Entropy Change Calculator
ΔS°rxn from absolute entropies, where elements are not zero.
ΔS°rxn from absolute entropies, where elements are not zero. Enter each product and reactant as its absolute entropy and coefficient.
Absolute entropies in J/(mol·K), written as value:coefficient.
Elements are NOT zero here — every substance has a positive S° at 298 K.
Standard entropy of reaction
-242.9 J/(mol·K)
Entropy decreases — unfavourable on its own
The trap here is assuming elements have S° = 0, by analogy with enthalpies of formation. They do not. The third law puts zero entropy at a perfect crystal at absolute zero, so every substance at 298 K has a positive absolute entropy — oxygen gas is 205.2 J/(mol·K), not nothing. The sign is usually predictable before you calculate: more moles of gas on the right means positive ΔS, fewer means negative. A reaction consuming gas to make a liquid or solid almost always loses entropy. Note the units. Entropy is tabulated in J/(mol·K) while enthalpy is in kJ/mol, and mixing them in ΔG = ΔH − TΔS gives an answer out by a factor of a thousand.
How the Entropy Change Calculator works
Enter each product and reactant as its absolute entropy and coefficient. The trap here is assuming elements have S° = 0 by analogy with enthalpies of formation. They do not, because the third law puts zero at a perfect crystal at absolute zero.
Also known as: delta s calculator · standard entropy of reaction calculator · entropy calculator for chemistry · how to calculate entropy change
Where to go next
The Entropy Change question rarely arrives on its own. These are the ones that usually come with it:
- Gibbs Free Energy Calculator — ΔG = ΔH − TΔS, with the unit mismatch handled.
- Enthalpy of Reaction Calculator — ΔH°rxn from standard enthalpies of formation.
- Hess's Law Calculator — Combine reaction steps, reversing and scaling as needed.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How do I calculate entropy change?
ΔS°rxn = Σ n·S°(products) − Σ n·S°(reactants), using absolute entropies in J/(mol·K). Same structure as the enthalpy calculation, different table.
Why is the entropy of an element not zero?
Because S° is an absolute entropy, measured from the third-law zero of a perfect crystal at 0 K. Every substance at 298 K has a positive value — oxygen gas is 205.2 J/(mol·K).
How can I predict the sign of ΔS?
Count moles of gas. More gas on the right means entropy increases; fewer means it falls. A reaction consuming gas to make a liquid or solid almost always loses entropy.
Why do the units matter here?
Because entropy is tabulated in J/(mol·K) while enthalpy is in kJ/mol. Combining them in ΔG = ΔH − TΔS without converting gives an answer out by a factor of a thousand.
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Related calculators
Gibbs Free Energy Calculator
ΔG = ΔH − TΔS, with the unit mismatch handled.
OpenEnthalpy of Reaction Calculator
ΔH°rxn from standard enthalpies of formation.
OpenHess's Law Calculator
Combine reaction steps, reversing and scaling as needed.
OpenSpeed Distance Time Calculator
Any one of the three from the other two, in four units.
Open