9.2 Absolute Entropy and Entropy Change

Absolute Entropy

Entropy (S) measures the number of microstates (dispersal of matter/energy) in a system. Unlike enthalpy, substances have absolute entropy values (S°, not ΔS°) because a perfect crystal at 0 K has zero entropy (third law of thermodynamics). Entropy generally increases: solid < liquid < gas, with more complex molecules and higher temperatures having greater entropy.

Predicting Entropy Change (ΔS)

ΔS is positive when: a solid/liquid becomes a gas, moles of gas increase, a substance dissolves, or temperature increases. ΔS is negative for the reverse changes. For reactions, ΔS°rxn = ΣS°products − ΣS°reactants, weighted by stoichiometric coefficients.

Entropy and Molecular Complexity

More complex molecules and larger/heavier molecules tend to have higher entropy due to more possible microstates (vibrational, rotational modes). Mixing substances (like gases or solutions) also increases entropy compared to separated pure components, since mixing increases the number of possible arrangements.

Key terms

Entropy (S)

A measure of the number of microstates or the dispersal of matter and energy in a system; higher entropy means more disorder/possible arrangements.

Third law of thermodynamics

A perfect crystal at 0 K has zero entropy, giving entropy an absolute scale (unlike enthalpy).

General entropy trend by phase

Solid < liquid < gas; entropy increases as matter becomes more disordered.

ΔS°rxn formula

ΔS°rxn = ΣS°products − ΣS°reactants, weighted by stoichiometric coefficients.

Factors that increase entropy

Increasing temperature, increasing moles of gas, dissolving a solute, mixing substances, and greater molecular complexity.

Try an AP-style question

What is ΔS° for N₂ + 3H₂ → 2NH₃?

1. +385.0 J/K
2. +198.7 J/K
3. −129.8 J/K
4. −198.7 J/K