Entropy measures how many ways the particles and their energy can be arranged. Gases have far more than liquids, liquids more than solids, so you can predict the sign of an entropy change by counting states and moles of gas, then calculate it from standard entropies.
Entropy, S, is a measure of the dispersal of energy in a system: the more ways the particles and their energy can be arranged, the greater the entropy. Its units are J K−1 mol−1, not kJ. Entropy increases from solid to liquid to gas (ice 48, water 70, steam 189), when temperature rises, when a solid dissolves, and when a reaction produces more moles of gas. A positive ΔS means energy becomes more dispersed. The standard entropy change of a reaction is ΔS° = ΣS°(products) − ΣS°(reactants), using the stoichiometric coefficients; for CaCO3 → CaO + CO2 it is 40 + 214 − 93 = +161 J K−1 mol−1.
Compare the before and after particle pictures in the simulator and watch how freely the particles move. More freedom means more possible arrangements, so more entropy.
| Change | Example | Sign of ΔS |
|---|---|---|
| melting, boiling, sublimation | H2O(l) → H2O(g) | + (+119) |
| freezing, condensing | H2O(l) → H2O(s) | − (−22) |
| raising temperature | particles have a wider spread of energies | + |
| dissolving a solid | NaCl(s) → Na+(aq) + Cl−(aq) | usually + |
| more moles of gas formed | CaCO3(s) → CaO(s) + CO2(g) | + |
| fewer moles of gas formed | N2(g) + 3H2(g) → 2NH3(g) | − (4 mol gas → 2 mol) |
Work through these with the standard entropy values given. Multiply each value by its coefficient in the equation.
| Substance | S° / J K−1 mol−1 | Substance | S° / J K−1 mol−1 |
|---|---|---|---|
| CaCO3(s) | 93 | N2(g) | 192 |
| CaO(s) | 40 | H2(g) | 131 |
| CO2(g) | 214 | NH3(g) | 193 |
| Mg(s) | 33 | O2(g) | 205 |
| MgO(s) | 27 | CH4(g) | 186 |
Entropy values are in J K−1 mol−1, not kJ. Elements do not have zero entropy, so O2, H2 and N2 must be included. Multiply by the coefficients, and always do products minus reactants. The state symbol matters: H2O(l) is 70 but H2O(g) is 189.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What are the units of standard entropy?
Which change has a positive entropy change?
Which substance has the greatest standard entropy?
What is the standard entropy of O2(g)?
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Using S°: N2 192, H2 131, NH3 193 J K−1 mol−1, what is ΔS° for N2 + 3H2 → 2NH3?
Using S°: CaCO3 93, CaO 40, CO2 214 J K−1 mol−1, what is ΔS° for CaCO3 → CaO + CO2?
Which reaction has the most negative entropy change?
Using S°: CH4(g) 186, O2(g) 205, CO2(g) 214, H2O(l) 70 J K−1 mol−1, what is ΔS° for CH4 + 2O2 → CO2 + 2H2O(l)?
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