute value, unlike enthalpy, which is only ever measured as a change. Absolute entropy is found by measuring how a substance’s heat capacity changes as it is warmed from near absolute zero. It increases with temperature and rises sharply when a solid melts or a liquid boils.
Key facts
| Symbol | S |
|---|---|
| Units | J/(mol·K) |
| Zero reference | Perfect crystal at 0 K (third law) |
| Field | Physical chemistry (thermodynamics) |
The standard molar entropy of liquid water is about 70 J/(mol·K), while water vapor, being far more disordered, is about 189 J/(mol·K).
Frequently asked questions
Why can entropy have an absolute value but enthalpy cannot?
The third law of thermodynamics fixes the entropy of a perfect crystal at absolute zero as zero, giving a true reference point. Enthalpy has no such natural zero, so only enthalpy changes can be measured.