Clausius-Clapeyron (Enthalpy of Vaporization)
The phase-equilibrium member the lab phys-chem set (ideal gas, Arrhenius, Nernst) was missing.
Example
You enter
- Vapor pressure P1 (any unit) 760
- Temperature 1 (°C) 100
- Vapor pressure P2 (same unit as P1) 525.9
- Temperature 2 (°C) 90
You get
- Enthalpy of vaporization dHvap 41.48
Details, formula, and sources
From two vapor-pressure/temperature points: dHvap = R ln(P2/P1) / (1/T1 - 1/T2), R = 8.314 J/(mol*K), T in kelvin. Only the pressure RATIO enters, so any consistent unit (kPa, mmHg, atm, psi) works. Water at 760 mmHg / 100 C and 525.9 mmHg / 90 C returns about 41.5 kJ/mol, close to the tabulated 40.7 (the small excess is the constant-enthalpy, ideal-vapor approximation over a 10 C span). Reports the enthalpy and the ln(P) vs 1/T slope (-dHvap/R). Apply the same equation with this enthalpy to predict a vapor pressure or boiling point at another temperature. A first-principles chemistry aid.
ln(P2/P1) = -(dHvap/R)(1/T2 - 1/T1), so dHvap = R ln(P2/P1) / (1/T1 - 1/T2); R = 8.314 J/(mol*K), T in kelvin. Only the pressure ratio enters. Slope of ln(P) vs 1/T is -dHvap/R.
The Clausius-Clapeyron equation; standard physical chemistry / thermodynamics. First principles.
The Clausius-Clapeyron equation and the gas constant R are public first-principles chemistry; the vapor pressures and temperatures are the user's own measurements.
Verify protocol against your lab's SOP before pipetting. A miscalculated dilution can ruin a run or a sample.
Field names used by the API: pressure1, temp1_c, pressure2, temp2_c, enthalpy_kj_mol
- Clausius-Clapeyron dHvap = R ln(P2/P1)/(1/T1 - 1/T2), T in kelvinClausius-Clapeyron
- Unit-agnostic only the pressure ratio enters; P1 and P2 may be in any consistent unitratio form
- Scope assumes constant dHvap and an ideal vapor over the interval (a curved ln(P) vs 1/T signals variation)scope of this tile