Entropy Change Calculator
ΔS°rxn from absolute entropies — where elements are not zero.
Work out Entropy Change. ΔS°rxn from absolute entropies — where elements are not zero. Names the sign error before you make it.
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
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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