Entropy Change Calculator
Calculate entropy change for heat transfer, isothermal gas expansion, or temperature change processes.
Includes universe entropy check.
Entropy Change
How Entropy Change Is Calculated
Entropy (S) measures disorder or randomness in a system. The entropy change for a process tells us whether a reaction is thermodynamically favorable when combined with enthalpy changes.
Entropy Change Formula:
ΔS = S_products − S_reactants
For a reversible process at constant temperature:
ΔS = Q_rev / T
Where:
- ΔS = entropy change in J/mol·K
- Q_rev = heat absorbed reversibly in Joules
- T = absolute temperature in Kelvin
Standard Entropy Values (J/mol·K at 25°C):
- H₂O(l): 69.9
- H₂O(g): 188.7
- CO₂(g): 213.8
- O₂(g): 205.2
- Fe(s): 27.3
- NaCl(s): 72.1
Worked Example — Vaporizing Water at 100°C:
- Q = 40,700 J/mol (enthalpy of vaporization)
- T = 373 K
- ΔS = 40,700 / 373 = 109.1 J/mol·K
Positive ΔS confirms the liquid → gas transition increases disorder.
Rules for Estimating Sign of ΔS:
- Solids → liquids → gases: ΔS > 0 (increases)
- More moles of gas produced: ΔS > 0
- Dissolving most solids in water: ΔS > 0
- Complex → simpler molecules: ΔS > 0
- Gases → solids/liquids: ΔS < 0
Gibbs Free Energy Connection: ΔG = ΔH − TΔS. Reaction is spontaneous when ΔG < 0.