Clausius-Clapeyron Equation Calculator

The Clausius-Clapeyron equation relates the vapor pressure of a liquid to its temperature and molar enthalpy of vaporization. Its integrated form is: ln(P₂/P₁) = −(ΔHvap/R) × (1/T₂ − 1/T₁), where R is the ideal gas constant (8.314 J/mol·K). It allows you to predict vapor pressure at a new temperature if you know it at a reference temperature.

Vapor pressure is the pressure exerted by the vapor of a substance in equilibrium with its liquid phase in a closed system. It reflects how readily molecules escape from the liquid surface. Substances with higher vapor pressures are more volatile. Vapor pressure increases with temperature.

The enthalpy of vaporization, also called the heat of vaporization, is the energy required to convert one mole of a liquid into a gas at constant pressure. For water, ΔHvap ≈ 40,700 J/mol at 100 °C. Higher values indicate stronger intermolecular forces and lower vapor pressures at a given temperature.

The Clausius-Clapeyron equation uses absolute temperature ratios in its derivation from thermodynamic principles. Celsius or Fahrenheit scales include arbitrary zero points that make the math incorrect. Always convert: K = °C + 273.15 before entering values into this calculator.

A liquid boils when its vapor pressure equals the surrounding atmospheric pressure. If atmospheric pressure decreases (e.g., at high altitude), the boiling point drops because the liquid reaches that lower pressure at a lower temperature. This is why water boils at less than 100 °C on a mountain.

R is the universal gas constant, equal to 8.314 J/(mol·K). It appears in the Clausius-Clapeyron equation because the derivation assumes the vapor behaves as an ideal gas and that the molar volume of the liquid is negligible compared to that of the vapor.

Some common values: water ≈ 40,700 J/mol, ethanol ≈ 38,600 J/mol, benzene ≈ 30,700 J/mol, and diethyl ether ≈ 26,000 J/mol. Substances with stronger hydrogen bonding or higher molecular weight generally have larger enthalpies of vaporization.

The equation assumes: (1) the vapor behaves as an ideal gas, (2) the molar volume of the liquid is negligible compared to the vapor, and (3) ΔHvap is constant over the temperature range considered. For large temperature ranges or non-ideal gases, more advanced equations like the Antoine equation may give better accuracy.