Entropy Change Calculator
The entropy change of a reaction, a temperature change or a phase transition — the disorder term in every spontaneity calculation.
Three cases.
How the entropy change calculator works
Three cases. For a reaction, ΔS = ΣS°(products) − ΣS°(reactants). For heating at constant pressure, ΔS = mc ln(T₂/T₁). For a phase change, ΔS = ΔH/T at the transition temperature.
The logarithm in the heating case is why entropy rises fast at low temperature and slowly at high — doubling from 100 K to 200 K adds the same entropy as doubling from 200 K to 400 K.
Formula: ΔS = ΣS°(prod) − ΣS°(react); ΔS = mc ln(T₂/T₁); ΔS = ΔH/T
Worked examples
| Inputs | Entropy change | Note |
|---|---|---|
| Methane combustion | -5.3 | −5.3 J/mol·K — barely changes |
| Heating a kilogram of water | 939.2705 | +941 J/K |
| Boiling water | 109.0714 | +109 J/mol·K |
FAQFrequently asked questions
What is entropy change?
How much the disorder of a system changes. Positive means more disorder; negative means more order.
Why does boiling increase entropy so much?
Because a gas has vastly more accessible arrangements than a liquid. Vaporization typically adds around 90 to 110 J/mol·K — Trouton's rule.
Why is heating logarithmic?
Because adding heat at a low temperature disorders the system more than the same heat at a high one. ΔS = ∫dq/T, and the 1/T is what produces the logarithm.
Can entropy decrease?
For a system, yes — freezing water is strongly negative. The second law only requires that the total, including the surroundings, does not fall.
How does this connect to spontaneity?
Through ΔG = ΔH − TΔS. A large positive ΔS can drive an endothermic reaction, and the higher the temperature the more it can do so.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.