Entropy Change Calculator
Calculate entropy change for heat transfer, isothermal gas expansion, or temperature change processes.
Includes universe entropy check.
Entropy (S) is a measure of disorder or the number of possible microstates of a system. The Second Law of Thermodynamics states that the total entropy of the universe never decreases.
Three key entropy formulas:
1. Heat transfer (reversible process):
ΔS = q_rev / T
where q is heat in joules and T is absolute temperature in Kelvin.
2. Isothermal expansion of ideal gas:
ΔS = nR ln(V₂/V₁) = nR ln(P₁/P₂)
where n = moles, R = 8.314 J/mol·K.
3. Temperature change at constant pressure:
ΔS = nCp ln(T₂/T₁)
or at constant volume:
ΔS = nCv ln(T₂/T₁)
Units: J/K (absolute entropy) or J/mol·K (molar entropy)
Universe entropy:
ΔS_universe = ΔS_system + ΔS_surroundings
ΔS_surroundings = -q_system / T
For a spontaneous process: ΔS_universe > 0.
Molar heat capacities (Cp) at 25°C:
- Monatomic ideal gas: Cp = 5/2 R = 20.8 J/mol·K (He, Ne, Ar)
- Diatomic ideal gas: Cp = 7/2 R = 29.1 J/mol·K (N₂, O₂, H₂)
- Water (liquid): Cp = 75.3 J/mol·K
- Metals (Dulong-Petit): Cp ≈ 25 J/mol·K
Standard molar entropies (S°) at 25°C:
- H₂O(l): 69.9 J/mol·K
- CO₂(g): 213.7 J/mol·K
- N₂(g): 191.6 J/mol·K
- Fe(s): 27.3 J/mol·K