Entropy Calculator

Calculate entropy change delta S for chemical reactions using standard molar entropies. Determine if disorder increases or decreases and predict temperature dependence of spontaneity.

⚗️ Thermodynamics📐 ΔS° = ΣS°(products) - ΣS°(reactants)🧪 Chemistry

Sum S° of products (J/mol·K)

Sum S° of reactants (J/mol·K)

Please enter valid values.

Formula & Reference

Variable	Symbol	Formula	Units
Entropy Calculator	See formula	ΔS° = ΣS°(products) - ΣS°(reactants)	J/mol·K

Step-by-Step Examples

Example 1

Combustion of Methane

CH4+2O2 to CO2+2H2O. S deg: CH4=186.3, O2=205.2, CO2=213.8, H2O(g)=188.7 J/molK.

Products: 213.8+2(188.7)=591.2 J/molK
Reactants: 186.3+2(205.2)=596.7 J/molK
delta-S = 591.2-596.7 = -5.5 J/molK

✓ delta-S = -5.5 J/molK (slight decrease in disorder)

Example 2

Vaporization of Water

H2O(l) to H2O(g). S: liquid=69.9, gas=188.7 J/molK.

delta-S = 188.7-69.9 = +118.8 J/molK
Entropy increases: liquid to gas (more disorder)

✓ delta-S = +118.8 J/molK

Example 3

Ammonia Synthesis

N2+3H2 to 2NH3. S: N2=191.6, H2=130.7, NH3=192.5 J/molK.

Products: 2(192.5)=385.0
Reactants: 191.6+3(130.7)=583.7
delta-S = 385.0-583.7 = -198.7 J/molK (4 mol gas to 2 mol gas)

✓ delta-S = -198.7 J/molK (fewer moles of gas)

Real-World Applications

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Thermodynamics

Entropy is the measure of disorder; central to the second law.

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Gibbs Energy

dG = dH - TdS. Entropy determines temperature dependence of spontaneity.

🌍

Environmental Chemistry

Entropy explains why pollutants disperse and why cleanup requires energy.

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Biology

Protein folding decreases entropy; driven by enthalpy and solvent effects.

Common Mistakes to Avoid

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Entropy is in J not kJ

Entropy is in J/molK while enthalpy is in kJ/mol. Convert dS to kJ when computing dG = dH - TdS: divide by 1000.

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Standard entropy of elements is not zero

Unlike dHf, standard molar entropy S is never zero (except at 0 K). H2(g)=130.7, O2(g)=205.2 J/molK.

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Count mol of gas as a guide

More moles of gas in products vs reactants generally means positive dS.

Frequently Asked Questions

What is the second law of thermodynamics? ▾

The total entropy of the universe always increases for spontaneous processes. dS_universe > 0 for all real processes.

Why do elements have non-zero S? ▾

Third law: entropy of perfect crystal at 0 K = 0. At 25 C, elements have thermal entropy from atomic vibrations.

How does phase change affect entropy? ▾

S(gas) >> S(liquid) > S(solid). Vaporization always has large positive dS; freezing always negative dS.

What controls entropy for reactions? ▾

Key factor: change in moles of gas (delta-n). More gas moles in products = positive dS. Structural complexity also matters.

What is residual entropy? ▾

Some materials retain entropy at 0 K due to structural disorder. CO and ice have small residual entropies from random orientations.

Related Chemistry Calculators

Formula Explorer connections

Interpretation: This relationship tracks energy transfer, state-function change or the balance between enthalpy and entropy in a chemical process. Assumption: Keep energy units compatible, use kelvin for absolute temperature, and match standard states and reaction stoichiometry. Thermodynamic favorability does not determine reaction speed.

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