How this calculator works
Choose the physical process before entering values. Reaction mode subtracts reactant entropy totals from product totals, with stoichiometric multipliers already included. Reversible heat mode assumes a constant absolute temperature. Ideal-gas modes assume fixed gas amount and isothermal expansion or compression, so pressure and volume ratios are reciprocal. All totals refer to the same system or reaction extent. If starting from molar entropies, multiply by the corresponding amounts first.
Formula
DeltaSreaction = Sproducts - Sreactants; DeltaS = qrev/T; DeltaS = (DeltaH - DeltaG)/T; DeltaSgas = nR ln(V2/V1) = nR ln(P1/P2).
Worked example
Product and reactant totals of 300 and 200 J/K give DeltaS = 100 J/K. Reversibly absorbing 600 J at 300 K gives 2 J/K. Doubling the volume of 1 mol ideal gas adds about 5.76315 J/K.
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Frequently asked questions
Can I use arbitrary heat for qrev?
No. Use heat along a reversible path connecting the same states.
Does compression lower gas entropy?
For isothermal ideal-gas compression at fixed amount, yes.
Do I enter molar entropy or total entropy?
This page uses total J/K. Apply stoichiometric amounts to molar data before entry.