Entropy Change
可逆相变或传热过程的熵变等于可逆热量除以温度。
| Item | Formula / Note | Source / Standard |
|---|---|---|
| Entropy change | ΔS = ∫δQ_rev/T | Clausius 1865 |
| Ideal gas | ΔS = c_v ln(T₂/T₁) + R ln(V₂/V₁) | Thermodynamic formula |
| Reversibility | ΔS depends only on end states | State function |
| Second law | ΔS ≥0 for isolated system | Second law |
| Units | J/K or J/(mol·K) | SI units |
| Irreversible | Compute via reversible path | Thermodynamic method |
Constants used on this page and their authoritative sources, for verification and citation
| Constant | Value | Unit | Source |
|---|---|---|---|
| Molar gas constant R | 8.314462618 | J·mol⁻¹·K⁻¹ | New SI (derived from exact N_A·k) |
| Boltzmann constant k | 1.380649e-23 | J·K⁻¹ | SI defining constant (exact) |
| Boiling point of water (1 atm) | 373.15 | K | ITS-90 defining value |
| Molar enthalpy of vaporisation of water at 100 ℃ | 40.656 | kJ·mol⁻¹ | NIST Chemistry WebBook / IAPWS-IF97 |
Note: the molar entropy of vaporisation of water is ΔS = 40656 J/mol ÷ 373.15 K ≈ 108.96 J·mol⁻¹·K⁻¹ (about 10.9 J/K for 0.1 mol).